For review: user_namespace(7) man page

Hello Eric et al.,

For various reasons, my work on the namespaces man pages fell off the table a while back. Nevertheless, the pages have been close to completion for a while now, and I recently restarted, in an effort to finish them. As you also noted to me f2f, there have been recently been some small namespace changes that you may affect the content of the pages. Therefore, I'll take the opportunity to send the namespace-related pages out for further (final?) review.

So, here, I start with the user_namespaces(7) page, which is shown in rendered form below, with source attached to this mail. I'll send various other pages in follow-on mails.

Review comments/suggestions for improvements / bug fixes welcome.

Cheers,

Michael

==

NAME
user_namespaces - overview of Linux user_namespaces

DESCRIPTION
For an overview of namespaces, see namespaces(7).

User namespaces isolate security-related identifiers and
attributes, in particular, user IDs and group IDs (see creden‐ tials(7), the root directory, keys (see keyctl(2)), and capabili‐ ties (see capabilities(7)). A process's user and group IDs can be different inside and outside a user namespace. In particular, a process can have a normal unprivileged user ID outside a user namespace while at the same time having a user ID of 0 inside the namespace; in other words, the process has full privileges for operations inside the user namespace, but is unprivileged for operations outside the namespace.

Nested namespaces, namespace membership
User namespaces can be nested; that is, each user namespace— except the initial ("root") namespace—has a parent user names‐ pace, and can have zero or more child user namespaces. The par‐ ent user namespace is the user namespace of the process that cre‐ ates the user namespace via a call to unshare(2) or clone(2) with the CLONE_NEWUSER flag.

The kernel imposes (since version 3.11) a limit of 32 nested lev‐ els of user namespaces. Calls to unshare(2) or clone(2) that would cause this limit to be exceeded fail with the error EUSERS.

Each process is a member of exactly one user namespace. A process created via fork(2) or clone(2) without the CLONE_NEWUSER flag is a member of the same user namespace as its parent. A process can join another user namespace with setns(2) if it has the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a full set of capabilities in that namespace.

A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag makes the new child process (for clone(2)) or the caller (for unshare(2)) a member of the new user namespace created by the call.

Capabilities
The child process created by clone(2) with the CLONE_NEWUSER flag starts out with a complete set of capabilities in the new user namespace. Likewise, a process that creates a new user namespace using unshare(2) or joins an existing user namespace using setns(2) gains a full set of capabilities in that namespace. On the other hand, that process has no capabilities in the parent (in the case of clone(2)) or previous (in the case of unshare(2) and setns(2)) user namespace, even if the new namespace is cre‐ ated or joined by the root user (i.e., a process with user ID 0 in the root namespace).

Note that a call to execve(2) will cause a process to lose any capabilities that it has, unless it has a user ID of 0 within the namespace. See the discussion of user and group ID mappings, below.

Hello Eric et al.,

For various reasons, my work on the namespaces man pages fell off the table a while back. Nevertheless, the pages have been close to completion for a while now, and I recently restarted, in an effort to finish them. As you also noted to me f2f, there have been recently been some small namespace changes that you may affect the content of the pages. Therefore, I'll take the opportunity to send the namespace-related pages out for further (final?) review.

So, here, I start with the user_namespaces(7) page, which is shown in rendered form below, with source attached to this mail. I'll send various other pages in follow-on mails.

Review comments/suggestions for improvements / bug fixes welcome.

Cheers,

Michael

==

NAME
user_namespaces - overview of Linux user_namespaces

DESCRIPTION
For an overview of namespaces, see namespaces(7).

User namespaces isolate security-related identifiers and
attributes, in particular, user IDs and group IDs (see creden‐ tials(7), the root directory, keys (see keyctl(2)), and capabili‐ ties (see capabilities(7)). A process's user and group IDs can be different inside and outside a user namespace. In particular, a process can have a normal unprivileged user ID outside a user namespace while at the same time having a user ID of 0 inside the namespace; in other words, the process has full privileges for operations inside the user namespace, but is unprivileged for operations outside the namespace.

Nested namespaces, namespace membership
User namespaces can be nested; that is, each user namespace— except the initial ("root") namespace—has a parent user names‐ pace, and can have zero or more child user namespaces. The par‐ ent user namespace is the user namespace of the process that cre‐ ates the user namespace via a call to unshare(2) or clone(2) with the CLONE_NEWUSER flag.

The kernel imposes (since version 3.11) a limit of 32 nested lev‐ els of user namespaces. Calls to unshare(2) or clone(2) that would cause this limit to be exceeded fail with the error EUSERS.

Each process is a member of exactly one user namespace. A process created via fork(2) or clone(2) without the CLONE_NEWUSER flag is a member of the same user namespace as its parent. A

process can join another user namespace with setns(2) if it has the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a full set of capabilities in that namespace.

A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag makes the new child process (for clone(2)) or the caller (for unshare(2)) a member of the new user namespace created by the call.

Capabilities
The child process created by clone(2) with the CLONE_NEWUSER flag starts out with a complete set of capabilities in the new user namespace. Likewise, a process that creates a new user namespace using unshare(2) or joins an existing user namespace using setns(2) gains a full set of capabilities in that namespace. On the other hand, that process has no capabilities in the parent (in the case of clone(2)) or previous (in the case of unshare(2) and setns(2)) user namespace, even if the new namespace is cre‐ ated or joined by the root user (i.e., a process with user ID 0 in the root namespace).

Note that a call to execve(2) will cause a process to lose any capabilities that it has, unless it has a user ID of 0 within the namespace. See the discussion of user and group ID mappings, below.

A call to clone(2), unshare(2), or setns(2) using the
CLONE_NEWUSER flag sets the "securebits" flags (see capabili‐ ties(7)) to their default values (all flags disabled) in the child (for clone(2)) or caller (for unshare(2), or setns(2)). Note that because the caller no longer has capabilities in its original user namespace after a call to setns(2), it is not pos‐ sible for a process to reset its "securebits" flags while retain‐ ing its user namespace membership by using a pair of setns(2) calls to move to another user namespace and then return to its original user namespace.

Having a capability inside a user namespace permits a process to perform operations (that require privilege) only on resources governed by that namespace. The rules for determining whether or not a process has a capability in a particular user namespace are as follows:

1. A process has a capability inside a user namespace if it is a member of that namespace and it has the capability in its effective capability set. A process can gain capabilities in its effective capability set in various ways. For example, it may execute a set-user-ID program or an executable with asso‐
ciated file capabilities. In addition, a process may gain
capabilities via the effect of clone(2), unshare(2), or setns(2), as already described.

2. If a process has a capability in a user namespace, then it has that capability in all child (and further removed descendant) namespaces as well.

3. When a user namespace is created, the kernel records the effective user ID of the creating process as being the "owner" of the namespace. A process that resides in the parent of the user namespace and whose effective user ID matches the owner of the namespace has all capabilities in the namespace. By virtue of the previous rule, this means that the process has all capabilities in all further removed descendant user names‐ paces as well.

Interaction of user namespaces and other types of namespaces
Starting in Linux 3.8, unprivileged processes can create user namespaces, and mount, PID, IPC, network, and UTS namespaces can be created with just the CAP_SYS_ADMIN capability in the caller's user namespace.

If CLONE_NEWUSER is specified along with other CLONE_NEW* flags in a single clone(2) or unshare(2) call, the user namespace is guaranteed to be created first, giving the child (clone(2)) or caller (unshare(2)) privileges over the remaining namespaces cre‐ ated by the call. Thus, it is possible for an unprivileged call‐ er to specify this combination of flags.

When a new IPC, mount, network, PID, or UTS namespace is created via clone(2) or unshare(2), the kernel records the user namespace of the creating process against the new namespace. (This associ‐
ation can't be changed.) When a process in the new namespace
subsequently performs privileged operations that operate on global resources isolated by the namespace, the permission checks are performed according to the process's capabilities in the user namespace that the kernel associated with the new namespace.

User and group ID mappings: uid_map and gid_map
When a user namespace is created, it starts out without a mapping
of user IDs (group IDs) to the parent user namespace. The
/proc/[pid]/uid_map and /proc/[pid]/gid_map files (available
since Linux 3.5) expose the mappings for user and group IDs inside the user namespace for the process pid. These files can be read to view the mappings in a user namespace and written to (once) to define the mappings.

The description in the following paragraphs explains the details for uid_map; gid_map is exactly the same, but each instance of "user ID" is replaced by "group ID".

The uid_map file exposes the mapping of user IDs from the user namespace of the process pid to the user namespace of the process that opened uid_map (but see a qualification to this point below). In other words, processes that are in different user namespaces will potentially see different values when reading from a particular uid_map file, depending on the user ID mappings for the user namespaces of the reading processes.

Each line in the uid_map file specifies a 1-to-1 mapping of a
range of contiguous user IDs between two user namespaces. (When
a user namespace is first created, this file is empty.) The specification in each line takes the form of three numbers delim‐ ited by white space. The first two numbers specify the starting
user ID in each of the two user namespaces. The third number
specifies the length of the mapped range. In detail, the fields are interpreted as follows:

(1) The start of the range of user IDs in the user namespace of the process pid.

(2) The start of the range of user IDs to which the user IDs specified by field one map. How field two is interpreted depends on whether the process that opened uid_map and the process pid are in the same user namespace, as follows:

a) If the two processes are in different user namespaces:
field two is the start of a range of user IDs in the user namespace of the process that opened uid_map.

b) If the two processes are in the same user namespace: field two is the start of the range of user IDs in the parent user namespace of the process pid. This case enables the opener of uid_map (the common case here is opening /proc/self/uid_map) to see the mapping of user IDs into the user namespace of the process that created this user namespace.

(3) The length of the range of user IDs that is mapped between the two user namespaces.

System calls that return user IDs (group IDs)—for example, getuid(2), getgid(2), and the credential fields in the structure returned by stat(2)—return the user ID (group ID) mapped into the caller's user namespace.

When a process accesses a file, its user and group IDs are mapped into the initial user namespace for the purpose of permission checking and assigning IDs when creating a file. When a process retrieves file user and group IDs via stat(2), the IDs are mapped in the opposite direction, to produce values relative to the process user and group ID mappings.

The initial user namespace has no parent namespace, but, for con‐ sistency, the kernel provides dummy user and group ID mapping files for this namespace. Looking at the uid_map file (gid_map is the same) from a shell in the initial namespace shows:

$ cat /proc/$$/uid_map
0 0 4294967295

This mapping tells us that the range starting at user ID 0 in this namespace maps to a range starting at 0 in the (nonexistent) parent namespace, and the length of the range is the largest 32-bit unsigned integer.

Defining user and group ID mappings: writing to uid_map and gid_map
After the creation of a new user namespace, the uid_map file of one of the processes in the namespace may be written to once to define the mapping of user IDs in the new user namespace. An attempt to write more than once to a uid_map file in a user namespace fails with the error EPERM. Similar rules apply for gid_map files.

The lines written to uid_map (gid_map) must conform to the fol‐ lowing rules:

* The three fields must be valid numbers, and the last field must be greater than 0.

* Lines are terminated by newline characters.

* There is an (arbitrary) limit on the number of lines in the file. As at Linux 3.8, the limit is five lines. In addition, the number of bytes written to the file must be less than the system page size, and the write must be performed at the start of the file (i.e., lseek(2) and pwrite(2) can't be used to write to nonzero offsets in the file).

* The range of user IDs (group IDs) specified in each line can‐ not overlap with the ranges in any other lines. In the ini‐ tial implementation (Linux 3.8), this requirement was satis‐ fied by a simplistic implementation that imposed the further requirement that the values in both field 1 and field 2 of successive lines must be in ascending numerical order, which prevented some otherwise valid maps from being created. Linux 3.9 and later fix this limitation, allowing any valid set of nonoverlapping maps.

* At least one line must be written to the file.

Writes that violate the above rules fail with the error EINVAL.

In order for a process to write to the /proc/[pid]/uid_map (/proc/[pid]/gid_map) file, all of the following requirements must be met:

1. The writing process must have the CAP_SETUID (CAP_SETGID) capability in the user namespace of the process pid.

2. The writing process must be in either the user namespace of the process pid or inside the parent user namespace of the process pid.

3. The mapped user IDs (group IDs) must in turn have a mapping in the parent user namespace.

4. One of the following is true:

* The data written to uid_map (gid_map) consists of a single line that maps the writing process's filesystem user ID (group ID) in the parent user namespace to a user ID (group ID) in the user namespace. The usual case here is that this single line provides a mapping for user ID of the process that created the namespace.

* The process has the CAP_SETUID (CAP_SETGID) capability in the parent user namespace. Thus, a privileged process can make mappings to arbitrary user IDs (group IDs) in the par‐ ent user namespace.

Writes that violate the above rules fail with the error EPERM.

Unmapped user and group IDs
There are various places where an unmapped user ID (group ID) may be exposed to user space. For example, the first process in a new user namespace may call getuid() before a user ID mapping has been defined for the namespace. In most such cases, an unmapped user ID is converted to the overflow user ID (group ID); the
default value for the overflow user ID (group ID) is 65534. See
the descriptions of /proc/sys/kernel/overflowuid and
/proc/sys/kernel/overflowgid in proc(5).

The cases where unmapped IDs are mapped in this fashion include system calls that return user IDs (getuid(2) getgid(2), and simi‐ lar), credentials passed over a UNIX domain socket, credentials returned by stat(2), waitid(2), and the System V IPC "ctl" IPC_STAT operations, credentials exposed by /proc/PID/status and the files in /proc/sysvipc/*, credentials returned via the si_uid field in the siginfo_t received with a signal (see sigaction(2)), credentials written to the process accounting file (see acct(5)), and credentials returned with POSIX message queue notifications (see mq_notify(3)).

There is one notable case where unmapped user and group IDs are not converted to the corresponding overflow ID value. When view‐ ing a uid_map or gid_map file in which there is no mapping for the second field, that field is displayed as 4294967295 (-1 as an unsigned integer);

Set-user-ID and set-group-ID programs
When a process inside a user namespace executes a set-user-ID (set-group-ID) program, the process's effective user (group) ID inside the namespace is changed to whatever value is mapped for the user (group) ID of the file. However, if either the user or the group ID of the file has no mapping inside the namespace, the set-user-ID (set-group-ID) bit is silently ignored: the new pro‐ gram is executed, but the process's effective user (group) ID is left unchanged. (This mirrors the semantics of executing a set- user-ID or set-group-ID program that resides on a filesystem that was mounted with the MS_NOSUID flag, as described in mount(2).)

Miscellaneous
When a process's user and group IDs are passed over a UNIX domain socket to a process in a different user namespace (see the description of SCM_CREDENTIALS in unix(7)), they are translated into the corresponding values as per the receiving process's user and group ID mappings.

CONFORMING TO
Namespaces are a Linux-specific feature.

NOTES
Over the years, there have been a lot of features that have been added to the Linux kernel that have been made available only to privileged users because of their potential to confuse set-user-
ID-root applications. In general, it becomes safe to allow the
root user in a user namespace to use those features because it is impossible, while in a user namespace, to gain more privilege than the root user of a user namespace has.

Availability
Use of user namespaces requires a kernel that is configured with the CONFIG_USER_NS option. User namespaces require support in a range of subsystems across the kernel. When an unsupported sub‐ system is configured into the kernel, it is not possible to con‐ figure user namespaces support.

As at Linux 3.8, most relevant subsystems supported user names‐ paces, but a number of filesystems did not have the infrastruc‐ ture needed to map user and group IDs between user namespaces. Linux 3.9 added the required infrastructure support for many of the remaining unsupported filesystems (Plan 9 (9P), Andrew File System (AFS), Ceph, CIFS, CODA, NFS, and OCFS2). Linux 3.11 added support the last of the unsupported major filesystems, XFS.

EXAMPLE
The program below is designed to allow experimenting with user namespaces, as well as other types of namespaces. It creates namespaces as specified by command-line options and then executes
a command inside those namespaces. The comments and usage()
function inside the program provide a full explanation of the program. The following shell session demonstrates its use.

First, we look at the run-time environment:

$ uname -rs # Need Linux 3.8 or later
Linux 3.8.0
$ id -u # Running as unprivileged user
1000
$ id -g
1000

Now start a new shell in new user (-U), mount (-m), and PID (-p) namespaces, with user ID (-M) and group ID (-G) 1000 mapped to 0 inside the user namespace:

$ ./userns_child_exec -p -m -U -M '0 1000 1' -G '0 1000 1' bash

The shell has PID 1, because it is the first process in the new PID namespace:

bash$ echo $$
1

Inside the user namespace, the shell has user and group ID 0, and a full set of permitted and effective capabilities:

bash$ cat /proc/$$/status | egrep '^[UG]id'
Uid: 0 0 0 0
Gid: 0 0 0 0
bash$ cat /proc/$$/status | egrep '^Cap(Prm|Inh|Eff)'
CapInh: 0000000000000000
CapPrm: 0000001fffffffff
CapEff: 0000001fffffffff

Mounting a new /proc filesystem and listing all of the processes visible in the new PID namespace shows that the shell can't see any processes outside the PID namespace:

bash$ mount -t proc proc /proc
bash$ ps ax
PID TTY STAT TIME COMMAND
1 pts/3 S 0:00 bash
22 pts/3 R+ 0:00 ps ax

   Program source

       /* userns_child_exec.c

          Licensed under GNU General Public License v2 or later

          Create a child process that executes a shell command in new
          namespace(s); allow UID and GID mappings to be specified when
          creating a user namespace.
       */
       #define _GNU_SOURCE
       #include <sched.h>
       #include <unistd.h>
       #include <stdlib.h>
       #include <sys/wait.h>
       #include <signal.h>
       #include <fcntl.h>
       #include <stdio.h>
       #include <string.h>
       #include <limits.h>
       #include <errno.h>

       /* A simple error-handling function: print an error message based
          on the value in 'errno' and terminate the calling process */

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)

       struct child_args {
           char **argv;        /* Command to be executed by child, with args */
           int    pipe_fd[2];  /* Pipe used to synchronize parent and child */
       };

       static int verbose;

       static void
       usage(char *pname)
       {
           fprintf(stderr, "Usage: %s [options] cmd [arg...]\n\n", pname);
           fprintf(stderr, "Create a child process that executes a shell "
                   "command in a new user namespace,\n"
                   "and possibly also other new namespace(s).\n\n");
           fprintf(stderr, "Options can be:\n\n");
       #define fpe(str) fprintf(stderr, "    %s", str);
           fpe("-i          New IPC namespace\n");
           fpe("-m          New mount namespace\n");
           fpe("-n          New network namespace\n");
           fpe("-p          New PID namespace\n");
           fpe("-u          New UTS namespace\n");
           fpe("-U          New user namespace\n");
           fpe("-M uid_map  Specify UID map for user namespace\n");
           fpe("-G gid_map  Specify GID map for user namespace\n");
           fpe("-z          Map user's UID and GID to 0 in user namespace\n");
           fpe("            (equivalent to: -M '0 <uid> 1' -G '0 <gid> 1')\n");
           fpe("-v          Display verbose messages\n");
           fpe("\n");
           fpe("If -z, -M, or -G is specified, -U is required.\n");
           fpe("It is not permitted to specify both -z and either -M or -G.\n");
           fpe("\n");
           fpe("Map strings for -M and -G consist of records of the form:\n");
           fpe("\n");
           fpe("    ID-inside-ns   ID-outside-ns   len\n");
           fpe("\n");
           fpe("A map string can contain multiple records, separated"
               " by commas;\n");
           fpe("the commas are replaced by newlines before writing"
               " to map files.\n");

           exit(EXIT_FAILURE);
       }

       /* Update the mapping file 'map_file', with the value provided in
          'mapping', a string that defines a UID or GID mapping. A UID or
          GID mapping consists of one or more newline-delimited records
          of the form:

              ID_inside-ns    ID-outside-ns   length

          Requiring the user to supply a string that contains newlines is
          of course inconvenient for command-line use. Thus, we permit the
          use of commas to delimit records in this string, and replace them
          with newlines before writing the string to the file. */

       static void
       update_map(char *mapping, char *map_file)
       {
           int fd, j;
           size_t map_len;     /* Length of 'mapping' */

           /* Replace commas in mapping string with newlines */

           map_len = strlen(mapping);
           for (j = 0; j < map_len; j++)
               if (mapping[j] == ',')
                   mapping[j] = '\n';

           fd = open(map_file, O_RDWR);
           if (fd == -1) {
               fprintf(stderr, "ERROR: open %s: %s\n", map_file,
                       strerror(errno));
               exit(EXIT_FAILURE);
           }

           if (write(fd, mapping, map_len) != map_len) {
               fprintf(stderr, "ERROR: write %s: %s\n", map_file,
                       strerror(errno));
               exit(EXIT_FAILURE);
           }

           close(fd);
       }

       static int              /* Start function for cloned child */
       childFunc(void *arg)
       {
           struct child_args *args = (struct child_args *) arg;
           char ch;

           /* Wait until the parent has updated the UID and GID mappings.
              See the comment in main(). We wait for end of file on a
              pipe that will be closed by the parent process once it has
              updated the mappings. */

           close(args->pipe_fd[1]);    /* Close our descriptor for the write
                                          end of the pipe so that we see EOF
                                          when parent closes its descriptor */
           if (read(args->pipe_fd[0], &ch, 1) != 0) {
               fprintf(stderr,
                       "Failure in child: read from pipe returned != 0\n");
               exit(EXIT_FAILURE);
           }

           /* Execute a shell command */

           printf("About to exec %s\n", args->argv[0]);
           execvp(args->argv[0], args->argv);
           errExit("execvp");
       }

       #define STACK_SIZE (1024 * 1024)

       static char child_stack[STACK_SIZE];    /* Space for child's stack */

       int
       main(int argc, char *argv[])
       {
           int flags, opt, map_zero;
           pid_t child_pid;
           struct child_args args;
           char *uid_map, *gid_map;
           const int MAP_BUF_SIZE = 100;
           char map_buf[MAP_BUF_SIZE];
           char map_path[PATH_MAX];

           /* Parse command-line options. The initial '+' character in
              the final getopt() argument prevents GNU-style permutation
              of command-line options. That's useful, since sometimes
              the 'command' to be executed by this program itself
              has command-line options. We don't want getopt() to treat
              those as options to this program. */

           flags = 0;
           verbose = 0;
           gid_map = NULL;
           uid_map = NULL;
           map_zero = 0;
           while ((opt = getopt(argc, argv, "+imnpuUM:G:zv")) != -1) {
               switch (opt) {
               case 'i': flags |= CLONE_NEWIPC;        break;
               case 'm': flags |= CLONE_NEWNS;         break;
               case 'n': flags |= CLONE_NEWNET;        break;
               case 'p': flags |= CLONE_NEWPID;        break;
               case 'u': flags |= CLONE_NEWUTS;        break;
               case 'v': verbose = 1;                  break;
               case 'z': map_zero = 1;                 break;
               case 'M': uid_map = optarg;             break;
               case 'G': gid_map = optarg;             break;
               case 'U': flags |= CLONE_NEWUSER;       break;
               default:  usage(argv[0]);
               }
           }

           /* -M or -G without -U is nonsensical */

           if (((uid_map != NULL || gid_map != NULL || map_zero) &&
                       !(flags & CLONE_NEWUSER)) ||
                   (map_zero && (uid_map != NULL || gid_map != NULL)))
               usage(argv[0]);

           args.argv = &argv[optind];

           /* We use a pipe to synchronize the parent and child, in order to
              ensure that the parent sets the UID and GID maps before the child
              calls execve(). This ensures that the child maintains its
              capabilities during the execve() in the common case where we
              want to map the child's effective user ID to 0 in the new user
              namespace. Without this synchronization, the child would lose
              its capabilities if it performed an execve() with nonzero
              user IDs (see the capabilities(7) man page for details of the
              transformation of a process's capabilities during execve()). */

           if (pipe(args.pipe_fd) == -1)
               errExit("pipe");

           /* Create the child in new namespace(s) */

           child_pid = clone(childFunc, child_stack + STACK_SIZE,
                             flags | SIGCHLD, &args);
           if (child_pid == -1)
               errExit("clone");

           /* Parent falls through to here */

           if (verbose)
               printf("%s: PID of child created by clone() is %ld\n",
                       argv[0], (long) child_pid);

           /* Update the UID and GID maps in the child */

           if (uid_map != NULL || map_zero) {
               snprintf(map_path, PATH_MAX, "/proc/%ld/uid_map",
                       (long) child_pid);
               if (map_zero) {
                   snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1", (long) getuid());
                   uid_map = map_buf;
               }
               update_map(uid_map, map_path);
           }
           if (gid_map != NULL || map_zero) {
               snprintf(map_path, PATH_MAX, "/proc/%ld/gid_map",
                       (long) child_pid);
               if (map_zero) {
                   snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1", (long) getgid());
                   gid_map = map_buf;
               }
               update_map(gid_map, map_path);
           }

           /* Close the write end of the pipe, to signal to the child that we
              have updated the UID and GID maps */

           close(args.pipe_fd[1]);

           if (waitpid(child_pid, NULL, 0) == -1)      /* Wait for child */
               errExit("waitpid");

           if (verbose)
               printf("%s: terminating\n", argv[0]);

           exit(EXIT_SUCCESS);
       }

SEE ALSO

       newgidmap(1),   newuidmap(1),   clone(2),  setns(2),  unshare(2),
       proc(5), subgid(5), subuid(5),  credentials(7),  capabilities(7),
       namespaces(7), pid_namespaces(7)

       The  kernel  source  file  Documentation/namespaces/resource-con‐
       trol.txt.

Quoted 75 lines · Michael Kerrisk

Hello Eric et al.,

For various reasons, my work on the namespaces man pages fell off the table a while back. Nevertheless, the pages have been close to completion for a while now, and I recently restarted, in an effort to finish them. As you also noted to me f2f, there have been recently been some small namespace changes that you may affect the content of the pages. Therefore, I'll take the opportunity to send the namespace-related pages out for further (final?) review.

So, here, I start with the user_namespaces(7) page, which is shown in rendered form below, with source attached to this mail. I'll send various other pages in follow-on mails.

Review comments/suggestions for improvements / bug fixes welcome.

Cheers,

Michael

==

NAME
user_namespaces - overview of Linux user_namespaces

DESCRIPTION
For an overview of namespaces, see namespaces(7).

User namespaces isolate security-related identifiers and
attributes, in particular, user IDs and group IDs (see creden‐ tials(7), the root directory, keys (see keyctl(2)), and capabili‐ ties (see capabilities(7)). A process's user and group IDs can be different inside and outside a user namespace. In particular, a process can have a normal unprivileged user ID outside a user namespace while at the same time having a user ID of 0 inside the namespace; in other words, the process has full privileges for operations inside the user namespace, but is unprivileged for operations outside the namespace.

Nested namespaces, namespace membership
User namespaces can be nested; that is, each user namespace— except the initial ("root") namespace—has a parent user names‐ pace, and can have zero or more child user namespaces. The par‐ ent user namespace is the user namespace of the process that cre‐ ates the user namespace via a call to unshare(2) or clone(2) with the CLONE_NEWUSER flag.

The kernel imposes (since version 3.11) a limit of 32 nested lev‐ els of user namespaces. Calls to unshare(2) or clone(2) that would cause this limit to be exceeded fail with the error EUSERS.

Each process is a member of exactly one user namespace. A process created via fork(2) or clone(2) without the CLONE_NEWUSER flag is a member of the same user namespace as its parent. A process can join another user namespace with setns(2) if it has the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a full set of capabilities in that namespace.

A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag makes the new child process (for clone(2)) or the caller (for unshare(2)) a member of the new user namespace created by the call.

Capabilities
The child process created by clone(2) with the CLONE_NEWUSER flag starts out with a complete set of capabilities in the new user namespace. Likewise, a process that creates a new user namespace using unshare(2) or joins an existing user namespace using setns(2) gains a full set of capabilities in that namespace. On the other hand, that process has no capabilities in the parent (in the case of clone(2)) or previous (in the case of unshare(2) and setns(2)) user namespace, even if the new namespace is cre‐ ated or joined by the root user (i.e., a process with user ID 0 in the root namespace).

Quoted 4 lines

Note that a call to execve(2) will cause a process to lose any capabilities that it has, unless it has a user ID of 0 within the namespace. See the discussion of user and group ID mappings, below.

The above is an approximation, but a bit misleading. On exec, the task capability set is recalculated according to the usual rules. So if the file being executed has file capabilities, the result task may end up with capabilities even if it is not root (even if it is uid -1).

Perhaps it should be phrased as:

Note that a call to execve(2) will cause a process' capabilities to be recalculated (see capabilities(7)), so that usually, unless it has a user ID of 0 within the namespace, it will lose all capabilities. See the discussion of user and group ID mappings, below.

Quoted 47 lines · Michael Kerrisk (man-pages)

Hello Eric et al.,

For various reasons, my work on the namespaces man pages fell off the table a while back. Nevertheless, the pages have been close to completion for a while now, and I recently restarted, in an effort to finish them. As you also noted to me f2f, there have been recently been some small namespace changes that you may affect the content of the pages. Therefore, I'll take the opportunity to send the namespace-related pages out for further (final?) review.

So, here, I start with the user_namespaces(7) page, which is shown in rendered form below, with source attached to this mail. I'll send various other pages in follow-on mails.

Review comments/suggestions for improvements / bug fixes welcome.

Cheers,

Michael

==

NAME
user_namespaces - overview of Linux user_namespaces

DESCRIPTION
For an overview of namespaces, see namespaces(7).

User namespaces isolate security-related identifiers and
attributes, in particular, user IDs and group IDs (see creden‐ tials(7), the root directory, keys (see keyctl(2)), and capabili‐ ties (see capabilities(7)). A process's user and group IDs can be different inside and outside a user namespace. In particular, a process can have a normal unprivileged user ID outside a user namespace while at the same time having a user ID of 0 inside the namespace; in other words, the process has full privileges for operations inside the user namespace, but is unprivileged for operations outside the namespace.

Nested namespaces, namespace membership
User namespaces can be nested; that is, each user namespace— except the initial ("root") namespace—has a parent user names‐ pace, and can have zero or more child user namespaces. The par‐ ent user namespace is the user namespace of the process that cre‐ ates the user namespace via a call to unshare(2) or clone(2) with the CLONE_NEWUSER flag.

Quoted 8 lines

The kernel imposes (since version 3.11) a limit of 32 nested lev‐ els of user namespaces. Calls to unshare(2) or clone(2) that would cause this limit to be exceeded fail with the error EUSERS.

Each process is a member of exactly one user namespace. A process created via fork(2) or clone(2) without the CLONE_NEWUSER flag is a member of the same user namespace as its parent. A

^ single-threaded

Because of chroot and other things multi-threaded processes are not allowed to join a user namespace. For the documentation just saying single-threaded sounds like enough here.

Quoted 78 lines

process can join another user namespace with setns(2) if it has the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a full set of capabilities in that namespace.

A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag makes the new child process (for clone(2)) or the caller (for unshare(2)) a member of the new user namespace created by the call.

Capabilities
The child process created by clone(2) with the CLONE_NEWUSER flag starts out with a complete set of capabilities in the new user namespace. Likewise, a process that creates a new user namespace using unshare(2) or joins an existing user namespace using setns(2) gains a full set of capabilities in that namespace. On the other hand, that process has no capabilities in the parent (in the case of clone(2)) or previous (in the case of unshare(2) and setns(2)) user namespace, even if the new namespace is cre‐ ated or joined by the root user (i.e., a process with user ID 0 in the root namespace).

Note that a call to execve(2) will cause a process to lose any capabilities that it has, unless it has a user ID of 0 within the namespace. See the discussion of user and group ID mappings, below.

A call to clone(2), unshare(2), or setns(2) using the
CLONE_NEWUSER flag sets the "securebits" flags (see capabili‐ ties(7)) to their default values (all flags disabled) in the child (for clone(2)) or caller (for unshare(2), or setns(2)). Note that because the caller no longer has capabilities in its original user namespace after a call to setns(2), it is not pos‐ sible for a process to reset its "securebits" flags while retain‐ ing its user namespace membership by using a pair of setns(2) calls to move to another user namespace and then return to its original user namespace.

Having a capability inside a user namespace permits a process to perform operations (that require privilege) only on resources governed by that namespace. The rules for determining whether or not a process has a capability in a particular user namespace are as follows:

1. A process has a capability inside a user namespace if it is a member of that namespace and it has the capability in its effective capability set. A process can gain capabilities in its effective capability set in various ways. For example, it may execute a set-user-ID program or an executable with asso‐
ciated file capabilities. In addition, a process may gain
capabilities via the effect of clone(2), unshare(2), or setns(2), as already described.

2. If a process has a capability in a user namespace, then it has that capability in all child (and further removed descendant) namespaces as well.

3. When a user namespace is created, the kernel records the effective user ID of the creating process as being the "owner" of the namespace. A process that resides in the parent of the user namespace and whose effective user ID matches the owner of the namespace has all capabilities in the namespace. By virtue of the previous rule, this means that the process has all capabilities in all further removed descendant user names‐ paces as well.

Interaction of user namespaces and other types of namespaces
Starting in Linux 3.8, unprivileged processes can create user namespaces, and mount, PID, IPC, network, and UTS namespaces can be created with just the CAP_SYS_ADMIN capability in the caller's user namespace.

If CLONE_NEWUSER is specified along with other CLONE_NEW* flags in a single clone(2) or unshare(2) call, the user namespace is guaranteed to be created first, giving the child (clone(2)) or caller (unshare(2)) privileges over the remaining namespaces cre‐ ated by the call. Thus, it is possible for an unprivileged call‐ er to specify this combination of flags.

Quoted 8 lines

When a new IPC, mount, network, PID, or UTS namespace is created via clone(2) or unshare(2), the kernel records the user namespace of the creating process against the new namespace. (This associ‐
ation can't be changed.) When a process in the new namespace
subsequently performs privileged operations that operate on global resources isolated by the namespace, the permission checks are performed according to the process's capabilities in the user namespace that the kernel associated with the new namespace.

Restrictions on mount namespaces.

- A mount namespace has a owner user namespace. A mount namespace whose owner user namespace is different than the owerner user namespace of it's parent mount namespace is considered a less privileged mount namespace.

- When creating a less privileged mount namespace shared mounts are reduced to slave mounts. This ensures that mappings performed in less privileged mount namespaces will not propogate to more privielged mount namespaces.

- Mounts that come as a single unit from more privileged mount are locked together and may not be separated in a less privielged mount namespace.

- The mount flags readonly, nodev, nosuid, noexec, and the mount atime settings when propogated from a more privielged to a less privileged mount namespace become locked, and may not be changed in the less privielged mount namespace.

- (As of 3.18-rc1 (in todays Al Viros vfs.git#for-next tree)) A file or directory that is a mountpoint in one namespace that is not a mount point in another namespace, may be renamed, unlinked, or rmdired in the mount namespace in which it is not a mount namespace if the ordinary permission checks pass.

Previously attemping to rmdir, unlink or rename a file or directory that was a mount point in another mount namespace would result in -EBUSY. This behavior had technical problems of enforcement (nfs) and resulted in a nice denial of servial attack against more privileged users. (Aka preventing individual files from being updated by bind mounting on top of them).

Quoted 47 lines · Michael Kerrisk (man-pages)

Hello Eric et al.,

For various reasons, my work on the namespaces man pages fell off the table a while back. Nevertheless, the pages have been close to completion for a while now, and I recently restarted, in an effort to finish them. As you also noted to me f2f, there have been recently been some small namespace changes that you may affect the content of the pages. Therefore, I'll take the opportunity to send the namespace-related pages out for further (final?) review.

So, here, I start with the user_namespaces(7) page, which is shown in rendered form below, with source attached to this mail. I'll send various other pages in follow-on mails.

Review comments/suggestions for improvements / bug fixes welcome.

Cheers,

Michael

==

NAME
user_namespaces - overview of Linux user_namespaces

DESCRIPTION
For an overview of namespaces, see namespaces(7).

User namespaces isolate security-related identifiers and
attributes, in particular, user IDs and group IDs (see creden‐ tials(7), the root directory, keys (see keyctl(2)), and capabili‐ ties (see capabilities(7)). A process's user and group IDs can be different inside and outside a user namespace. In particular, a process can have a normal unprivileged user ID outside a user namespace while at the same time having a user ID of 0 inside the namespace; in other words, the process has full privileges for operations inside the user namespace, but is unprivileged for operations outside the namespace.

Nested namespaces, namespace membership
User namespaces can be nested; that is, each user namespace— except the initial ("root") namespace—has a parent user names‐ pace, and can have zero or more child user namespaces. The par‐ ent user namespace is the user namespace of the process that cre‐ ates the user namespace via a call to unshare(2) or clone(2) with the CLONE_NEWUSER flag.

Quoted 8 lines

The kernel imposes (since version 3.11) a limit of 32 nested lev‐ els of user namespaces. Calls to unshare(2) or clone(2) that would cause this limit to be exceeded fail with the error EUSERS.

Each process is a member of exactly one user namespace. A process created via fork(2) or clone(2) without the CLONE_NEWUSER flag is a member of the same user namespace as its parent. A

^ single-threaded

Because of chroot and other things multi-threaded processes are not allowed to join a user namespace. For the documentation just saying single-threaded sounds like enough here.

Quoted 78 lines

process can join another user namespace with setns(2) if it has the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a full set of capabilities in that namespace.

A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag makes the new child process (for clone(2)) or the caller (for unshare(2)) a member of the new user namespace created by the call.

Capabilities
The child process created by clone(2) with the CLONE_NEWUSER flag starts out with a complete set of capabilities in the new user namespace. Likewise, a process that creates a new user namespace using unshare(2) or joins an existing user namespace using setns(2) gains a full set of capabilities in that namespace. On the other hand, that process has no capabilities in the parent (in the case of clone(2)) or previous (in the case of unshare(2) and setns(2)) user namespace, even if the new namespace is cre‐ ated or joined by the root user (i.e., a process with user ID 0 in the root namespace).

Note that a call to execve(2) will cause a process to lose any capabilities that it has, unless it has a user ID of 0 within the namespace. See the discussion of user and group ID mappings, below.

A call to clone(2), unshare(2), or setns(2) using the
CLONE_NEWUSER flag sets the "securebits" flags (see capabili‐ ties(7)) to their default values (all flags disabled) in the child (for clone(2)) or caller (for unshare(2), or setns(2)). Note that because the caller no longer has capabilities in its original user namespace after a call to setns(2), it is not pos‐ sible for a process to reset its "securebits" flags while retain‐ ing its user namespace membership by using a pair of setns(2) calls to move to another user namespace and then return to its original user namespace.

Having a capability inside a user namespace permits a process to perform operations (that require privilege) only on resources governed by that namespace. The rules for determining whether or not a process has a capability in a particular user namespace are as follows:

1. A process has a capability inside a user namespace if it is a member of that namespace and it has the capability in its effective capability set. A process can gain capabilities in its effective capability set in various ways. For example, it may execute a set-user-ID program or an executable with asso‐
ciated file capabilities. In addition, a process may gain
capabilities via the effect of clone(2), unshare(2), or setns(2), as already described.

2. If a process has a capability in a user namespace, then it has that capability in all child (and further removed descendant) namespaces as well.

3. When a user namespace is created, the kernel records the effective user ID of the creating process as being the "owner" of the namespace. A process that resides in the parent of the user namespace and whose effective user ID matches the owner of the namespace has all capabilities in the namespace. By virtue of the previous rule, this means that the process has all capabilities in all further removed descendant user names‐ paces as well.

Interaction of user namespaces and other types of namespaces
Starting in Linux 3.8, unprivileged processes can create user namespaces, and mount, PID, IPC, network, and UTS namespaces can be created with just the CAP_SYS_ADMIN capability in the caller's user namespace.

If CLONE_NEWUSER is specified along with other CLONE_NEW* flags in a single clone(2) or unshare(2) call, the user namespace is guaranteed to be created first, giving the child (clone(2)) or caller (unshare(2)) privileges over the remaining namespaces cre‐ ated by the call. Thus, it is possible for an unprivileged call‐ er to specify this combination of flags.

Quoted 8 lines

When a new IPC, mount, network, PID, or UTS namespace is created via clone(2) or unshare(2), the kernel records the user namespace of the creating process against the new namespace. (This associ‐
ation can't be changed.) When a process in the new namespace
subsequently performs privileged operations that operate on global resources isolated by the namespace, the permission checks are performed according to the process's capabilities in the user namespace that the kernel associated with the new namespace.

Restrictions on mount namespaces.

- A mount namespace has a owner user namespace. A mount namespace whose owner user namespace is different than the owerner user namespace of it's parent mount namespace is considered a less privileged mount namespace.

- When creating a less privileged mount namespace shared mounts are reduced to slave mounts. This ensures that mappings performed in less privileged mount namespaces will not propogate to more privielged mount namespaces.

- Mounts that come as a single unit from more privileged mount are locked together and may not be separated in a less privielged mount namespace.

- The mount flags readonly, nodev, nosuid, noexec, and the mount atime settings when propogated from a more privielged to a less privileged mount namespace become locked, and may not be changed in the less privielged mount namespace.

- (As of 3.18-rc1 (in todays Al Viros vfs.git#for-next tree)) A file or directory that is a mountpoint in one namespace that is not a mount point in another namespace, may be renamed, unlinked, or rmdired in the mount namespace in which it is not a mount namespace if the ordinary permission checks pass.

Previously attemping to rmdir, unlink or rename a file or directory that was a mount point in another mount namespace would result in -EBUSY. This behavior had technical problems of enforcement (nfs) and resulted in a nice denial of servial attack against more privileged users. (Aka preventing individual files from being updated by bind mounting on top of them).

Hello Eric et al.,

For various reasons, my work on the namespaces man pages fell off the table a while back. Nevertheless, the pages have been close to completion for a while now, and I recently restarted, in an effort to finish them. As you also noted to me f2f, there have been recently been some small namespace changes that you may affect the content of the pages. Therefore, I'll take the opportunity to send the namespace-related pages out for further (final?) review.

So, here, I start with the user_namespaces(7) page, which is shown in rendered form below, with source attached to this mail. I'll send various other pages in follow-on mails.

Review comments/suggestions for improvements / bug fixes welcome.

Cheers,

Michael

==

NAME
user_namespaces - overview of Linux user_namespaces

DESCRIPTION
For an overview of namespaces, see namespaces(7).

User namespaces isolate security-related identifiers and
attributes, in particular, user IDs and group IDs (see creden‐ tials(7), the root directory, keys (see keyctl(2)), and capabili‐

ties (see capabilities(7)). A process's user and group IDs can be different inside and outside a user namespace. In particular, a process can have a normal unprivileged user ID outside a user namespace while at the same time having a user ID of 0 inside the namespace; in other words, the process has full privileges for operations inside the user namespace, but is unprivileged for operations outside the namespace.

Nested namespaces, namespace membership
User namespaces can be nested; that is, each user namespace— except the initial ("root") namespace—has a parent user names‐ pace, and can have zero or more child user namespaces. The par‐ ent user namespace is the user namespace of the process that cre‐ ates the user namespace via a call to unshare(2) or clone(2) with the CLONE_NEWUSER flag.

The kernel imposes (since version 3.11) a limit of 32 nested lev‐ els of user namespaces. Calls to unshare(2) or clone(2) that would cause this limit to be exceeded fail with the error EUSERS.

Each process is a member of exactly one user namespace. A process created via fork(2) or clone(2) without the CLONE_NEWUSER flag is a member of the same user namespace as its parent. A process can join another user namespace with setns(2) if it has the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a full set of capabilities in that namespace.

A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag makes the new child process (for clone(2)) or the caller (for unshare(2)) a member of the new user namespace created by the call.

Capabilities
The child process created by clone(2) with the CLONE_NEWUSER flag starts out with a complete set of capabilities in the new user namespace. Likewise, a process that creates a new user namespace using unshare(2) or joins an existing user namespace using setns(2) gains a full set of capabilities in that namespace. On the other hand, that process has no capabilities in the parent (in the case of clone(2)) or previous (in the case of unshare(2) and setns(2)) user namespace, even if the new namespace is cre‐ ated or joined by the root user (i.e., a process with user ID 0 in the root namespace).

Note that a call to execve(2) will cause a process to lose any capabilities that it has, unless it has a user ID of 0 within the namespace.

           $ cat /proc/$$/uid_map
                    0          0 4294967295

This mapping tells us that the range starting at user ID 0 in this namespace maps to a range starting at 0 in the (nonexistent) parent namespace, and the length of the range is the largest 32-bit unsigned integer.

Defining user and group ID mappings: writing to uid_map and gid_map
After the creation of a new user namespace, the uid_map file of one of the processes in the namespace may be written to once to define the mapping of user IDs in the new user namespace. An attempt to write more than once to a uid_map file in a user namespace fails with the error EPERM. Similar rules apply for gid_map files.

The lines written to uid_map (gid_map) must conform to the fol‐ lowing rules:

* The three fields must be valid numbers, and the last field must be greater than 0.

* Lines are terminated by newline characters.

* There is an (arbitrary) limit on the number of lines in the file. As at Linux 3.8, the limit is five lines. In addition, the number of bytes written to the file must be less than the system page size, and the write must be performed at the start of the file (i.e., lseek(2) and pwrite(2) can't be used to write to nonzero offsets in the file).

* The range of user IDs (group IDs) specified in each line can‐ not overlap with the ranges in any other lines. In the ini‐ tial implementation (Linux 3.8), this requirement was satis‐ fied by a simplistic implementation that imposed the further requirement that the values in both field 1 and field 2 of successive lines must be in ascending numerical order, which prevented some otherwise valid maps from being created. Linux 3.9 and later fix this limitation, allowing any valid set of nonoverlapping maps.

* At least one line must be written to the file.

Writes that violate the above rules fail with the error EINVAL.

In order for a process to write to the /proc/[pid]/uid_map (/proc/[pid]/gid_map) file, all of the following requirements must be met:

1. The writing process must have the CAP_SETUID (CAP_SETGID) capability in the user namespace of the process pid.

2. The writing process must be in either the user namespace of the process pid or inside the parent user namespace of the process pid.

3. The mapped user IDs (group IDs) must in turn have a mapping in the parent user namespace.

4. One of the following is true:

* The data written to uid_map (gid_map) consists of a single line that maps the writing process's filesystem user ID (group ID) in the parent user namespace to a user ID (group ID) in the user namespace. The usual case here is that this single line provides a mapping for user ID of the process that created the namespace.

* The process has the CAP_SETUID (CAP_SETGID) capability in the parent user namespace. Thus, a privileged process can make mappings to arbitrary user IDs (group IDs) in the par‐ ent user namespace.

Quoted 55 lines · Eric W. Biederman

Quoted 45 lines · Michael Kerrisk (man-pages)

For various reasons, my work on the namespaces man pages fell off the table a while back. Nevertheless, the pages have been close to completion for a while now, and I recently restarted, in an effort to finish them. As you also noted to me f2f, there have been recently been some small namespace changes that you may affect the content of the pages. Therefore, I'll take the opportunity to send the namespace-related pages out for further (final?) review.

So, here, I start with the user_namespaces(7) page, which is shown in rendered form below, with source attached to this mail. I'll send various other pages in follow-on mails.

Review comments/suggestions for improvements / bug fixes welcome.

Cheers,

Michael

==

NAME
user_namespaces - overview of Linux user_namespaces

DESCRIPTION
For an overview of namespaces, see namespaces(7).

User namespaces isolate security-related identifiers and
attributes, in particular, user IDs and group IDs (see creden‐ tials(7), the root directory, keys (see keyctl(2)), and capabili‐ ties (see capabilities(7)). A process's user and group IDs can be different inside and outside a user namespace. In particular, a process can have a normal unprivileged user ID outside a user namespace while at the same time having a user ID of 0 inside the namespace; in other words, the process has full privileges for operations inside the user namespace, but is unprivileged for operations outside the namespace.

Nested namespaces, namespace membership
User namespaces can be nested; that is, each user namespace— except the initial ("root") namespace—has a parent user names‐ pace, and can have zero or more child user namespaces. The par‐ ent user namespace is the user namespace of the process that cre‐ ates the user namespace via a call to unshare(2) or clone(2) with the CLONE_NEWUSER flag.

Quoted 8 lines

The kernel imposes (since version 3.11) a limit of 32 nested lev‐ els of user namespaces. Calls to unshare(2) or clone(2) that would cause this limit to be exceeded fail with the error EUSERS.

Each process is a member of exactly one user namespace. A process created via fork(2) or clone(2) without the CLONE_NEWUSER flag is a member of the same user namespace as its parent. A

Quoted 5 lines

^ single-threaded

Because of chroot and other things multi-threaded processes are not allowed to join a user namespace. For the documentation just saying single-threaded sounds like enough here.

Thanks. Fixed.

Quoted 114 lines

Quoted 78 lines

process can join another user namespace with setns(2) if it has the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a full set of capabilities in that namespace.

A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag makes the new child process (for clone(2)) or the caller (for unshare(2)) a member of the new user namespace created by the call.

Capabilities
The child process created by clone(2) with the CLONE_NEWUSER flag starts out with a complete set of capabilities in the new user namespace. Likewise, a process that creates a new user namespace using unshare(2) or joins an existing user namespace using setns(2) gains a full set of capabilities in that namespace. On the other hand, that process has no capabilities in the parent (in the case of clone(2)) or previous (in the case of unshare(2) and setns(2)) user namespace, even if the new namespace is cre‐ ated or joined by the root user (i.e., a process with user ID 0 in the root namespace).

Note that a call to execve(2) will cause a process to lose any capabilities that it has, unless it has a user ID of 0 within the namespace. See the discussion of user and group ID mappings, below.

A call to clone(2), unshare(2), or setns(2) using the
CLONE_NEWUSER flag sets the "securebits" flags (see capabili‐ ties(7)) to their default values (all flags disabled) in the child (for clone(2)) or caller (for unshare(2), or setns(2)). Note that because the caller no longer has capabilities in its original user namespace after a call to setns(2), it is not pos‐ sible for a process to reset its "securebits" flags while retain‐ ing its user namespace membership by using a pair of setns(2) calls to move to another user namespace and then return to its original user namespace.

Having a capability inside a user namespace permits a process to perform operations (that require privilege) only on resources governed by that namespace. The rules for determining whether or not a process has a capability in a particular user namespace are as follows:

1. A process has a capability inside a user namespace if it is a member of that namespace and it has the capability in its effective capability set. A process can gain capabilities in its effective capability set in various ways. For example, it may execute a set-user-ID program or an executable with asso‐
ciated file capabilities. In addition, a process may gain
capabilities via the effect of clone(2), unshare(2), or setns(2), as already described.

2. If a process has a capability in a user namespace, then it has that capability in all child (and further removed descendant) namespaces as well.

3. When a user namespace is created, the kernel records the effective user ID of the creating process as being the "owner" of the namespace. A process that resides in the parent of the user namespace and whose effective user ID matches the owner of the namespace has all capabilities in the namespace. By virtue of the previous rule, this means that the process has all capabilities in all further removed descendant user names‐ paces as well.

Interaction of user namespaces and other types of namespaces
Starting in Linux 3.8, unprivileged processes can create user namespaces, and mount, PID, IPC, network, and UTS namespaces can be created with just the CAP_SYS_ADMIN capability in the caller's user namespace.

If CLONE_NEWUSER is specified along with other CLONE_NEW* flags in a single clone(2) or unshare(2) call, the user namespace is guaranteed to be created first, giving the child (clone(2)) or caller (unshare(2)) privileges over the remaining namespaces cre‐ ated by the call. Thus, it is possible for an unprivileged call‐ er to specify this combination of flags.

Quoted 8 lines

When a new IPC, mount, network, PID, or UTS namespace is created via clone(2) or unshare(2), the kernel records the user namespace of the creating process against the new namespace. (This associ‐
ation can't be changed.) When a process in the new namespace
subsequently performs privileged operations that operate on global resources isolated by the namespace, the permission checks are performed according to the process's capabilities in the user namespace that the kernel associated with the new namespace.

Restrictions on mount namespaces.

- A mount namespace has a owner user namespace. A mount namespace whose owner user namespace is different than the owerner user namespace of it's parent mount namespace is considered a less privileged mount namespace.

- When creating a less privileged mount namespace shared mounts are reduced to slave mounts. This ensures that mappings performed in less privileged mount namespaces will not propogate to more privielged mount namespaces.

- Mounts that come as a single unit from more privileged mount are locked together and may not be separated in a less privielged mount namespace.

- The mount flags readonly, nodev, nosuid, noexec, and the mount atime settings when propogated from a more privielged to a less privileged mount namespace become locked, and may not be changed in the less privielged mount namespace.

- (As of 3.18-rc1 (in todays Al Viros vfs.git#for-next tree)) A file or directory that is a mountpoint in one namespace that is not a mount point in another namespace, may be renamed, unlinked, or rmdired in the mount namespace in which it is not a mount namespace if the ordinary permission checks pass.

Quoted 6 lines

Previously attemping to rmdir, unlink or rename a file or directory that was a mount point in another mount namespace would result in -EBUSY. This behavior had technical problems of enforcement (nfs) and resulted in a nice denial of servial attack against more privileged users. (Aka preventing individual files from being updated by bind mounting on top of them).

I need some help here. What is your intention for the above text. Do you mean I should add it pretty much as is under a subheading "Restrictions on mount namespaces"?

Quoted 25 lines · Michael Kerrisk (man-pages)

Hello Eric et al.,

For various reasons, my work on the namespaces man pages fell off the table a while back. Nevertheless, the pages have been close to completion for a while now, and I recently restarted, in an effort to finish them. As you also noted to me f2f, there have been recently been some small namespace changes that you may affect the content of the pages. Therefore, I'll take the opportunity to send the namespace-related pages out for further (final?) review.

So, here, I start with the user_namespaces(7) page, which is shown in rendered form below, with source attached to this mail. I'll send various other pages in follow-on mails.

Review comments/suggestions for improvements / bug fixes welcome.

Cheers,

Michael

==

NAME
user_namespaces - overview of Linux user_namespaces

Quoted 8 lines

When a new IPC, mount, network, PID, or UTS namespace is created via clone(2) or unshare(2), the kernel records the user namespace of the creating process against the new namespace. (This associ‐
ation can't be changed.) When a process in the new namespace
subsequently performs privileged operations that operate on global resources isolated by the namespace, the permission checks are performed according to the process's capabilities in the user namespace that the kernel associated with the new namespace.

Restrictions on mount namespaces.

- A mount namespace has a owner user namespace. A mount namespace whose owner user namespace is different than the owerner user namespace of it's parent mount namespace is considered a less privileged mount namespace.

- When creating a less privileged mount namespace shared mounts are reduced to slave mounts. This ensures that mappings performed in less privileged mount namespaces will not propogate to more privielged mount namespaces.

- Mounts that come as a single unit from more privileged mount are locked together and may not be separated in a less privielged mount namespace.

- The mount flags readonly, nodev, nosuid, noexec, and the mount atime settings when propogated from a more privielged to a less privileged mount namespace become locked, and may not be changed in the less privielged mount namespace.

- (As of 3.18-rc1 (in todays Al Viros vfs.git#for-next tree)) A file or directory that is a mountpoint in one namespace that is not a mount point in another namespace, may be renamed, unlinked, or rmdired in the mount namespace in which it is not a mount namespace if the ordinary permission checks pass.

Previously attemping to rmdir, unlink or rename a file or directory that was a mount point in another mount namespace would result in -EBUSY. This behavior had technical problems of enforcement (nfs) and resulted in a nice denial of servial attack against more privileged users. (Aka preventing individual files from being updated by bind mounting on top of them).

"Michael Kerrisk (man-pages)" <[email protected]> writes:

Quoted 5 lines

The initial user namespace has no parent namespace, but, for con‐ sistency, the kernel provides dummy user and group ID mapping files for this namespace. Looking at the uid_map file (gid_map is the same) from a shell in the initial namespace shows:

Quoted 7 lines

$ cat /proc/$$/uid_map
0 0 4294967295

This mapping tells us that the range starting at user ID 0 in this namespace maps to a range starting at 0 in the (nonexistent) parent namespace, and the length of the range is the largest 32-bit unsigned integer.

Which deliberately leaves 4294967295 32bit (-1) unmapped. (uid_t)-1 is used in several interfaces (like setreuid) as a way to specify no uid leaving it unmapped and unusuable guarantees that there will be no confusion when using those kernel methods.

On a related note. One thing that has come up recently (in 3 separate implementations is that mount(MS_REMOUNT|...,...) must include all of
the mount flags that need to be preserved. People creating read-only
bind mounts tend to miss that and the locked flags in mount namespaces. That issue was flushed out now that the kernel is now not allowing most mount flags to be cleared in mount namespaces.

Quoted 28 lines · Michael Kerrisk (man-pages)

Hello Eric et al.,

For various reasons, my work on the namespaces man pages fell off the table a while back. Nevertheless, the pages have been close to completion for a while now, and I recently restarted, in an effort to finish them. As you also noted to me f2f, there have been recently been some small namespace changes that you may affect the content of the pages. Therefore, I'll take the opportunity to send the namespace-related pages out for further (final?) review.

So, here, I start with the user_namespaces(7) page, which is shown in rendered form below, with source attached to this mail. I'll send various other pages in follow-on mails.

Review comments/suggestions for improvements / bug fixes welcome.

Cheers,

Michael

==

NAME
user_namespaces - overview of Linux user_namespaces

DESCRIPTION
For an overview of namespaces, see namespaces(7).

Quoted 3 lines

User namespaces isolate security-related identifiers and
attributes, in particular, user IDs and group IDs (see creden‐ tials(7), the root directory, keys (see keyctl(2)), and capabili‐

Putting "root directory" here is odd -- that's really part of a different namespace. But user namespaces sort of isolate the other namespaces from each other.

Also, ugh, keys. How did keyctl(2) ever make it through any kind of review?

Quoted 44 lines

ties (see capabilities(7)). A process's user and group IDs can be different inside and outside a user namespace. In particular, a process can have a normal unprivileged user ID outside a user namespace while at the same time having a user ID of 0 inside the namespace; in other words, the process has full privileges for operations inside the user namespace, but is unprivileged for operations outside the namespace.

Nested namespaces, namespace membership
User namespaces can be nested; that is, each user namespace— except the initial ("root") namespace—has a parent user names‐ pace, and can have zero or more child user namespaces. The par‐ ent user namespace is the user namespace of the process that cre‐ ates the user namespace via a call to unshare(2) or clone(2) with the CLONE_NEWUSER flag.

The kernel imposes (since version 3.11) a limit of 32 nested lev‐ els of user namespaces. Calls to unshare(2) or clone(2) that would cause this limit to be exceeded fail with the error EUSERS.

Each process is a member of exactly one user namespace. A process created via fork(2) or clone(2) without the CLONE_NEWUSER flag is a member of the same user namespace as its parent. A process can join another user namespace with setns(2) if it has the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a full set of capabilities in that namespace.

A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag makes the new child process (for clone(2)) or the caller (for unshare(2)) a member of the new user namespace created by the call.

Capabilities
The child process created by clone(2) with the CLONE_NEWUSER flag starts out with a complete set of capabilities in the new user namespace. Likewise, a process that creates a new user namespace using unshare(2) or joins an existing user namespace using setns(2) gains a full set of capabilities in that namespace. On the other hand, that process has no capabilities in the parent (in the case of clone(2)) or previous (in the case of unshare(2) and setns(2)) user namespace, even if the new namespace is cre‐ ated or joined by the root user (i.e., a process with user ID 0 in the root namespace).

Quoted 3 lines

Note that a call to execve(2) will cause a process to lose any capabilities that it has, unless it has a user ID of 0 within the namespace.

Or unless file capabilities have a non-empty inheritable mask.

It may be worth mentioning that execve in a user namespace works exactly like execve outside a userns.

Quoted 50 lines

           $ cat /proc/$$/uid_map
                    0          0 4294967295

This mapping tells us that the range starting at user ID 0 in this namespace maps to a range starting at 0 in the (nonexistent) parent namespace, and the length of the range is the largest 32-bit unsigned integer.

Defining user and group ID mappings: writing to uid_map and gid_map
After the creation of a new user namespace, the uid_map file of one of the processes in the namespace may be written to once to define the mapping of user IDs in the new user namespace. An attempt to write more than once to a uid_map file in a user namespace fails with the error EPERM. Similar rules apply for gid_map files.

The lines written to uid_map (gid_map) must conform to the fol‐ lowing rules:

* The three fields must be valid numbers, and the last field must be greater than 0.

* Lines are terminated by newline characters.

* There is an (arbitrary) limit on the number of lines in the file. As at Linux 3.8, the limit is five lines. In addition, the number of bytes written to the file must be less than the system page size, and the write must be performed at the start of the file (i.e., lseek(2) and pwrite(2) can't be used to write to nonzero offsets in the file).

* The range of user IDs (group IDs) specified in each line can‐ not overlap with the ranges in any other lines. In the ini‐ tial implementation (Linux 3.8), this requirement was satis‐ fied by a simplistic implementation that imposed the further requirement that the values in both field 1 and field 2 of successive lines must be in ascending numerical order, which prevented some otherwise valid maps from being created. Linux 3.9 and later fix this limitation, allowing any valid set of nonoverlapping maps.

* At least one line must be written to the file.

Writes that violate the above rules fail with the error EINVAL.

In order for a process to write to the /proc/[pid]/uid_map (/proc/[pid]/gid_map) file, all of the following requirements must be met:

Quoted 2 lines

1. The writing process must have the CAP_SETUID (CAP_SETGID) capability in the user namespace of the process pid.

This checked for the opening process (and I don't actually remember whether it's checked for the writing process).

Quoted 17 lines

2. The writing process must be in either the user namespace of the process pid or inside the parent user namespace of the process pid.

3. The mapped user IDs (group IDs) must in turn have a mapping in the parent user namespace.

4. One of the following is true:

* The data written to uid_map (gid_map) consists of a single line that maps the writing process's filesystem user ID (group ID) in the parent user namespace to a user ID (group ID) in the user namespace. The usual case here is that this single line provides a mapping for user ID of the process that created the namespace.

Quoted 4 lines

* The process has the CAP_SETUID (CAP_SETGID) capability in the parent user namespace. Thus, a privileged process can make mappings to arbitrary user IDs (group IDs) in the par‐ ent user namespace.

The opening process.

One other thing that could be worth mentioning it: any non-user namespace that's created is owned by the user namespace of the process that created it at the time of creation. Actions on those namespaces require capabilities in the corresponding user namespace.

Thanks for doing this!

Hi Eric,

Quoted 27 lines · Eric W. Biederman

Quoted 25 lines · Michael Kerrisk (man-pages)

Hello Eric et al.,

For various reasons, my work on the namespaces man pages fell off the table a while back. Nevertheless, the pages have been close to completion for a while now, and I recently restarted, in an effort to finish them. As you also noted to me f2f, there have been recently been some small namespace changes that you may affect the content of the pages. Therefore, I'll take the opportunity to send the namespace-related pages out for further (final?) review.

So, here, I start with the user_namespaces(7) page, which is shown in rendered form below, with source attached to this mail. I'll send various other pages in follow-on mails.

Review comments/suggestions for improvements / bug fixes welcome.

Cheers,

Michael

==

NAME
user_namespaces - overview of Linux user_namespaces

[...]

Quoted 21 lines

Quoted 8 lines

When a new IPC, mount, network, PID, or UTS namespace is created via clone(2) or unshare(2), the kernel records the user namespace of the creating process against the new namespace. (This associ‐
ation can't be changed.) When a process in the new namespace
subsequently performs privileged operations that operate on global resources isolated by the namespace, the permission checks are performed according to the process's capabilities in the user namespace that the kernel associated with the new namespace.

Restrictions on mount namespaces.

- A mount namespace has a owner user namespace. A mount namespace whose owner user namespace is different than the owerner user namespace of it's parent mount namespace is considered a less privileged mount namespace.

- When creating a less privileged mount namespace shared mounts are reduced to slave mounts. This ensures that mappings performed in less privileged mount namespaces will not propogate to more privielged mount namespaces.

Quoted 3 lines

- Mounts that come as a single unit from more privileged mount are locked together and may not be separated in a less privielged mount namespace.

Could you clarify what you mean by "Mounts that come as a single unit"?

Quoted 11 lines

- The mount flags readonly, nodev, nosuid, noexec, and the mount atime settings when propogated from a more privielged to a less privileged mount namespace become locked, and may not be changed in the less privielged mount namespace.

- (As of 3.18-rc1 (in todays Al Viros vfs.git#for-next tree)) A file or directory that is a mountpoint in one namespace that is not a mount point in another namespace, may be renamed, unlinked, or rmdired in the mount namespace in which it is not a mount namespace if the ordinary permission checks pass.

Quoted 6 lines

Previously attemping to rmdir, unlink or rename a file or directory that was a mount point in another mount namespace would result in -EBUSY. This behavior had technical problems of enforcement (nfs) and resulted in a nice denial of servial attack against more privileged users. (Aka preventing individual files from being updated by bind mounting on top of them).

I have reworked the text above a little so that now we have the following. Aside from question above, does it look okay?

Restrictions on mount namespaces
Note the following points with respect to mount namespaces:

* A mount namespace has na owner user namespace. A mount

namespace whose owner user namespace is different from the owner user namespace of its parent mount namespace is con‐ sidered a less privileged mount namespace.

* When creating a less privileged mount namespace, shared mounts are reduced to slave mounts. This ensures that map‐ pings performed in less privileged mount namespaces will not propagate to more privileged mount namespaces.

* Mounts that come as a single unit from more privileged mount

are locked together and may not be separated in a less priv‐ ileged mount namespace.

* The mount(2) flags MS_RDONLY, MS_NOSUID, MS_NOEXEC, and the "atime" flags (MS_NOATIME, MS_NODIRATIME, MS_RELATIME) set‐ tings become locked when propagated from a more privileged to a less privileged mount namespace, and may not be changed in the less privileged mount namespace.

* A file or directory that is a mount point in one namespace that is not a mount point in another namespace, may be renamed, unlinked, or removed (rmdir(2)) in the mount names‐ pace in which it is not a mount point (subject to the usual permission checks).

Previously, attempting to unlink, rename, or remove a file or directory that was a mount point in another mount names‐ pace would result in the error EBUSY. That behavior had technical problems of enforcement (e.g., for NFS) and per‐ mitted denial-of-service attacks against more privileged
users. (i.e., preventing individual files from being
updated by bind mounting on top of them).

Quoted 1 line · Eric W. Biederman

"Michael Kerrisk (man-pages)" <[email protected]> writes:

[...]

Quoted 13 lines

Quoted 5 lines

The initial user namespace has no parent namespace, but, for con‐ sistency, the kernel provides dummy user and group ID mapping files for this namespace. Looking at the uid_map file (gid_map is the same) from a shell in the initial namespace shows:

Quoted 7 lines

$ cat /proc/$$/uid_map
0 0 4294967295

This mapping tells us that the range starting at user ID 0 in this namespace maps to a range starting at 0 in the (nonexistent) parent namespace, and the length of the range is the largest 32-bit unsigned integer.

Quoted 4 lines

Which deliberately leaves 4294967295 32bit (-1) unmapped. (uid_t)-1 is used in several interfaces (like setreuid) as a way to specify no uid leaving it unmapped and unusuable guarantees that there will be no confusion when using those kernel methods.

So, I worked that piece into the text to give:

This mapping tells us that the range starting at user ID 0 in this namespace maps to a range starting at 0 in the (nonexis‐ tent) parent namespace, and the length of the range is the
largest 32-bit unsigned integer. (This deliberately leaves
4294967295 (the 32-bit signed -1 value) unmapped. This is
deliberate: (uid_t) -1 is used in several interfaces (e.g., setreuid(2)) as a way to specify "no user ID". Leaving setreuid(2)) unmapped and unusuable guarantees that there will

be no confusion when using these interfaces.

Okay?

Hi Andy, and Eric,

Quoted 34 lines · Andy Lutomirski

Quoted 31 lines · Michael Kerrisk (man-pages)

Hello Eric et al.,

For various reasons, my work on the namespaces man pages fell off the table a while back. Nevertheless, the pages have been close to completion for a while now, and I recently restarted, in an effort to finish them. As you also noted to me f2f, there have been recently been some small namespace changes that you may affect the content of the pages. Therefore, I'll take the opportunity to send the namespace-related pages out for further (final?) review.

So, here, I start with the user_namespaces(7) page, which is shown in rendered form below, with source attached to this mail. I'll send various other pages in follow-on mails.

Review comments/suggestions for improvements / bug fixes welcome.

Cheers,

Michael

==

NAME
user_namespaces - overview of Linux user_namespaces

DESCRIPTION
For an overview of namespaces, see namespaces(7).

User namespaces isolate security-related identifiers and
attributes, in particular, user IDs and group IDs (see creden‐ tials(7), the root directory, keys (see keyctl(2)), and capabili‐

Quoted 3 lines

Putting "root directory" here is odd -- that's really part of a different namespace. But user namespaces sort of isolate the other namespaces from each other.

I'm trying to remember the details here. I think this piece originally came after a discussion with Eric, but I am not sure. Eric?

Quoted 46 lines

Also, ugh, keys. How did keyctl(2) ever make it through any kind of review?

Quoted 44 lines

ties (see capabilities(7)). A process's user and group IDs can be different inside and outside a user namespace. In particular, a process can have a normal unprivileged user ID outside a user namespace while at the same time having a user ID of 0 inside the namespace; in other words, the process has full privileges for operations inside the user namespace, but is unprivileged for operations outside the namespace.

Nested namespaces, namespace membership
User namespaces can be nested; that is, each user namespace— except the initial ("root") namespace—has a parent user names‐ pace, and can have zero or more child user namespaces. The par‐ ent user namespace is the user namespace of the process that cre‐ ates the user namespace via a call to unshare(2) or clone(2) with the CLONE_NEWUSER flag.

The kernel imposes (since version 3.11) a limit of 32 nested lev‐ els of user namespaces. Calls to unshare(2) or clone(2) that would cause this limit to be exceeded fail with the error EUSERS.

Each process is a member of exactly one user namespace. A process created via fork(2) or clone(2) without the CLONE_NEWUSER flag is a member of the same user namespace as its parent. A process can join another user namespace with setns(2) if it has the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a full set of capabilities in that namespace.

A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag makes the new child process (for clone(2)) or the caller (for unshare(2)) a member of the new user namespace created by the call.

Capabilities
The child process created by clone(2) with the CLONE_NEWUSER flag starts out with a complete set of capabilities in the new user namespace. Likewise, a process that creates a new user namespace using unshare(2) or joins an existing user namespace using setns(2) gains a full set of capabilities in that namespace. On the other hand, that process has no capabilities in the parent (in the case of clone(2)) or previous (in the case of unshare(2) and setns(2)) user namespace, even if the new namespace is cre‐ ated or joined by the root user (i.e., a process with user ID 0 in the root namespace).

Quoted 8 lines

Quoted 3 lines

Note that a call to execve(2) will cause a process to lose any capabilities that it has, unless it has a user ID of 0 within the namespace.

Or unless file capabilities have a non-empty inheritable mask.

It may be worth mentioning that execve in a user namespace works exactly like execve outside a userns.

I';ve reworded that para to say:

Note that a call to execve(2) will cause a process's capabili‐ ties to be recalculated in the usual way (see capabilities(7)), so that usually, unless it has a user ID of 0 within the names‐ pace or the executable file has a nonempty inheritable capabil‐ ities mask, it will lose all capabilities. See the discussion of user and group ID mappings, below.

Okay?

Quoted 49 lines

Quoted 49 lines

           $ cat /proc/$$/uid_map
                    0          0 4294967295

This mapping tells us that the range starting at user ID 0 in this namespace maps to a range starting at 0 in the (nonexistent) parent namespace, and the length of the range is the largest 32-bit unsigned integer.

Defining user and group ID mappings: writing to uid_map and gid_map
After the creation of a new user namespace, the uid_map file of one of the processes in the namespace may be written to once to define the mapping of user IDs in the new user namespace. An attempt to write more than once to a uid_map file in a user namespace fails with the error EPERM. Similar rules apply for gid_map files.

The lines written to uid_map (gid_map) must conform to the fol‐ lowing rules:

* The three fields must be valid numbers, and the last field must be greater than 0.

* Lines are terminated by newline characters.

* There is an (arbitrary) limit on the number of lines in the file. As at Linux 3.8, the limit is five lines. In addition, the number of bytes written to the file must be less than the system page size, and the write must be performed at the start of the file (i.e., lseek(2) and pwrite(2) can't be used to write to nonzero offsets in the file).

* The range of user IDs (group IDs) specified in each line can‐ not overlap with the ranges in any other lines. In the ini‐ tial implementation (Linux 3.8), this requirement was satis‐ fied by a simplistic implementation that imposed the further requirement that the values in both field 1 and field 2 of successive lines must be in ascending numerical order, which prevented some otherwise valid maps from being created. Linux 3.9 and later fix this limitation, allowing any valid set of nonoverlapping maps.

* At least one line must be written to the file.

Writes that violate the above rules fail with the error EINVAL.

In order for a process to write to the /proc/[pid]/uid_map (/proc/[pid]/gid_map) file, all of the following requirements must be met:

Quoted 5 lines

Quoted 2 lines

1. The writing process must have the CAP_SETUID (CAP_SETGID) capability in the user namespace of the process pid.

This checked for the opening process (and I don't actually remember whether it's checked for the writing process).

Eric, can you comment?

Quoted 16 lines

Quoted 16 lines

2. The writing process must be in either the user namespace of the process pid or inside the parent user namespace of the process pid.

3. The mapped user IDs (group IDs) must in turn have a mapping in the parent user namespace.

4. One of the following is true:

* The data written to uid_map (gid_map) consists of a single line that maps the writing process's filesystem user ID (group ID) in the parent user namespace to a user ID (group ID) in the user namespace. The usual case here is that this single line provides a mapping for user ID of the process that created the namespace.

Quoted 6 lines

Quoted 4 lines

* The process has the CAP_SETUID (CAP_SETGID) capability in the parent user namespace. Thus, a privileged process can make mappings to arbitrary user IDs (group IDs) in the par‐ ent user namespace.

The opening process.

Fixed.

Quoted 4 lines

One other thing that could be worth mentioning it: any non-user namespace that's created is owned by the user namespace of the process that created it at the time of creation. Actions on those namespaces require capabilities in the corresponding user namespace.

I added:

[[
When a non-user-namespace is created,
it is owned by the user namespace in which the creating process was a member at the time of the creation of the namespace. Actions on the non-user-namespace
require capabilities in the corresponding user namespace. ]]

Quoted 1 line

Thanks for doing this!

You're welcome. Thanks for the review!

Hi Eric,

Quoted 6 lines

On a related note. One thing that has come up recently (in 3 separate implementations is that mount(MS_REMOUNT|...,...) must include all of
the mount flags that need to be preserved. People creating read-only
bind mounts tend to miss that and the locked flags in mount namespaces. That issue was flushed out now that the kernel is now not allowing most mount flags to be cleared in mount namespaces.

So, are you meaning that something needs to be added to the page regarding this point?

We may also want to discuss the specific restrictions on chroot.

The text about chroot at least gives people a strong hint that the chroot rules are affected by user namespaces.

The restrictions that we have settled on to avoid chroot being a problem are the creator of a user namespace must not be chrooted in their current mount namespace, and the creator of the user namespace must not be threaded.

Andy can you check me on this it looks like unshare is currently buggy in that it will allow a threaded application to create a user namespace.

Quoted 6 lines · Eric W. Biederman

We may also want to discuss the specific restrictions on chroot.

The text about chroot at least gives people a strong hint that the chroot rules are affected by user namespaces.

Quoted 7 lines

The restrictions that we have settled on to avoid chroot being a problem are the creator of a user namespace must not be chrooted in their current mount namespace, and the creator of the user namespace must not be threaded.

Andy can you check me on this it looks like unshare is currently buggy in that it will allow a threaded application to create a user namespace.

I think it's this code in unshare:

    /*
     * If unsharing a user namespace must also unshare the thread.
     */
    if (unshare_flags & CLONE_NEWUSER)
        unshare_flags |= CLONE_THREAD | CLONE_FS;

I suppose that this should be documented.

CLONE_FS prevents the chroot from leaking out of the namespace. (But see the other thread that I'm about to start...)

Quoted 9 lines

Quoted 9 lines

On a related note. One thing that has come up recently (in 3 separate

Quoted 5 lines

Quoted 5 lines

implementations is that mount(MS_REMOUNT|...,...) must include all of
the mount flags that need to be preserved. People creating read-only
bind mounts tend to miss that and the locked flags in mount namespaces. That issue was flushed out now that the kernel is now not allowing most mount flags to be cleared in mount namespaces.

So, are you meaning that something needs to be added to the page regarding this point?

Yes. The interface is non-intuitive and we should at least document the weirdness.

Quoted 57 lines · Michael Kerrisk (man-pages)

Hi Eric,

Quoted 27 lines · Eric W. Biederman

Quoted 25 lines · Michael Kerrisk (man-pages)

Hello Eric et al.,

For various reasons, my work on the namespaces man pages fell off the table a while back. Nevertheless, the pages have been close to completion for a while now, and I recently restarted, in an effort to finish them. As you also noted to me f2f, there have been recently been some small namespace changes that you may affect the content of the pages. Therefore, I'll take the opportunity to send the namespace-related pages out for further (final?) review.

So, here, I start with the user_namespaces(7) page, which is shown in rendered form below, with source attached to this mail. I'll send various other pages in follow-on mails.

Review comments/suggestions for improvements / bug fixes welcome.

Cheers,

Michael

==

NAME
user_namespaces - overview of Linux user_namespaces

[...]

Quoted 24 lines

Quoted 8 lines

When a new IPC, mount, network, PID, or UTS namespace is created via clone(2) or unshare(2), the kernel records the user namespace of the creating process against the new namespace. (This associ‐
ation can't be changed.) When a process in the new namespace
subsequently performs privileged operations that operate on global resources isolated by the namespace, the permission checks are performed according to the process's capabilities in the user namespace that the kernel associated with the new namespace.

Restrictions on mount namespaces.

- A mount namespace has a owner user namespace. A mount namespace whose owner user namespace is different than the owerner user namespace of it's parent mount namespace is considered a less privileged mount namespace.

- When creating a less privileged mount namespace shared mounts are reduced to slave mounts. This ensures that mappings performed in less privileged mount namespaces will not propogate to more privielged mount namespaces.

- Mounts that come as a single unit from more privileged mount are locked together and may not be separated in a less privielged mount namespace.

Quoted 2 lines

Could you clarify what you mean by "Mounts that come as a single unit"?

unshare(CLONE_NEWNS) brings across all of the mounts from the original mount namespace as a single unit.

recursive mounts that propogate between mount namespaces propogate as a single unit.

The importance of this is allow the global root to mount over things and not have to worry that someone from a user namespace root can peek underneath.

Quoted 18 lines

Quoted 11 lines

- The mount flags readonly, nodev, nosuid, noexec, and the mount atime settings when propogated from a more privielged to a less privileged mount namespace become locked, and may not be changed in the less privielged mount namespace.

- (As of 3.18-rc1 (in todays Al Viros vfs.git#for-next tree)) A file or directory that is a mountpoint in one namespace that is not a mount point in another namespace, may be renamed, unlinked, or rmdired in the mount namespace in which it is not a mount namespace if the ordinary permission checks pass.

Quoted 6 lines

Previously attemping to rmdir, unlink or rename a file or directory that was a mount point in another mount namespace would result in -EBUSY. This behavior had technical problems of enforcement (nfs) and resulted in a nice denial of servial attack against more privileged users. (Aka preventing individual files from being updated by bind mounting on top of them).

Quoted 7 lines

I have reworked the text above a little so that now we have the following. Aside from question above, does it look okay?

Restrictions on mount namespaces
Note the following points with respect to mount namespaces:

* A mount namespace has na owner user namespace. A mount

                                     ^s/na/an/

Quoted 10 lines

namespace whose owner user namespace is different from the owner user namespace of its parent mount namespace is con‐ sidered a less privileged mount namespace.

* When creating a less privileged mount namespace, shared mounts are reduced to slave mounts. This ensures that map‐ pings performed in less privileged mount namespaces will not propagate to more privileged mount namespaces.

* Mounts that come as a single unit from more privileged mount

^ namespaces

Quoted 15 lines

are locked together and may not be separated in a less priv‐ ileged mount namespace.

* The mount(2) flags MS_RDONLY, MS_NOSUID, MS_NOEXEC, and the "atime" flags (MS_NOATIME, MS_NODIRATIME, MS_RELATIME) set‐ tings become locked when propagated from a more privileged to a less privileged mount namespace, and may not be changed in the less privileged mount namespace.

* A file or directory that is a mount point in one namespace that is not a mount point in another namespace, may be renamed, unlinked, or removed (rmdir(2)) in the mount names‐ pace in which it is not a mount point (subject to the usual permission checks).

Quoted 7 lines

Previously, attempting to unlink, rename, or remove a file or directory that was a mount point in another mount names‐ pace would result in the error EBUSY. That behavior had technical problems of enforcement (e.g., for NFS) and per‐ mitted denial-of-service attacks against more privileged
users. (i.e., preventing individual files from being
updated by bind mounting on top of them).

Subject to tiny typo corrections that looks fine.

Quoted 25 lines · Michael Kerrisk (man-pages)

Quoted 1 line · Eric W. Biederman

"Michael Kerrisk (man-pages)" <[email protected]> writes:

[...]

Quoted 13 lines

Quoted 5 lines

The initial user namespace has no parent namespace, but, for con‐ sistency, the kernel provides dummy user and group ID mapping files for this namespace. Looking at the uid_map file (gid_map is the same) from a shell in the initial namespace shows:

Quoted 7 lines

$ cat /proc/$$/uid_map
0 0 4294967295

This mapping tells us that the range starting at user ID 0 in this namespace maps to a range starting at 0 in the (nonexistent) parent namespace, and the length of the range is the largest 32-bit unsigned integer.

Quoted 4 lines

Which deliberately leaves 4294967295 32bit (-1) unmapped. (uid_t)-1 is used in several interfaces (like setreuid) as a way to specify no uid leaving it unmapped and unusuable guarantees that there will be no confusion when using those kernel methods.

So, I worked that piece into the text to give:

Quoted 8 lines

This mapping tells us that the range starting at user ID 0 in this namespace maps to a range starting at 0 in the (nonexis‐ tent) parent namespace, and the length of the range is the
largest 32-bit unsigned integer. (This deliberately leaves
4294967295 (the 32-bit signed -1 value) unmapped. This is
deliberate: (uid_t) -1 is used in several interfaces (e.g., setreuid(2)) as a way to specify "no user ID". Leaving setreuid(2)) unmapped and unusuable guarantees that there will

         ^^^^ (uid_t) -1 (not setreuid(2)

Quoted 3 lines

be no confusion when using these interfaces.

Okay?

Other than the typo fix above this looks good.

Quoted 37 lines · Michael Kerrisk (man-pages)

Hi Andy, and Eric,

Quoted 34 lines · Andy Lutomirski

Quoted 31 lines · Michael Kerrisk (man-pages)

Hello Eric et al.,

For various reasons, my work on the namespaces man pages fell off the table a while back. Nevertheless, the pages have been close to completion for a while now, and I recently restarted, in an effort to finish them. As you also noted to me f2f, there have been recently been some small namespace changes that you may affect the content of the pages. Therefore, I'll take the opportunity to send the namespace-related pages out for further (final?) review.

So, here, I start with the user_namespaces(7) page, which is shown in rendered form below, with source attached to this mail. I'll send various other pages in follow-on mails.

Review comments/suggestions for improvements / bug fixes welcome.

Cheers,

Michael

==

NAME
user_namespaces - overview of Linux user_namespaces

DESCRIPTION
For an overview of namespaces, see namespaces(7).

User namespaces isolate security-related identifiers and
attributes, in particular, user IDs and group IDs (see creden‐ tials(7), the root directory, keys (see keyctl(2)), and capabili‐

Quoted 6 lines

Quoted 3 lines

Putting "root directory" here is odd -- that's really part of a different namespace. But user namespaces sort of isolate the other namespaces from each other.

I'm trying to remember the details here. I think this piece originally came after a discussion with Eric, but I am not sure. Eric?

Probably.

I am not certain what the best way to say it but we do need to document that an unprivileged user that creates a user namespace can now call chroot.

We may also want to discuss the specific restrictions on chroot.

The text about chroot at least gives people a strong hint that the chroot rules are affected by user namespaces.

The restrictions that we have settled on to avoid chroot being a problem are the creator of a user namespace must not be chrooted in their current mount namespace, and the creator of the user namespace must not be threaded.

Andy can you check me on this it looks like unshare is currently buggy in that it will allow a threaded application to create a user namespace.

Quoted 58 lines

Quoted 46 lines

Also, ugh, keys. How did keyctl(2) ever make it through any kind of review?

Quoted 44 lines

ties (see capabilities(7)). A process's user and group IDs can be different inside and outside a user namespace. In particular, a process can have a normal unprivileged user ID outside a user namespace while at the same time having a user ID of 0 inside the namespace; in other words, the process has full privileges for operations inside the user namespace, but is unprivileged for operations outside the namespace.

Nested namespaces, namespace membership
User namespaces can be nested; that is, each user namespace— except the initial ("root") namespace—has a parent user names‐ pace, and can have zero or more child user namespaces. The par‐ ent user namespace is the user namespace of the process that cre‐ ates the user namespace via a call to unshare(2) or clone(2) with the CLONE_NEWUSER flag.

The kernel imposes (since version 3.11) a limit of 32 nested lev‐ els of user namespaces. Calls to unshare(2) or clone(2) that would cause this limit to be exceeded fail with the error EUSERS.

Each process is a member of exactly one user namespace. A process created via fork(2) or clone(2) without the CLONE_NEWUSER flag is a member of the same user namespace as its parent. A process can join another user namespace with setns(2) if it has the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a full set of capabilities in that namespace.

A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag makes the new child process (for clone(2)) or the caller (for unshare(2)) a member of the new user namespace created by the call.

Capabilities
The child process created by clone(2) with the CLONE_NEWUSER flag starts out with a complete set of capabilities in the new user namespace. Likewise, a process that creates a new user namespace using unshare(2) or joins an existing user namespace using setns(2) gains a full set of capabilities in that namespace. On the other hand, that process has no capabilities in the parent (in the case of clone(2)) or previous (in the case of unshare(2) and setns(2)) user namespace, even if the new namespace is cre‐ ated or joined by the root user (i.e., a process with user ID 0 in the root namespace).

Quoted 8 lines

Quoted 3 lines

Note that a call to execve(2) will cause a process to lose any capabilities that it has, unless it has a user ID of 0 within the namespace.

Or unless file capabilities have a non-empty inheritable mask.

It may be worth mentioning that execve in a user namespace works exactly like execve outside a userns.

I';ve reworded that para to say:

Quoted 8 lines

Note that a call to execve(2) will cause a process's capabili‐ ties to be recalculated in the usual way (see capabilities(7)), so that usually, unless it has a user ID of 0 within the names‐ pace or the executable file has a nonempty inheritable capabil‐ ities mask, it will lose all capabilities. See the discussion of user and group ID mappings, below.

Okay?

That seems reasonable to me.

Quoted 49 lines

Quoted 49 lines

Quoted 49 lines

           $ cat /proc/$$/uid_map
                    0          0 4294967295

This mapping tells us that the range starting at user ID 0 in this namespace maps to a range starting at 0 in the (nonexistent) parent namespace, and the length of the range is the largest 32-bit unsigned integer.

Defining user and group ID mappings: writing to uid_map and gid_map
After the creation of a new user namespace, the uid_map file of one of the processes in the namespace may be written to once to define the mapping of user IDs in the new user namespace. An attempt to write more than once to a uid_map file in a user namespace fails with the error EPERM. Similar rules apply for gid_map files.

The lines written to uid_map (gid_map) must conform to the fol‐ lowing rules:

* The three fields must be valid numbers, and the last field must be greater than 0.

* Lines are terminated by newline characters.

* There is an (arbitrary) limit on the number of lines in the file. As at Linux 3.8, the limit is five lines. In addition, the number of bytes written to the file must be less than the system page size, and the write must be performed at the start of the file (i.e., lseek(2) and pwrite(2) can't be used to write to nonzero offsets in the file).

* The range of user IDs (group IDs) specified in each line can‐ not overlap with the ranges in any other lines. In the ini‐ tial implementation (Linux 3.8), this requirement was satis‐ fied by a simplistic implementation that imposed the further requirement that the values in both field 1 and field 2 of successive lines must be in ascending numerical order, which prevented some otherwise valid maps from being created. Linux 3.9 and later fix this limitation, allowing any valid set of nonoverlapping maps.

* At least one line must be written to the file.

Writes that violate the above rules fail with the error EINVAL.

In order for a process to write to the /proc/[pid]/uid_map (/proc/[pid]/gid_map) file, all of the following requirements must be met:

Quoted 7 lines

Quoted 5 lines

Quoted 2 lines

1. The writing process must have the CAP_SETUID (CAP_SETGID) capability in the user namespace of the process pid.

This checked for the opening process (and I don't actually remember whether it's checked for the writing process).

Eric, can you comment?

We have to check for the opening processes and that changes was made after I implemented my interface. Pieces of the code appear to also examine the writing process and verify everything applies to it as well.

I goofed when I designed the interface originall and had not realized what a classic design error it can be to not restrict by the opening process.

Quoted 37 lines · Michael Kerrisk (man-pages)

Hi Andy, and Eric,

Quoted 34 lines · Andy Lutomirski

Quoted 31 lines · Michael Kerrisk (man-pages)

Hello Eric et al.,

For various reasons, my work on the namespaces man pages fell off the table a while back. Nevertheless, the pages have been close to completion for a while now, and I recently restarted, in an effort to finish them. As you also noted to me f2f, there have been recently been some small namespace changes that you may affect the content of the pages. Therefore, I'll take the opportunity to send the namespace-related pages out for further (final?) review.

So, here, I start with the user_namespaces(7) page, which is shown in rendered form below, with source attached to this mail. I'll send various other pages in follow-on mails.

Review comments/suggestions for improvements / bug fixes welcome.

Cheers,

Michael

==

NAME
user_namespaces - overview of Linux user_namespaces

DESCRIPTION
For an overview of namespaces, see namespaces(7).

User namespaces isolate security-related identifiers and
attributes, in particular, user IDs and group IDs (see creden‐ tials(7), the root directory, keys (see keyctl(2)), and capabili‐

Quoted 6 lines

Quoted 3 lines

Putting "root directory" here is odd -- that's really part of a different namespace. But user namespaces sort of isolate the other namespaces from each other.

I'm trying to remember the details here. I think this piece originally came after a discussion with Eric, but I am not sure. Eric?

Probably.

I am not certain what the best way to say it but we do need to document that an unprivileged user that creates a user namespace can now call chroot.

We may also want to discuss the specific restrictions on chroot.

The text about chroot at least gives people a strong hint that the chroot rules are affected by user namespaces.

The restrictions that we have settled on to avoid chroot being a problem are the creator of a user namespace must not be chrooted in their current mount namespace, and the creator of the user namespace must not be threaded.

Andy can you check me on this it looks like unshare is currently buggy in that it will allow a threaded application to create a user namespace.

Quoted 58 lines

Quoted 46 lines

Also, ugh, keys. How did keyctl(2) ever make it through any kind of review?

Quoted 44 lines

ties (see capabilities(7)). A process's user and group IDs can be different inside and outside a user namespace. In particular, a process can have a normal unprivileged user ID outside a user namespace while at the same time having a user ID of 0 inside the namespace; in other words, the process has full privileges for operations inside the user namespace, but is unprivileged for operations outside the namespace.

Nested namespaces, namespace membership
User namespaces can be nested; that is, each user namespace— except the initial ("root") namespace—has a parent user names‐ pace, and can have zero or more child user namespaces. The par‐ ent user namespace is the user namespace of the process that cre‐ ates the user namespace via a call to unshare(2) or clone(2) with the CLONE_NEWUSER flag.

The kernel imposes (since version 3.11) a limit of 32 nested lev‐ els of user namespaces. Calls to unshare(2) or clone(2) that would cause this limit to be exceeded fail with the error EUSERS.

Each process is a member of exactly one user namespace. A process created via fork(2) or clone(2) without the CLONE_NEWUSER flag is a member of the same user namespace as its parent. A process can join another user namespace with setns(2) if it has the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a full set of capabilities in that namespace.

A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag makes the new child process (for clone(2)) or the caller (for unshare(2)) a member of the new user namespace created by the call.

Capabilities
The child process created by clone(2) with the CLONE_NEWUSER flag starts out with a complete set of capabilities in the new user namespace. Likewise, a process that creates a new user namespace using unshare(2) or joins an existing user namespace using setns(2) gains a full set of capabilities in that namespace. On the other hand, that process has no capabilities in the parent (in the case of clone(2)) or previous (in the case of unshare(2) and setns(2)) user namespace, even if the new namespace is cre‐ ated or joined by the root user (i.e., a process with user ID 0 in the root namespace).

Quoted 8 lines

Quoted 3 lines

Note that a call to execve(2) will cause a process to lose any capabilities that it has, unless it has a user ID of 0 within the namespace.

Or unless file capabilities have a non-empty inheritable mask.

It may be worth mentioning that execve in a user namespace works exactly like execve outside a userns.

I';ve reworded that para to say:

Quoted 8 lines

Note that a call to execve(2) will cause a process's capabili‐ ties to be recalculated in the usual way (see capabilities(7)), so that usually, unless it has a user ID of 0 within the names‐ pace or the executable file has a nonempty inheritable capabil‐ ities mask, it will lose all capabilities. See the discussion of user and group ID mappings, below.

Okay?

That seems reasonable to me.

Quoted 49 lines

Quoted 49 lines

Quoted 49 lines

           $ cat /proc/$$/uid_map
                    0          0 4294967295

This mapping tells us that the range starting at user ID 0 in this namespace maps to a range starting at 0 in the (nonexistent) parent namespace, and the length of the range is the largest 32-bit unsigned integer.

Defining user and group ID mappings: writing to uid_map and gid_map
After the creation of a new user namespace, the uid_map file of one of the processes in the namespace may be written to once to define the mapping of user IDs in the new user namespace. An attempt to write more than once to a uid_map file in a user namespace fails with the error EPERM. Similar rules apply for gid_map files.

The lines written to uid_map (gid_map) must conform to the fol‐ lowing rules:

* The three fields must be valid numbers, and the last field must be greater than 0.

* Lines are terminated by newline characters.

* There is an (arbitrary) limit on the number of lines in the file. As at Linux 3.8, the limit is five lines. In addition, the number of bytes written to the file must be less than the system page size, and the write must be performed at the start of the file (i.e., lseek(2) and pwrite(2) can't be used to write to nonzero offsets in the file).

* The range of user IDs (group IDs) specified in each line can‐ not overlap with the ranges in any other lines. In the ini‐ tial implementation (Linux 3.8), this requirement was satis‐ fied by a simplistic implementation that imposed the further requirement that the values in both field 1 and field 2 of successive lines must be in ascending numerical order, which prevented some otherwise valid maps from being created. Linux 3.9 and later fix this limitation, allowing any valid set of nonoverlapping maps.

* At least one line must be written to the file.

Writes that violate the above rules fail with the error EINVAL.

In order for a process to write to the /proc/[pid]/uid_map (/proc/[pid]/gid_map) file, all of the following requirements must be met:

Quoted 7 lines

Quoted 5 lines

Quoted 2 lines

1. The writing process must have the CAP_SETUID (CAP_SETGID) capability in the user namespace of the process pid.

This checked for the opening process (and I don't actually remember whether it's checked for the writing process).

Eric, can you comment?

We have to check for the opening processes and that changes was made after I implemented my interface. Pieces of the code appear to also examine the writing process and verify everything applies to it as well.

I goofed when I designed the interface originall and had not realized what a classic design error it can be to not restrict by the opening process.

Quoted 6 lines · Eric W. Biederman

We may also want to discuss the specific restrictions on chroot.

The text about chroot at least gives people a strong hint that the chroot rules are affected by user namespaces.

Quoted 7 lines

The restrictions that we have settled on to avoid chroot being a problem are the creator of a user namespace must not be chrooted in their current mount namespace, and the creator of the user namespace must not be threaded.

Andy can you check me on this it looks like unshare is currently buggy in that it will allow a threaded application to create a user namespace.

I think it's this code in unshare:

    /*
     * If unsharing a user namespace must also unshare the thread.
     */
    if (unshare_flags & CLONE_NEWUSER)
        unshare_flags |= CLONE_THREAD | CLONE_FS;

I suppose that this should be documented.

CLONE_FS prevents the chroot from leaking out of the namespace. (But see the other thread that I'm about to start...)

Hi Eric,

Quoted 15 lines · Eric W. Biederman

Quoted 8 lines · Michael Kerrisk (man-pages)

Hi Andy, and Eric,

Quoted 5 lines

Quoted 2 lines

1. The writing process must have the CAP_SETUID (CAP_SETGID) capability in the user namespace of the process pid.

This checked for the opening process (and I don't actually remember whether it's checked for the writing process).

Eric, can you comment?

We have to check for the opening processes and that changes was made after I implemented my interface. Pieces of the code appear to also examine the writing process and verify everything applies to it as well.

Quoted 3 lines

I goofed when I designed the interface originall and had not realized what a classic design error it can be to not restrict by the opening process.

So, I still need some help here. Should the sentence above just read:

1. The *opening* process must have the CAP_SETUID (CAP_SETGID) capability in the user namespace of the process pid.

or must something also be said about the writing process? (If so, i'd appreciate a completely formed sentence or two that I can just drop into the man page..)

So, in the current draft of the setns(2) page, there is

CLONE_NEWNS
...
Since Linux 3.9, CLONE_NEWUSER also automatically implies CLONE_FS.

Does that cover your point? Or did you mean that more needs to be said?

Quoted 18 lines · Michael Kerrisk (man-pages)

Hi Eric,

Quoted 15 lines · Eric W. Biederman

Quoted 8 lines · Michael Kerrisk (man-pages)

Hi Andy, and Eric,

Quoted 5 lines

Quoted 2 lines

1. The writing process must have the CAP_SETUID (CAP_SETGID) capability in the user namespace of the process pid.

This checked for the opening process (and I don't actually remember whether it's checked for the writing process).

Eric, can you comment?

We have to check for the opening processes and that changes was made after I implemented my interface. Pieces of the code appear to also examine the writing process and verify everything applies to it as well.

Quoted 8 lines

Quoted 3 lines

I goofed when I designed the interface originall and had not realized what a classic design error it can be to not restrict by the opening process.

So, I still need some help here. Should the sentence above just read:

1. The *opening* process must have the CAP_SETUID (CAP_SETGID) capability in the user namespace of the process pid.

I think this is sufficient.

Quoted 1 line

Quoted 3 lines

or must something also be said about the writing process? (If so, i'd appreciate a completely formed sentence or two that I can just drop into the man page..)

There might be a restriction there, too, but I think it could be removed, and I also think that it's unlikely that anyone will encounter it.

Quoted 18 lines · Michael Kerrisk (man-pages)

Hi Eric,

Quoted 15 lines · Eric W. Biederman

Quoted 8 lines · Michael Kerrisk (man-pages)

Hi Andy, and Eric,

Quoted 5 lines

Quoted 2 lines

1. The writing process must have the CAP_SETUID (CAP_SETGID) capability in the user namespace of the process pid.

This checked for the opening process (and I don't actually remember whether it's checked for the writing process).

Eric, can you comment?

We have to check for the opening processes and that changes was made after I implemented my interface. Pieces of the code appear to also examine the writing process and verify everything applies to it as well.

Quoted 8 lines

Quoted 3 lines

I goofed when I designed the interface originall and had not realized what a classic design error it can be to not restrict by the opening process.

So, I still need some help here. Should the sentence above just read:

1. The *opening* process must have the CAP_SETUID (CAP_SETGID) capability in the user namespace of the process pid.

I think this is sufficient.

Quoted 1 line

Quoted 3 lines

or must something also be said about the writing process? (If so, i'd appreciate a completely formed sentence or two that I can just drop into the man page..)

There might be a restriction there, too, but I think it could be removed, and I also think that it's unlikely that anyone will encounter it.

Quoted 8 lines · Michael Kerrisk (man-pages)

So, in the current draft of the setns(2) page, there is

CLONE_NEWNS
...
Since Linux 3.9, CLONE_NEWUSER also automatically implies CLONE_FS.

Quoted 1 line

Does that cover your point? Or did you mean that more needs to be said?

Looks good, although you could add CLONE_THREAD and the rest of the things implied by CLONE_THREAD if you want to be fancier.

Quoted 8 lines · Michael Kerrisk (man-pages)

So, in the current draft of the setns(2) page, there is

CLONE_NEWNS
...
Since Linux 3.9, CLONE_NEWUSER also automatically implies CLONE_FS.

Quoted 1 line

Does that cover your point? Or did you mean that more needs to be said?

Looks good, although you could add CLONE_THREAD and the rest of the things implied by CLONE_THREAD if you want to be fancier.