namespace man page updates for 3.8

Document the user namespace files that report the mapping of uids and gids between user namespaces.

Signed-off-by "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/[email protected]>

---
 man5/proc.5 |   50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 50 insertions(+), 0 deletions(-)

man5/proc.5 +1 -0

index fb70d2b..840480d 100644
@@ -317,6 +317,31 @@ The files in this directory are readable only by the owner of the process.
 .\" .TP
 .\" .IR /proc/[pid]/io " (since kernel 2.6.20)"
 .TP
+.IR /proc/[pid]/gid_map " (since kernel 3.6)"

man5/proc.5 +32 -0

+
+Each line contains three numbers.  The start of the range of gids in
+the user namespace of the process specifed by
+.IR pid.
+The start of the range of gids in the user namespace of the process that
+opened
+.IR /proc/[pid]/gid_map.
+The number of gids in the range of numbers that is mapped between to two
+user namespaces.
+
+After the creation of a new user namespace this file may be written to
+exactly once to specify the mapping of gids in the new user namespace.
+
+.TP
 .IR /proc/[pid]/limits " (since kernel 2.6.24)"
 This file displays the soft limit, hard limit, and units of measurement
 for each of the process's resource limits (see

@@ -1169,6 +1194,31 @@ directory are not available if the main thread has already terminated
 (typically by calling
 .BR pthread_exit (3)).
 .TP
+.IR /proc/[pid]/uid_map " (since kernel 3.6)"
+This file reports the mapping of uids from the user namespace of the process specified by
+.IR pid
+to the user namespace of the process that opened
+.IR /proc/[pid]/uid_map .
+
+Each line specifies a 1 to 1 mapping of a range of contiguous uids from
+the user namespace of the process specified by
+.IR pid
+to the user namespace of the process that opened
+.IR /proc/[pid]/uid_map.
+
+Each line contains three numbers.  The start of the range of uids in
+the user namespace of the process specifed by
+.IR pid.
+The start of the range of uids in the user namespace of the process that
+opened
+.IR /proc/[pid]/uid_map.

Hi Eric,

Thanks for this patch. I have one question and a revised version f the text that I'd like you to review.

Quoted 27 lines · Eric W. Biederman

Document the user namespace files that report the mapping of uids and gids between user namespaces.

Signed-off-by "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/[email protected]>

---
 man5/proc.5 |   50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 50 insertions(+), 0 deletions(-)

man5/proc.5 +10 -0

index fb70d2b..840480d 100644
@@ -317,6 +317,31 @@ The files in this directory are readable only by the owner of the process.
 .\" .TP
 .\" .IR /proc/[pid]/io " (since kernel 2.6.20)"
 .TP
+.IR /proc/[pid]/gid_map " (since kernel 3.6)"
+This file reports the mapping of gids from the user namespace of the process specified by
+.IR pid
+to the user namespace of the process that opened
+.IR /proc/[pid]/gid_map .
+
+Each line specifies a 1 to 1 mapping of a range of contiguous gids from
+the user namespace of the process specified by
+.IR pid
+to the user namespace of the process that opened

man5/proc.5 The files in this directory are readable only by the owner of the process. · line 330

Quoted 1 line

man5/proc.5 +1 -0

+.IR /proc/[pid]/gid_map.

I want to check the above point. What do you mean by "the process that opened uid_map"? Does that mean the process that opened uid_map to do the one-time write of the UID map? I had assumed that uid_map actually provided a mapping between the namespace of 'pid' and the 'parent' namespace, where the parent namespace is the namespace of the process that created this namespace via clone(CLONE_NEWUSER).

Quoted 38 lines

man5/proc.5 +31 -0

+Each line contains three numbers.  The start of the range of gids in
+the user namespace of the process specifed by
+.IR pid.
+The start of the range of gids in the user namespace of the process that
+opened
+.IR /proc/[pid]/gid_map.
+The number of gids in the range of numbers that is mapped between to two
+user namespaces.
+
+After the creation of a new user namespace this file may be written to
+exactly once to specify the mapping of gids in the new user namespace.
+
+.TP
 .IR /proc/[pid]/limits " (since kernel 2.6.24)"
 This file displays the soft limit, hard limit, and units of measurement
 for each of the process's resource limits (see

@@ -1169,6 +1194,31 @@ directory are not available if the main thread has already terminated
 (typically by calling
 .BR pthread_exit (3)).
 .TP
+.IR /proc/[pid]/uid_map " (since kernel 3.6)"
+This file reports the mapping of uids from the user namespace of the process specified by
+.IR pid
+to the user namespace of the process that opened
+.IR /proc/[pid]/uid_map .
+
+Each line specifies a 1 to 1 mapping of a range of contiguous uids from
+the user namespace of the process specified by
+.IR pid
+to the user namespace of the process that opened
+.IR /proc/[pid]/uid_map.
+
+Each line contains three numbers.  The start of the range of uids in
+the user namespace of the process specifed by
+.IR pid.
+The start of the range of uids in the user namespace of the process that
+opened
+.IR /proc/[pid]/uid_map.

man5/proc.5 directory are not available if the main thread has already terminated · line 1224

Quoted 10 lines

man5/proc.5 +7 -0

+The number of uids in the range of numbers that is mapped between to two
+user namespaces.
+
+After the creation of a new user namespace this file may be written to
+exactly once to specify the mapping of uids in the new user namespace.
+
+.TP
 .I /proc/apm
 Advanced power management version and battery information when
 .B CONFIG_APM

I revised your text quite a bit, and added a piece on the format od the uid_map files. Could you please read the following and let me know of errors:

[[
/proc/[pid]/uid_map, /proc/[pid]/gid_map (since Linux 3.6)
These files expose the mappings for user and group IDs
inside the user namespace for the process pid. The
description here 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 names‐ pace of the process that opened uid_map.

Each line in the file specifies a 1-to-1 mapping of a range of contiguous user IDs from the user namespace of the process pid to the user namespace of the process that opened uid_map.

Each line contains three numbers delimited by white space:

(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 in the user namespace of the process that opened uid_map.

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

After the creation of a new user namespace, this file may be written to exactly once to specify the mapping of
user IDs in the new user namespace. (An attempt to
write more than once to the file fails with the error EPERM.)

The lines written to uid_map must conform to the follow‐ ing rules:

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

* Lines are terminated by newline characters.

* The file can contain a maximum of five lines.

* The values in both field 1 and field 2 of each line must be in ascending numerical order.

* The range of user IDs specified in each line cannot overlap with the ranges in any other lines.

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

Thanks,

Michael

Quoted 34 lines · Michael Kerrisk (man-pages)

Hi Eric,

Thanks for this patch. I have one question and a revised version f the text that I'd like you to review.

Quoted 27 lines · Eric W. Biederman

Document the user namespace files that report the mapping of uids and gids between user namespaces.

Signed-off-by "Eric W. Biederman" <[email protected]>

---
 man5/proc.5 |   50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 50 insertions(+), 0 deletions(-)

man5/proc.5 +10 -0

index fb70d2b..840480d 100644
@@ -317,6 +317,31 @@ The files in this directory are readable only by the owner of the process.
 .\" .TP
 .\" .IR /proc/[pid]/io " (since kernel 2.6.20)"
 .TP
+.IR /proc/[pid]/gid_map " (since kernel 3.6)"
+This file reports the mapping of gids from the user namespace of the process specified by
+.IR pid
+to the user namespace of the process that opened
+.IR /proc/[pid]/gid_map .
+
+Each line specifies a 1 to 1 mapping of a range of contiguous gids from
+the user namespace of the process specified by
+.IR pid
+to the user namespace of the process that opened

Quoted 8 lines

man5/proc.5 The files in this directory are readable only by the owner of the process. · line 330

Quoted 1 line

man5/proc.5 +1 -0

+.IR /proc/[pid]/gid_map.

I want to check the above point. What do you mean by "the process that opened uid_map"? Does that mean the process that opened uid_map to do the one-time write of the UID map? I had assumed that uid_map actually provided a mapping between the namespace of 'pid' and the 'parent' namespace, where the parent namespace is the namespace of the process that created this namespace via clone(CLONE_NEWUSER).

I mean the process that opens uid_map for read or write.

For writing you are correct about the mapping to the parent (but that is not an exception that is a restriction on who can write to the file).

The complete rule is for the user namespace of the second value is:

- If the user namespace of the opener of the file and the user namespace of the process do not match. The user namespace of the opener of the file is used.

- If the user namespace of the opener of the file and the user namespace of the process are the same. The parent user namespace of the process is used for the second value.

While very wordy I think the rule makes a lot of intuitive and practical sense. Especially since it is non-trivial to come up with the chain of user namespaces a process is in.

Quoted 91 lines

Quoted 38 lines

man5/proc.5 +31 -0

+Each line contains three numbers.  The start of the range of gids in
+the user namespace of the process specifed by
+.IR pid.
+The start of the range of gids in the user namespace of the process that
+opened
+.IR /proc/[pid]/gid_map.
+The number of gids in the range of numbers that is mapped between to two
+user namespaces.
+
+After the creation of a new user namespace this file may be written to
+exactly once to specify the mapping of gids in the new user namespace.
+
+.TP
 .IR /proc/[pid]/limits " (since kernel 2.6.24)"
 This file displays the soft limit, hard limit, and units of measurement
 for each of the process's resource limits (see

@@ -1169,6 +1194,31 @@ directory are not available if the main thread has already terminated
 (typically by calling
 .BR pthread_exit (3)).
 .TP
+.IR /proc/[pid]/uid_map " (since kernel 3.6)"
+This file reports the mapping of uids from the user namespace of the process specified by
+.IR pid
+to the user namespace of the process that opened
+.IR /proc/[pid]/uid_map .
+
+Each line specifies a 1 to 1 mapping of a range of contiguous uids from
+the user namespace of the process specified by
+.IR pid
+to the user namespace of the process that opened
+.IR /proc/[pid]/uid_map.
+
+Each line contains three numbers.  The start of the range of uids in
+the user namespace of the process specifed by
+.IR pid.
+The start of the range of uids in the user namespace of the process that
+opened
+.IR /proc/[pid]/uid_map.

man5/proc.5 directory are not available if the main thread has already terminated · line 1224

Quoted 10 lines

man5/proc.5 +7 -0

+The number of uids in the range of numbers that is mapped between to two
+user namespaces.
+
+After the creation of a new user namespace this file may be written to
+exactly once to specify the mapping of uids in the new user namespace.
+
+.TP
 .I /proc/apm
 Advanced power management version and battery information when
 .B CONFIG_APM

I revised your text quite a bit, and added a piece on the format od the uid_map files. Could you please read the following and let me know of errors:

[[
/proc/[pid]/uid_map, /proc/[pid]/gid_map (since Linux 3.6)
These files expose the mappings for user and group IDs
inside the user namespace for the process pid. The
description here 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 names‐ pace of the process that opened uid_map.

Each line in the file specifies a 1-to-1 mapping of a range of contiguous user IDs from the user namespace of the process pid to the user namespace of the process that opened uid_map.

Each line contains three numbers delimited by white space:

(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 in the user namespace of the process that opened uid_map.

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

After the creation of a new user namespace, this file may be written to exactly once to specify the mapping of
user IDs in the new user namespace. (An attempt to
write more than once to the file fails with the error EPERM.)

The lines written to uid_map must conform to the follow‐ ing rules:

Quoted 6 lines

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

* Lines are terminated by newline characters.

* The file can contain a maximum of five lines.

A maximum of 5 lines is important to Document but it is a current arbitrary limit that may be changed in the future. Right now 5 extents are more than enough for any conceivable use case, and fit nicely within a single cache line.

It is probably better to say writes that exceed an arbitrary maximum length fail with -EINVAL. Currently the arbitrary maximum length is five lines.

Quoted 3 lines

* The values in both field 1 and field 2 of each line must be in ascending numerical order.

The rule is that the extents need to be non-overlapping. Ascending numerical order is how that is implemented but that is a misfeature, and there has already been one request to fix that. Removing the ascending numerical order limitation is on my todo list.

Hi Eric,

Quoted 45 lines · Eric W. Biederman

Quoted 34 lines · Michael Kerrisk (man-pages)

Hi Eric,

Thanks for this patch. I have one question and a revised version f the text that I'd like you to review.

Quoted 27 lines · Eric W. Biederman

Document the user namespace files that report the mapping of uids and gids between user namespaces.

Signed-off-by "Eric W. Biederman" <[email protected]>

---
 man5/proc.5 |   50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 50 insertions(+), 0 deletions(-)

man5/proc.5 +10 -0

index fb70d2b..840480d 100644
@@ -317,6 +317,31 @@ The files in this directory are readable only by the owner of the process.
 .\" .TP
 .\" .IR /proc/[pid]/io " (since kernel 2.6.20)"
 .TP
+.IR /proc/[pid]/gid_map " (since kernel 3.6)"
+This file reports the mapping of gids from the user namespace of the process specified by
+.IR pid
+to the user namespace of the process that opened
+.IR /proc/[pid]/gid_map .
+
+Each line specifies a 1 to 1 mapping of a range of contiguous gids from
+the user namespace of the process specified by
+.IR pid
+to the user namespace of the process that opened

Quoted 8 lines

man5/proc.5 The files in this directory are readable only by the owner of the process. · line 330

Quoted 1 line

man5/proc.5 +1 -0

+.IR /proc/[pid]/gid_map.

I want to check the above point. What do you mean by "the process that opened uid_map"? Does that mean the process that opened uid_map to do the one-time write of the UID map? I had assumed that uid_map actually provided a mapping between the namespace of 'pid' and the 'parent' namespace, where the parent namespace is the namespace of the process that created this namespace via clone(CLONE_NEWUSER).

Quoted 1 line

I mean the process that opens uid_map for read or write.

Thanks for the confirmation.

Quoted 2 lines

For writing you are correct about the mapping to the parent (but that is not an exception that is a restriction on who can write to the file).

So, by the way, I added this sentence to the page:

In order to write to the /proc/[pid]/uid_map
(/proc/[pid]/gid_map) file, a process must have the CAP_SETUID (CAP_SETGID) capability in the user namespace of the process pid.

Is that correct?

But, there appear to be more rules than this governing whether a process can write to the file (i.e., various other -EPERM cases). What are the rules?

Quoted 6 lines

The complete rule is for the user namespace of the second value is:

- If the user namespace of the opener of the file and the user namespace of the process do not match. The user namespace of the opener of the file is used.

Quoted 3 lines

- If the user namespace of the opener of the file and the user namespace of the process are the same. The parent user namespace of the process is used for the second value.

Could you give an example of the last case? (What I'm really seeking, I think, is clarification of "parent user namespace". Does that mean "user namespace of the process that created the user namespace of this process"?)

Quoted 102 lines

While very wordy I think the rule makes a lot of intuitive and practical sense. Especially since it is non-trivial to come up with the chain of user namespaces a process is in.

Quoted 91 lines

Quoted 38 lines

man5/proc.5 +31 -0

+Each line contains three numbers.  The start of the range of gids in
+the user namespace of the process specifed by
+.IR pid.
+The start of the range of gids in the user namespace of the process that
+opened
+.IR /proc/[pid]/gid_map.
+The number of gids in the range of numbers that is mapped between to two
+user namespaces.
+
+After the creation of a new user namespace this file may be written to
+exactly once to specify the mapping of gids in the new user namespace.
+
+.TP
 .IR /proc/[pid]/limits " (since kernel 2.6.24)"
 This file displays the soft limit, hard limit, and units of measurement
 for each of the process's resource limits (see

@@ -1169,6 +1194,31 @@ directory are not available if the main thread has already terminated
 (typically by calling
 .BR pthread_exit (3)).
 .TP
+.IR /proc/[pid]/uid_map " (since kernel 3.6)"
+This file reports the mapping of uids from the user namespace of the process specified by
+.IR pid
+to the user namespace of the process that opened
+.IR /proc/[pid]/uid_map .
+
+Each line specifies a 1 to 1 mapping of a range of contiguous uids from
+the user namespace of the process specified by
+.IR pid
+to the user namespace of the process that opened
+.IR /proc/[pid]/uid_map.
+
+Each line contains three numbers.  The start of the range of uids in
+the user namespace of the process specifed by
+.IR pid.
+The start of the range of uids in the user namespace of the process that
+opened
+.IR /proc/[pid]/uid_map.

man5/proc.5 directory are not available if the main thread has already terminated · line 1224

Quoted 10 lines

man5/proc.5 +7 -0

+The number of uids in the range of numbers that is mapped between to two
+user namespaces.
+
+After the creation of a new user namespace this file may be written to
+exactly once to specify the mapping of uids in the new user namespace.
+
+.TP
 .I /proc/apm
 Advanced power management version and battery information when
 .B CONFIG_APM

I revised your text quite a bit, and added a piece on the format od the uid_map files. Could you please read the following and let me know of errors:

[[
/proc/[pid]/uid_map, /proc/[pid]/gid_map (since Linux 3.6)
These files expose the mappings for user and group IDs
inside the user namespace for the process pid. The
description here 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 names‐ pace of the process that opened uid_map.

Each line in the file specifies a 1-to-1 mapping of a range of contiguous user IDs from the user namespace of the process pid to the user namespace of the process that opened uid_map.

Each line contains three numbers delimited by white space:

(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 in the user namespace of the process that opened uid_map.

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

After the creation of a new user namespace, this file may be written to exactly once to specify the mapping of
user IDs in the new user namespace. (An attempt to
write more than once to the file fails with the error EPERM.)

The lines written to uid_map must conform to the follow‐ ing rules:

Quoted 6 lines

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

* Lines are terminated by newline characters.

* The file can contain a maximum of five lines.

Quoted 8 lines

A maximum of 5 lines is important to Document but it is a current arbitrary limit that may be changed in the future. Right now 5 extents are more than enough for any conceivable use case, and fit nicely within a single cache line.

It is probably better to say writes that exceed an arbitrary maximum length fail with -EINVAL. Currently the arbitrary maximum length is five lines.

Okay -- reworded.

Quoted 4 lines

Quoted 3 lines

* The values in both field 1 and field 2 of each line must be in ascending numerical order.

Quoted 4 lines

The rule is that the extents need to be non-overlapping. Ascending numerical order is how that is implemented but that is a misfeature, and there has already been one request to fix that. Removing the ascending numerical order limitation is on my todo list.

Okay -- I've reworded some text here.

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

Quoted 11 lines

Quoted 2 lines

For writing you are correct about the mapping to the parent (but that is not an exception that is a restriction on who can write to the file).

So, by the way, I added this sentence to the page:

In order to write to the /proc/[pid]/uid_map
(/proc/[pid]/gid_map) file, a process must have the CAP_SETUID (CAP_SETGID) capability in the user namespace of the process pid.

Is that correct?

Yes.

Quoted 3 lines

But, there appear to be more rules than this governing whether a process can write to the file (i.e., various other -EPERM cases). What are the rules?

In general you must also have CAP_SETUID (CAP_SETGID) in the parent user namespace as well. The one exception to that is if you are mapping your current uid and gid.

A rose by any other name will smell as
sweet. In practice this means you must be root to map to uid or gids other than your own, which preserves the current limits on setuid and setgid.

Additionally the writer must see the map file with the lower user namespace being the parent user namespace. Which means you must be inside the user namespace itself or in the parent user namespace to write to the user namespaces mapping file.

For /proc/[pid]/projid_map which will be interesting once xfs has kuid/kgid support there are no capability checks because xfs let's anyone have any projid.

This is one of the few cases where it almost matters to understand how ns_capable works when you are not in the user namespace in question, and that goes to what is a parent user namespace. If you would like some more detail on that please ask.

Quoted 6 lines

Quoted 6 lines

The complete rule is for the user namespace of the second value is:

- If the user namespace of the opener of the file and the user namespace of the process do not match. The user namespace of the opener of the file is used.

Quoted 8 lines

Quoted 3 lines

- If the user namespace of the opener of the file and the user namespace of the process are the same. The parent user namespace of the process is used for the second value.

Could you give an example of the last case? (What I'm really seeking, I think, is clarification of "parent user namespace". Does that mean "user namespace of the process that created the user namespace of this process"?)

User namespaces form a tree. What you can do in one user namespace is a subset of what you can do in the parent user namespace.

The parent user namespace is the user namespace of the process that calls unshare or clone with CLONE_NEWUSER.

The last case is the common case of /proc/self/uid_map. And you see how your uids map into the user namespace of the creator of your user namespace.

With the default being just: 0 0 4294967295

Quoted 3 lines

Quoted 3 lines

While very wordy I think the rule makes a lot of intuitive and practical sense. Especially since it is non-trivial to come up with the chain of user namespaces a process is in.

Thank you very much for your time and patience in getting a good description of the user namespace.

Hi Eric,

Quoted 1 line · Eric W. Biederman

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

[...]

Quoted 18 lines

Quoted 11 lines

Quoted 2 lines

For writing you are correct about the mapping to the parent (but that is not an exception that is a restriction on who can write to the file).

So, by the way, I added this sentence to the page:

In order to write to the /proc/[pid]/uid_map
(/proc/[pid]/gid_map) file, a process must have the CAP_SETUID (CAP_SETGID) capability in the user namespace of the process pid.

Is that correct?

Yes.

Quoted 3 lines

But, there appear to be more rules than this governing whether a process can write to the file (i.e., various other -EPERM cases). What are the rules?

Quoted 3 lines

In general you must also have CAP_SETUID (CAP_SETGID) in the parent user namespace as well. The one exception to that is if you are mapping your current uid and gid.

Can you clarify what you mean by "mapping your own UID and GID" please (i.e., who is "you" in that sentence).

Quoted 9 lines

A rose by any other name will smell as
sweet. In practice this means you must be root to map to uid or gids other than your own, which preserves the current limits on setuid and setgid.

Additionally the writer must see the map file with the lower user namespace being the parent user namespace. Which means you must be inside the user namespace itself or in the parent user namespace to write to the user namespaces mapping file.

Okay -- I added some words on this point.

Quoted 24 lines

For /proc/[pid]/projid_map which will be interesting once xfs has kuid/kgid support there are no capability checks because xfs let's anyone have any projid.

This is one of the few cases where it almost matters to understand how ns_capable works when you are not in the user namespace in question, and that goes to what is a parent user namespace. If you would like some more detail on that please ask.

Quoted 6 lines

Quoted 6 lines

The complete rule is for the user namespace of the second value is:

- If the user namespace of the opener of the file and the user namespace of the process do not match. The user namespace of the opener of the file is used.

Quoted 8 lines

Quoted 3 lines

- If the user namespace of the opener of the file and the user namespace of the process are the same. The parent user namespace of the process is used for the second value.

Could you give an example of the last case? (What I'm really seeking, I think, is clarification of "parent user namespace". Does that mean "user namespace of the process that created the user namespace of this process"?)

Quoted 5 lines

User namespaces form a tree. What you can do in one user namespace is a subset of what you can do in the parent user namespace.

The parent user namespace is the user namespace of the process that calls unshare or clone with CLONE_NEWUSER.

Thanks.

Quoted 3 lines

The last case is the common case of /proc/self/uid_map. And you see how your uids map into the user namespace of the creator of your user namespace.

Okay -- got it now.

Quoted 1 line

With the default being just: 0 0 4294967295

Right.

Quoted 3 lines

Quoted 3 lines

Quoted 3 lines

While very wordy I think the rule makes a lot of intuitive and practical sense. Especially since it is non-trivial to come up with the chain of user namespaces a process is in.

Yes, I see what you mean.

[...]

Quoted 2 lines

Thank you very much for your time and patience in getting a good description of the user namespace.

Well, we're not done yet, but we're getting there. Below, I've pasted the current text from proc(5). Could you please take a look, and let me know of any errors or improvements.

Cheers,

Michael

/proc/[pid]/uid_map, /proc/[pid]/gid_map (since Linux 3.5)
These files expose the mappings for user and group IDs
inside the user namespace for the process pid. The
description here 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 names‐ pace of the process that opened uid_map (but see a qual‐ ification to this point below). In other words, pro‐ cesses that are in different user namespaces will poten‐ tially see different values when reading from a particu‐ lar uid_map file, depending on the user ID mappings for the user namespaces of the reading processes.

Each line in the file specifies a 1-to-1 mapping of a range of contiguous between two user namespaces. The specification in each line takes the form of three num‐ bers delimited by white space. The first two numbers specify the starting user ID in each user namespace. The third number specifies the length of the mapped range. In detail, the fields are interpreted as fol‐ lows:

(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 names‐
paces: 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 names‐
pace: field two is the start of the range of user IDs in the parent user namespace of the process pid. (The "parent user namespace" is the user namespace of the process that created a user namespace via a call to unshare(2) or clone(2) with the CLONE_NEWUSER flag.) 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.

After the creation of a new user namespace, the uid_map file may be written to exactly once to specify the map‐ ping of user IDs in the new user namespace. (An attempt to write more than once to the file fails with the error EPERM.)

The lines written to uid_map must conform to the follow‐ ing 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.

* The range of user IDs specified in each line cannot overlap with the ranges in any other lines. In the current implementation (Linux 3.8), this requirement is satisified by a simplistic implementation that imposes the further requirement that the values in both field 1 and field 2 of successive lines must be in ascending numerical order.

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, the fol‐ lowing requirements must be met:

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

* The process must have the CAP_SETUID (CAP_SETGID) capability in the parent user namespace.

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

Quoted 26 lines · Michael Kerrisk (man-pages)

Hi Eric,

Quoted 1 line · Eric W. Biederman

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

[...]

Quoted 18 lines

Quoted 11 lines

Quoted 2 lines

For writing you are correct about the mapping to the parent (but that is not an exception that is a restriction on who can write to the file).

So, by the way, I added this sentence to the page:

In order to write to the /proc/[pid]/uid_map
(/proc/[pid]/gid_map) file, a process must have the CAP_SETUID (CAP_SETGID) capability in the user namespace of the process pid.

Is that correct?

Yes.

Quoted 3 lines

But, there appear to be more rules than this governing whether a process can write to the file (i.e., various other -EPERM cases). What are the rules?

Quoted 6 lines

Quoted 3 lines

In general you must also have CAP_SETUID (CAP_SETGID) in the parent user namespace as well. The one exception to that is if you are mapping your current uid and gid.

Can you clarify what you mean by "mapping your own UID and GID" please (i.e., who is "you" in that sentence).

At the time of clone() or unshare() that creates a new user namespace, the kuid and the kgid of the process does not change.

setuid and setgid fail before any mappings are set up.

Therefore the caller is allowed to map any single uid to the uid of the caller in the parent user namespace. Likewise the caller is allowed to map any single gid to the gid of the caller in the parent user namespace.

Hi Eric,

Thanks for this patch. I have one question and a revised version f the text that I'd like you to review.

Quoted 27 lines · Eric W. Biederman

Document the user namespace files that report the mapping of uids and gids between user namespaces.

Signed-off-by "Eric W. Biederman" <[email protected]>

---
 man5/proc.5 |   50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 50 insertions(+), 0 deletions(-)

man5/proc.5 +10 -0

index fb70d2b..840480d 100644
@@ -317,6 +317,31 @@ The files in this directory are readable only by the owner of the process.
 .\" .TP
 .\" .IR /proc/[pid]/io " (since kernel 2.6.20)"
 .TP
+.IR /proc/[pid]/gid_map " (since kernel 3.6)"
+This file reports the mapping of gids from the user namespace of the process specified by
+.IR pid
+to the user namespace of the process that opened
+.IR /proc/[pid]/gid_map .
+
+Each line specifies a 1 to 1 mapping of a range of contiguous gids from
+the user namespace of the process specified by
+.IR pid
+to the user namespace of the process that opened

man5/proc.5 The files in this directory are readable only by the owner of the process. · line 330

Quoted 1 line

man5/proc.5 +1 -0

+.IR /proc/[pid]/gid_map.

I want to check the above point. What do you mean by "the process that opened uid_map"? Does that mean the process that opened uid_map to do the one-time write of the UID map? I had assumed that uid_map actually provided a mapping between the namespace of 'pid' and the 'parent' namespace, where the parent namespace is the namespace of the process that created this namespace via clone(CLONE_NEWUSER).

Quoted 38 lines

man5/proc.5 +31 -0

+Each line contains three numbers.  The start of the range of gids in
+the user namespace of the process specifed by
+.IR pid.
+The start of the range of gids in the user namespace of the process that
+opened
+.IR /proc/[pid]/gid_map.
+The number of gids in the range of numbers that is mapped between to two
+user namespaces.
+
+After the creation of a new user namespace this file may be written to
+exactly once to specify the mapping of gids in the new user namespace.
+
+.TP
 .IR /proc/[pid]/limits " (since kernel 2.6.24)"
 This file displays the soft limit, hard limit, and units of measurement
 for each of the process's resource limits (see

@@ -1169,6 +1194,31 @@ directory are not available if the main thread has already terminated
 (typically by calling
 .BR pthread_exit (3)).
 .TP
+.IR /proc/[pid]/uid_map " (since kernel 3.6)"
+This file reports the mapping of uids from the user namespace of the process specified by
+.IR pid
+to the user namespace of the process that opened
+.IR /proc/[pid]/uid_map .
+
+Each line specifies a 1 to 1 mapping of a range of contiguous uids from
+the user namespace of the process specified by
+.IR pid
+to the user namespace of the process that opened
+.IR /proc/[pid]/uid_map.
+
+Each line contains three numbers.  The start of the range of uids in
+the user namespace of the process specifed by
+.IR pid.
+The start of the range of uids in the user namespace of the process that
+opened
+.IR /proc/[pid]/uid_map.

man5/proc.5 directory are not available if the main thread has already terminated · line 1224

Quoted 10 lines

man5/proc.5 +7 -0

+The number of uids in the range of numbers that is mapped between to two
+user namespaces.
+
+After the creation of a new user namespace this file may be written to
+exactly once to specify the mapping of uids in the new user namespace.
+
+.TP
 .I /proc/apm
 Advanced power management version and battery information when
 .B CONFIG_APM

I revised your text quite a bit, and added a piece on the format od the uid_map files. Could you please read the following and let me know of errors:

[[
/proc/[pid]/uid_map, /proc/[pid]/gid_map (since Linux 3.6)
These files expose the mappings for user and group IDs
inside the user namespace for the process pid. The
description here 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 names‐ pace of the process that opened uid_map.

Each line in the file specifies a 1-to-1 mapping of a range of contiguous user IDs from the user namespace of the process pid to the user namespace of the process that opened uid_map.

Each line contains three numbers delimited by white space:

(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 in the user namespace of the process that opened uid_map.

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

After the creation of a new user namespace, this file may be written to exactly once to specify the mapping of
user IDs in the new user namespace. (An attempt to
write more than once to the file fails with the error EPERM.)

The lines written to uid_map must conform to the follow‐ ing rules:

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

* Lines are terminated by newline characters.

* The file can contain a maximum of five lines.

* The values in both field 1 and field 2 of each line must be in ascending numerical order.

* The range of user IDs specified in each line cannot overlap with the ranges in any other lines.

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

Thanks,

Michael

Hi Eric,

Thanks for this patch. I have one question and a revised version f the text that I'd like you to review.

Quoted 4 lines · Michael Kerrisk (man-pages)

Hi Eric,

Thanks for this patch. I have one question and a revised version f the text that I'd like you to review.

In this patch where I said 3.6 it should have been 3.5

Quoted 4 lines · Michael Kerrisk (man-pages)

Hi Eric,

Thanks for this patch. I have one question and a revised version f the text that I'd like you to review.

In this patch where I said 3.6 it should have been 3.5

Hi Eric,

Thanks for this patch. I have one question and a revised version f the text that I'd like you to review.

Document the user namespace files that report the mapping of uids and gids between user namespaces.

Signed-off-by "Eric W. Biederman" <[email protected]>

---
 man5/proc.5 |   50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 50 insertions(+), 0 deletions(-)

man5/proc.5 +1 -0

index fb70d2b..840480d 100644
@@ -317,6 +317,31 @@ The files in this directory are readable only by the owner of the process.
 .\" .TP
 .\" .IR /proc/[pid]/io " (since kernel 2.6.20)"
 .TP
+.IR /proc/[pid]/gid_map " (since kernel 3.6)"

man5/proc.5 +32 -0

+
+Each line contains three numbers.  The start of the range of gids in
+the user namespace of the process specifed by
+.IR pid.
+The start of the range of gids in the user namespace of the process that
+opened
+.IR /proc/[pid]/gid_map.
+The number of gids in the range of numbers that is mapped between to two
+user namespaces.
+
+After the creation of a new user namespace this file may be written to
+exactly once to specify the mapping of gids in the new user namespace.
+
+.TP
 .IR /proc/[pid]/limits " (since kernel 2.6.24)"
 This file displays the soft limit, hard limit, and units of measurement
 for each of the process's resource limits (see

@@ -1169,6 +1194,31 @@ directory are not available if the main thread has already terminated
 (typically by calling
 .BR pthread_exit (3)).
 .TP
+.IR /proc/[pid]/uid_map " (since kernel 3.6)"
+This file reports the mapping of uids from the user namespace of the process specified by
+.IR pid
+to the user namespace of the process that opened
+.IR /proc/[pid]/uid_map .
+
+Each line specifies a 1 to 1 mapping of a range of contiguous uids from
+the user namespace of the process specified by
+.IR pid
+to the user namespace of the process that opened
+.IR /proc/[pid]/uid_map.
+
+Each line contains three numbers.  The start of the range of uids in
+the user namespace of the process specifed by
+.IR pid.
+The start of the range of uids in the user namespace of the process that
+opened
+.IR /proc/[pid]/uid_map.

Signed-off-by "Eric W. Biederman" <[email protected]>

---
 man2/clone.2 |   39 +++++++++++++++++++++++++++++++++++++++
 1 files changed, 39 insertions(+), 0 deletions(-)

man2/clone.2 +0 -0

index 0582057..4566677 100644

man2/clone.2 +31 -0

+If
+.B CLONE_NEWUSER
+is set, the create the process in a new user namespace.  If this flag is not set, then (as with
+.BR fork (2)),
+the process is created in the same user namespace as the calling process.
+
+A user namespace provides an isolated environment for security related identifiers in particular
+uids, gids, keys (see
+.BR keyctl (2)),
+and capabilities.
+
+When a user namespace is created it initially starts out without a mapping of uids and gids
+to the parent user namespace.  The desired mapping of uids to the parent user namespace
+may be set by writting into
+.IR /proc/[pid]/uid_map.
+The desired mapping of gids to the parent user namespace may be set by writinng into
+.IR /proc/[pid]/gid_map.
+
+The first process in a user namespace starts out with a complete set of capabilities with
+respect to the new user namespace.
+
+syscalls that return uids and gids will either return the uid or gid mapped into the current
+user namespace if there is a mapping or depending on the context will return either
+the overflowuid (default 65534) or the overflowgid (default 65534). See
+.IR /proc/sys/kernel/overflowuid, /proc/sys/kernel/overflowgid
+
+As of Linux 3.8 no priviliges are needed to create a user namespace,
+and mount, pid, ipc, net, uts namespaces can be created with just
+CAP_SYS_ADMIN privileges in your current user namespace.
+
+Over the years there have been a lot of features that have been added

Hi Eric,

Quoted 6 lines · Eric W. Biederman

Signed-off-by "Eric W. Biederman" <[email protected]>

---
 man2/clone.2 |   39 +++++++++++++++++++++++++++++++++++++++
 1 files changed, 39 insertions(+), 0 deletions(-)

man2/clone.2 in the same · line 369

Quoted 9 lines

man2/clone.2 +1 -0

index 0582057..4566677 100644
@@ -366,6 +366,45 @@ in the same
 .BR clone ()
 call.
 .TP
+.BR CLONE_NEWUSER " (since Linux 3.6)"

Why "since Linux 3.6"? As fas as I can see, CLONE_NEWUSER first gained some meaning in 2.6.29.

Quoted 31 lines

man2/clone.2 +31 -0

+If
+.B CLONE_NEWUSER
+is set, the create the process in a new user namespace.  If this flag is not set, then (as with
+.BR fork (2)),
+the process is created in the same user namespace as the calling process.
+
+A user namespace provides an isolated environment for security related identifiers in particular
+uids, gids, keys (see
+.BR keyctl (2)),
+and capabilities.
+
+When a user namespace is created it initially starts out without a mapping of uids and gids
+to the parent user namespace.  The desired mapping of uids to the parent user namespace
+may be set by writting into
+.IR /proc/[pid]/uid_map.
+The desired mapping of gids to the parent user namespace may be set by writinng into
+.IR /proc/[pid]/gid_map.
+
+The first process in a user namespace starts out with a complete set of capabilities with
+respect to the new user namespace.
+
+syscalls that return uids and gids will either return the uid or gid mapped into the current
+user namespace if there is a mapping or depending on the context will return either
+the overflowuid (default 65534) or the overflowgid (default 65534). See
+.IR /proc/sys/kernel/overflowuid, /proc/sys/kernel/overflowgid
+
+As of Linux 3.8 no priviliges are needed to create a user namespace,
+and mount, pid, ipc, net, uts namespaces can be created with just
+CAP_SYS_ADMIN privileges in your current user namespace.
+
+Over the years there have been a lot of features that have been added

man2/clone.2 in the same · line 410

Quoted 10 lines

man2/clone.2 +7 -0

+to the linux kernel that are only available to privileged users
+because of their potential to confuse setuid 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.
+
+.TP
 .BR CLONE_NEWPID " (since Linux 2.6.24)"
 .\" This explanation draws a lot of details from
 .\" http://lwn.net/Articles/259217/

I reworked your text somewhat. Could you please review the following:

[[
CLONE_NEWUSER
(This flag first became meaningful for clone() in Linux 2.6.29, but the implementation of user namespaces was only completed in Linux 3.8.)

If CLONE_NEWUSER is set,
then create the process in a new user namespace. If
this flag is not set, then (as with fork(2)) the process is created in the same user namespace as the calling process.

A user namespace provides an isolated environment for security related identifiers, in particular, user IDs, group IDs, keys (see keyctl(2)), and capabilities.

When a user namespace is created, it starts out without a mapping of user IDs (group IDs) to the parent user
namespace. The desired mapping of user IDs (group IDs)
to the parent user namespace may be set by writing into /proc/[pid]/uid_map (/proc/[pid]/gid_map); see proc(5).

The first process in a user namespace starts out with a complete set of capabilities with respect to the new user namespace.

System calls that return user IDs (group IDs) will return either the user ID (group ID) mapped into the current user namespace if there is a mapping, or the overflow user ID (group ID); the default value for the overflow user ID (group ID) is 65534. See the descrip‐ tions of /proc/sys/kernel/overflowuid and /proc/sys/ker‐ nel/overflowgid in proc(5).

Starting with Linux 3.8, no privileges are needed to create a user namespace, and mount, PID, IPC, net, and
UTS namespaces can be created with just the
CAP_SYS_ADMIN capability in the caller's user namespace.

Over the years, there have been a lot of features that have been added to the Linux kernel that are only avail‐ able 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.

]]

Thanks,

Michael

Quoted 10 lines · Michael Kerrisk (man-pages)

Hi Eric,

Quoted 6 lines · Eric W. Biederman

Signed-off-by "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/[email protected]>

---
 man2/clone.2 |   39 +++++++++++++++++++++++++++++++++++++++
 1 files changed, 39 insertions(+), 0 deletions(-)

Quoted 12 lines

man2/clone.2 in the same · line 369

Quoted 9 lines

man2/clone.2 +1 -0

index 0582057..4566677 100644
@@ -366,6 +366,45 @@ in the same
 .BR clone ()
 call.
 .TP
+.BR CLONE_NEWUSER " (since Linux 3.6)"

Why "since Linux 3.6"? As fas as I can see, CLONE_NEWUSER first gained some meaning in 2.6.29.

Looking at it where I have said 3.6 that is wrong. I meant 3.5.

I think I made the same mistake in one or two other manpages. Nothing was merged in 3.6 unfortunately.

My intent was these are the semantics of user namespaces since 3.5, when my rework/refocusing of them was merged.

Since 3.5 all that has really happened with user namespaces is the uid/gid to kuid/kgid conversion, permission checks have been relaxed, and a few bugs have been fixed.

3.8 is huge from a usability standpoint. 3.8 is huge because setns(), and unshare() are now complete from a namespace perspective, and because enough permission checks have been relaxed in user namespaces that you can really start using them.

But semantically from a user namespace perspective nothing really has changed in 3.8.

Quoted 7 lines

I reworked your text somewhat. Could you please review the following:

[[
CLONE_NEWUSER
(This flag first became meaningful for clone() in Linux 2.6.29, but the implementation of user namespaces was only completed in Linux 3.8.)

Long rant about 2.6.29 vs 3.8 above. I think what we need to say is:

(This flag first became meaningful for clone() in Linux 2.6.29, the current semantics were merged present in 3.5, and user namespaces only really became usable in 3.8.)

Quoted 10 lines

If CLONE_NEWUSER is set,
then create the process in a new user namespace. If
this flag is not set, then (as with fork(2)) the process is created in the same user namespace as the calling process.

A user namespace provides an isolated environment for security related identifiers, in particular, user IDs, group IDs, keys (see keyctl(2)), and capabilities.

Quoted 5 lines

When a user namespace is created, it starts out without a mapping of user IDs (group IDs) to the parent user
namespace. The desired mapping of user IDs (group IDs)
to the parent user namespace may be set by writing into /proc/[pid]/uid_map (/proc/[pid]/gid_map); see proc(5).

/proc/[pid]/projid_map deserves a mention. Not that I am a fan of project is or that xfs where the are implemented has been converted yet but....

Quoted 17 lines

The first process in a user namespace starts out with a complete set of capabilities with respect to the new user namespace.

System calls that return user IDs (group IDs) will return either the user ID (group ID) mapped into the current user namespace if there is a mapping, or the overflow user ID (group ID); the default value for the overflow user ID (group ID) is 65534. See the descrip‐ tions of /proc/sys/kernel/overflowuid and /proc/sys/ker‐ nel/overflowgid in proc(5).

Starting with Linux 3.8, no privileges are needed to create a user namespace, and mount, PID, IPC, net, and
UTS namespaces can be created with just the
CAP_SYS_ADMIN capability in the caller's user namespace.

Quoted 8 lines

Over the years, there have been a lot of features that have been added to the Linux kernel that are only avail‐ able 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.

I don't have any problems with this bit of text.

It occurs to me that what is going on with capabilities and user namespaces needs to be documented better. There was a minor bug with them this release cycle and I realized while the current definition makes sense and isn't hard to understand in general. In detail the interaction of capabilities and user namespaces are hard to describe.

I think capabilities and user namespaces are the work of a future patch however.

Hi Eric,

Quoted 25 lines · Eric W. Biederman

Quoted 10 lines · Michael Kerrisk (man-pages)

Hi Eric,

Quoted 6 lines · Eric W. Biederman

Signed-off-by "Eric W. Biederman" <[email protected]>

---
 man2/clone.2 |   39 +++++++++++++++++++++++++++++++++++++++
 1 files changed, 39 insertions(+), 0 deletions(-)

Quoted 12 lines

man2/clone.2 in the same · line 369

Quoted 9 lines

man2/clone.2 +1 -0

index 0582057..4566677 100644
@@ -366,6 +366,45 @@ in the same
 .BR clone ()
 call.
 .TP
+.BR CLONE_NEWUSER " (since Linux 3.6)"

Why "since Linux 3.6"? As fas as I can see, CLONE_NEWUSER first gained some meaning in 2.6.29.

Quoted 1 line

Looking at it where I have said 3.6 that is wrong. I meant 3.5.

Okay.

Quoted 2 lines

I think I made the same mistake in one or two other manpages. Nothing was merged in 3.6 unfortunately.

I think the other cases have been fixed by now.

Quoted 15 lines

My intent was these are the semantics of user namespaces since 3.5, when my rework/refocusing of them was merged.

Since 3.5 all that has really happened with user namespaces is the uid/gid to kuid/kgid conversion, permission checks have been relaxed, and a few bugs have been fixed.

3.8 is huge from a usability standpoint. 3.8 is huge because setns(), and unshare() are now complete from a namespace perspective, and because enough permission checks have been relaxed in user namespaces that you can really start using them.

But semantically from a user namespace perspective nothing really has changed in 3.8.

[...]

Quoted 8 lines

Quoted 7 lines

I reworked your text somewhat. Could you please review the following:

[[
CLONE_NEWUSER
(This flag first became meaningful for clone() in Linux 2.6.29, but the implementation of user namespaces was only completed in Linux 3.8.)

Quoted 5 lines

Long rant about 2.6.29 vs 3.8 above. I think what we need to say is:

(This flag first became meaningful for clone() in Linux 2.6.29, the current semantics were merged present in 3.5, and user namespaces only really became usable in 3.8.)

Yup. I've done something like that now.

Quoted 16 lines

Quoted 15 lines

If CLONE_NEWUSER is set,
then create the process in a new user namespace. If
this flag is not set, then (as with fork(2)) the process is created in the same user namespace as the calling process.

A user namespace provides an isolated environment for security related identifiers, in particular, user IDs, group IDs, keys (see keyctl(2)), and capabilities.

When a user namespace is created, it starts out without a mapping of user IDs (group IDs) to the parent user
namespace. The desired mapping of user IDs (group IDs)
to the parent user namespace may be set by writing into /proc/[pid]/uid_map (/proc/[pid]/gid_map); see proc(5).

Quoted 3 lines

/proc/[pid]/projid_map deserves a mention. Not that I am a fan of project is or that xfs where the are implemented has been converted yet but....

Would you be able to send a patch documenting this in proc(5)?

Quoted 28 lines

Quoted 17 lines

The first process in a user namespace starts out with a complete set of capabilities with respect to the new user namespace.

System calls that return user IDs (group IDs) will return either the user ID (group ID) mapped into the current user namespace if there is a mapping, or the overflow user ID (group ID); the default value for the overflow user ID (group ID) is 65534. See the descrip‐ tions of /proc/sys/kernel/overflowuid and /proc/sys/ker‐ nel/overflowgid in proc(5).

Starting with Linux 3.8, no privileges are needed to create a user namespace, and mount, PID, IPC, net, and
UTS namespaces can be created with just the
CAP_SYS_ADMIN capability in the caller's user namespace.

Quoted 8 lines

Over the years, there have been a lot of features that have been added to the Linux kernel that are only avail‐ able 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.

I don't have any problems with this bit of text.

Quoted 8 lines

It occurs to me that what is going on with capabilities and user namespaces needs to be documented better. There was a minor bug with them this release cycle and I realized while the current definition makes sense and isn't hard to understand in general. In detail the interaction of capabilities and user namespaces are hard to describe.

I think capabilities and user namespaces are the work of a future patch however.

Okay. So, below, a new iteration of the text. Could you please check it over, and note any errors to be fixed or improvements to be made.

Thanks,

Michael

CLONE_NEWUSER
(This flag first became meaningful for clone() in Linux 2.6.23, the current clone() semantics were merged in Linux 3.5, and the final pieces to make the user names‐ paces completely usable were merged in Linux 3.8.)

If CLONE_NEWUSER is set, then create the process in a new user namespace. If this flag is not set, then (as with fork(2)) the process is created in the same user namespace as the calling process.

A user namespace provides an isolated environment for security related identifiers, in particular, user IDs, group IDs, keys (see keyctl(2)), and capabilities.

When a user namespace is created, it starts out without a mapping of user IDs (group IDs) to the parent user
namespace. The desired mapping of user IDs (group IDs)
to the parent user namespace may be set by writing into /proc/[pid]/uid_map (/proc/[pid]/gid_map); see proc(5).

The first process in a user namespace starts out with a complete set of capabilities with respect to the new user namespace.

System calls that return user IDs (group IDs) will return either the user ID (group ID) mapped into the current user namespace if there is a mapping, or the overflow user ID (group ID); the default value for the overflow user ID (group ID) is 65534. See the descrip‐ tions of /proc/sys/kernel/overflowuid and /proc/sys/ker‐ nel/overflowgid in proc(5).

Use of this flag requires a kernel configured with the
CONFIG_USER_NS option. Before Linux 3.8, use of
CLONE_NEWUSER required that the caller have three capa‐
bilities: CAP_SYS_ADMIN, CAP_SETUID, and CAP_SETGID. Starting with Linux 3.8, no privileges are needed to create a user namespace, and mount, PID, IPC, net, and
UTS namespaces can be created with just the
CAP_SYS_ADMIN capability in the caller's user namespace.

Over the years, there have been a lot of features that have been added to the Linux kernel that are only avail‐ able 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.

Quoted 10 lines · Michael Kerrisk (man-pages)

Hi Eric,

Quoted 6 lines · Eric W. Biederman

Signed-off-by "Eric W. Biederman" <[email protected]>

---
 man2/clone.2 |   39 +++++++++++++++++++++++++++++++++++++++
 1 files changed, 39 insertions(+), 0 deletions(-)

Quoted 12 lines

man2/clone.2 in the same · line 369

Quoted 9 lines

man2/clone.2 +1 -0

index 0582057..4566677 100644
@@ -366,6 +366,45 @@ in the same
 .BR clone ()
 call.
 .TP
+.BR CLONE_NEWUSER " (since Linux 3.6)"

Why "since Linux 3.6"? As fas as I can see, CLONE_NEWUSER first gained some meaning in 2.6.29.

Looking at it where I have said 3.6 that is wrong. I meant 3.5.

I think I made the same mistake in one or two other manpages. Nothing was merged in 3.6 unfortunately.

My intent was these are the semantics of user namespaces since 3.5, when my rework/refocusing of them was merged.

Since 3.5 all that has really happened with user namespaces is the uid/gid to kuid/kgid conversion, permission checks have been relaxed, and a few bugs have been fixed.

3.8 is huge from a usability standpoint. 3.8 is huge because setns(), and unshare() are now complete from a namespace perspective, and because enough permission checks have been relaxed in user namespaces that you can really start using them.

But semantically from a user namespace perspective nothing really has changed in 3.8.

Quoted 7 lines

I reworked your text somewhat. Could you please review the following:

[[
CLONE_NEWUSER
(This flag first became meaningful for clone() in Linux 2.6.29, but the implementation of user namespaces was only completed in Linux 3.8.)

Long rant about 2.6.29 vs 3.8 above. I think what we need to say is:

(This flag first became meaningful for clone() in Linux 2.6.29, the current semantics were merged present in 3.5, and user namespaces only really became usable in 3.8.)

Quoted 10 lines

If CLONE_NEWUSER is set,
then create the process in a new user namespace. If
this flag is not set, then (as with fork(2)) the process is created in the same user namespace as the calling process.

A user namespace provides an isolated environment for security related identifiers, in particular, user IDs, group IDs, keys (see keyctl(2)), and capabilities.

Quoted 5 lines

When a user namespace is created, it starts out without a mapping of user IDs (group IDs) to the parent user
namespace. The desired mapping of user IDs (group IDs)
to the parent user namespace may be set by writing into /proc/[pid]/uid_map (/proc/[pid]/gid_map); see proc(5).

/proc/[pid]/projid_map deserves a mention. Not that I am a fan of project is or that xfs where the are implemented has been converted yet but....

Quoted 17 lines

The first process in a user namespace starts out with a complete set of capabilities with respect to the new user namespace.

System calls that return user IDs (group IDs) will return either the user ID (group ID) mapped into the current user namespace if there is a mapping, or the overflow user ID (group ID); the default value for the overflow user ID (group ID) is 65534. See the descrip‐ tions of /proc/sys/kernel/overflowuid and /proc/sys/ker‐ nel/overflowgid in proc(5).

Starting with Linux 3.8, no privileges are needed to create a user namespace, and mount, PID, IPC, net, and
UTS namespaces can be created with just the
CAP_SYS_ADMIN capability in the caller's user namespace.

Quoted 8 lines

Over the years, there have been a lot of features that have been added to the Linux kernel that are only avail‐ able 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.

I don't have any problems with this bit of text.

It occurs to me that what is going on with capabilities and user namespaces needs to be documented better. There was a minor bug with them this release cycle and I realized while the current definition makes sense and isn't hard to understand in general. In detail the interaction of capabilities and user namespaces are hard to describe.

I think capabilities and user namespaces are the work of a future patch however.

There is one other bit that needs to be documented in clone, although I am not certain where/how.

The sequences:

unshare(CLONE_NEWPID).
clone(CLONE_VM)

setns(fd, CLONE_NEWPID).
clone(CLONE_VM).

Now fail.

Basically the rule is all threads must be in the same pid namespace.

The joy of reviews with good comments that come much later than hoped.

Hi Eric,

Quoted 9 lines · Eric W. Biederman

There is one other bit that needs to be documented in clone, although I am not certain where/how.

The sequences:

unshare(CLONE_NEWPID).
clone(CLONE_VM)

Quoted 4 lines

setns(fd, CLONE_NEWPID).
clone(CLONE_VM).

Now fail.

Can you define "now" please. Which kernel version?

Quoted 18 lines · Michael Kerrisk (man-pages)

Hi Eric,

Quoted 13 lines · Eric W. Biederman

There is one other bit that needs to be documented in clone, although I am not certain where/how.

The sequences:

unshare(CLONE_NEWPID).
clone(CLONE_VM)

setns(fd, CLONE_NEWPID).
clone(CLONE_VM).

Now fail.

Quoted 1 line

Can you define "now" please. Which kernel version?

3.8

The sequence was impossible in 3.7.

I think that change that made that impossible happened in the 3.8-rc1 to 3.8-rc2 window.

Hi Eric,

Quoted 27 lines · Eric W. Biederman

Quoted 13 lines · Michael Kerrisk (man-pages)

Hi Eric,

Quoted 9 lines · Eric W. Biederman

There is one other bit that needs to be documented in clone, although I am not certain where/how.

The sequences:

unshare(CLONE_NEWPID).
clone(CLONE_VM)

Quoted 6 lines

Quoted 4 lines

setns(fd, CLONE_NEWPID).
clone(CLONE_VM).

Now fail.

Can you define "now" please. Which kernel version?

3.8

The sequence was impossible in 3.7.

I think that change that made that impossible happened in the 3.8-rc1 to

Quoted 1 line

3.8-rc2 window.

Adding something along these lines to the man page would be fine, but we need some text to explain *why* these sequences fail. Could you send me a sentence or two about that?

Hi Eric,

Quoted 27 lines · Eric W. Biederman

Quoted 13 lines · Michael Kerrisk (man-pages)

Hi Eric,

Quoted 9 lines · Eric W. Biederman

There is one other bit that needs to be documented in clone, although I am not certain where/how.

The sequences:

unshare(CLONE_NEWPID).
clone(CLONE_VM)

Quoted 6 lines

Quoted 4 lines

setns(fd, CLONE_NEWPID).
clone(CLONE_VM).

Now fail.

Can you define "now" please. Which kernel version?

3.8

The sequence was impossible in 3.7.

I think that change that made that impossible happened in the 3.8-rc1 to

Quoted 1 line

3.8-rc2 window.

Adding something along these lines to the man page would be fine, but we need some text to explain *why* these sequences fail. Could you send me a sentence or two about that?

Quoted 18 lines · Michael Kerrisk (man-pages)

Hi Eric,

Quoted 13 lines · Eric W. Biederman

There is one other bit that needs to be documented in clone, although I am not certain where/how.

The sequences:

unshare(CLONE_NEWPID).
clone(CLONE_VM)

setns(fd, CLONE_NEWPID).
clone(CLONE_VM).

Now fail.

Quoted 1 line

Can you define "now" please. Which kernel version?

3.8

The sequence was impossible in 3.7.

I think that change that made that impossible happened in the 3.8-rc1 to 3.8-rc2 window.

Hi Eric,

Quoted 9 lines · Eric W. Biederman

There is one other bit that needs to be documented in clone, although I am not certain where/how.

The sequences:

unshare(CLONE_NEWPID).
clone(CLONE_VM)

Quoted 4 lines

setns(fd, CLONE_NEWPID).
clone(CLONE_VM).

Now fail.

Can you define "now" please. Which kernel version?

Signed-off-by "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/[email protected]>

---
 man5/proc.5 |   52 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 52 insertions(+), 0 deletions(-)

man5/proc.5 +52 -1

index 840480d..eb612b9 100644
@@ -581,6 +581,58 @@ even if all processes in the namespace terminate.
 The file descriptor can be passed to
 .BR setns (2).
 .TP
+.IR /proc/[pid]/ns/user " (since Linux 3.8)"
+Bind mounting this file (see
+.BR mount (2))
+to somewhere else in the filesystem keeps
+the user namespace of the process specified by
+.I pid
+alive even if all processes currently in the namespace terminate.
+
+Opening this file returns a file handle for the user namespace
+of the process specified by
+.IR pid .
+As long as this file descriptor remains open,
+the user namespace will remain alive,
+even if all processes in the namespace terminate.
+The file descriptor can be passed to
+.BR setns (2).
+.TP
+.IR /proc/[pid]/ns/pid " (since Linux 3.8)"
+Bind mounting this file (see
+.BR mount (2))
+to somewhere else in the filesystem keeps
+the PID namespace of the process specified by
+.I pid
+alive even if all processes currently in the namespace terminate.
+
+Opening this file returns a file handle for the PID namespace
+of the process specified by
+.IR pid .
+As long as this file descriptor remains open,
+the PID namespace will remain alive,
+even if all processes in the namespace terminate.
+The file descriptor can be passed to
+.BR setns (2).
+.TP
+.IR /proc/[pid]/ns/mnt " (since Linux 3.8)"
+Bind mounting this file (see
+.BR mount (2))
+to somewhere else in the filesystem keeps
+the mount namespace of the process specified by
+.I pid
+alive even if all processes currently in the namespace terminate.
+
+Opening this file returns a file handle for the mount namespace
+of the process specified by
+.IR pid .
+As long as this file descriptor remains open,
+the mount namespace will remain alive,
+even if all processes in the namespace terminate.
+The file descriptor can be passed to
+.BR setns (2).
+
+.TP
 .IR /proc/[pid]/numa_maps " (since Linux 2.6.14)"
 See
 .BR numa (7).
--

1.7.5.4

Signed-off-by "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/[email protected]>

---
 man2/setns.2 |   41 +++++++++++++++++++++++++++++++++--------
 1 files changed, 33 insertions(+), 8 deletions(-)

man2/setns.2 +12 -0

index 6aa01e1..63b04dc 100644
@@ -48,6 +48,18 @@ must refer to a network namespace.
 .BR CLONE_NEWUTS
 .I fd
 must refer to a UTS namespace.
+.TP
+.BR CLONE_NEWPID
+.I fd
+must refer to a PID namespace.
+.TP
+.BR CLONE_NEWUSER
+.I fd
+must refer to a user namespace.
+.TP
+.BR CLONE_NEWNS
+.I fd
+must refer to a mount namespace.
 .PP
 Specifying
 .I nstype

@@ -63,6 +75,25 @@ and wants to ensure that the namespace is of a particular type.
 .IR fd
 if the file descriptor was opened by another process and, for example,

or it does not possess both CAP_SYS_CHROOT privileges +and CAP_SYS_ADMIN rights over the target mount namespace.

.SH RETURN VALUE
On success,
.IR setns ()

man2/setns.2 +2 -1

@@ -94,7 +125,8 @@ for this operation.
 The
 .BR setns ()
 system call first appeared in Linux in kernel 3.0;
-library support was added to glibc in version 2.14.
+library support was added to glibc in version 2.14;
+Support for PID, user and mount namespaces first appeard in Linux in kernel 3.8.
 .SH CONFORMING TO
 The
 .BR setns ()

@@ -106,13 +138,6 @@ a new thread is created using
 can be changed using
 .BR setns ().
 .SH BUGS

Hi Eric,

Some questions below.

Quoted 36 lines · Eric W. Biederman

Signed-off-by "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/[email protected]>

---
 man2/setns.2 |   41 +++++++++++++++++++++++++++++++++--------
 1 files changed, 33 insertions(+), 8 deletions(-)

man2/setns.2 +15 -0

index 6aa01e1..63b04dc 100644
@@ -48,6 +48,18 @@ must refer to a network namespace.
 .BR CLONE_NEWUTS
 .I fd
 must refer to a UTS namespace.
+.TP
+.BR CLONE_NEWPID
+.I fd
+must refer to a PID namespace.
+.TP
+.BR CLONE_NEWUSER
+.I fd
+must refer to a user namespace.
+.TP
+.BR CLONE_NEWNS
+.I fd
+must refer to a mount namespace.
 .PP
 Specifying
 .I nstype

@@ -63,6 +75,25 @@ and wants to ensure that the namespace is of a particular type.
 .IR fd
 if the file descriptor was opened by another process and, for example,
 passed to the caller via a UNIX domain socket.)
+
+The pid namespace is a little different.  Reassociating the calling
+thread with a pid namespace only changes the pid namespace that the

man2/setns.2 and wants to ensure that the namespace is of a particular type. · line 86

Quoted 6 lines

man2/setns.2 +6 -0

+child processes will be created in.
+
+Changing the pid namespace for child processes is only allowed if the
+pid namespace specified by
+.IR fd
+is a child pid namespace of the pid namespace of the current thread.

I assume "current thread" above should be "calling thread", right?

man2/setns.2 and wants to ensure that the namespace is of a particular type.

Quoted 4 lines

man2/setns.2 +4 -0

+
+A multi-threaded process may not change user namespace with setns.  A
+process may not reassociate the thread with the current user
+namespace.

What do you mean by "the current user nsamesapce"?

man2/setns.2 and wants to ensure that the namespace is of a particular type.

Quoted 5 lines

The process reassociating itself with a user namespace

man2/setns.2 +3 -0

+must have CAP_SYS_ADMIN privileges in the target user namespace.
 
+A process may not be reassociated with a new mount namespace if it is
+multi-threaded

I tried to verify the precdeing two lines from the kernel source, but did not work out where this check is made. Where is it?

Quoted 2 lines

or it does not possess both CAP_SYS_CHROOT privileges +and CAP_SYS_ADMIN rights over the target mount namespace.

Could you please expand/clarify the preceding two lines. As they stand, I don't really understand them.

Quoted 17 lines

.SH RETURN VALUE
On success,
.IR setns ()

man2/setns.2 +2 -1

@@ -94,7 +125,8 @@ for this operation.
 The
 .BR setns ()
 system call first appeared in Linux in kernel 3.0;
-library support was added to glibc in version 2.14.
+library support was added to glibc in version 2.14;
+Support for PID, user and mount namespaces first appeard in Linux in kernel 3.8.
 .SH CONFORMING TO
 The
 .BR setns ()

@@ -106,13 +138,6 @@ a new thread is created using
 can be changed using
 .BR setns ().
 .SH BUGS

man2/setns.2 a new thread is created using · line 143

Quoted 10 lines

man2/setns.2 +0 -7

-The PID namespace and the mount namespace are not currently supported.
-(See the descriptions of
-.BR CLONE_NEWPID
-and
-.BR CLONE_NEWNS
-in
-.BR clone (2).)
 .SH SEE ALSO
 .BR clone (2),
 .BR fork (2),

Cheers,

Michael

Quoted 3 lines · Michael Kerrisk (man-pages)

Hi Eric,

Some questions below.

A quick note. Getting the permission checks correct has been a little more interesting that I would have preferred.

I had to add a nsown_capable(CAP_SYS_ADMIN) check to all of the setns() install methods except the user namespace. Not a change in pre 3.8 behavior but a change to my patch, and possibly a documentation change below.

Quoted 38 lines

Quoted 36 lines · Eric W. Biederman

Signed-off-by "Eric W. Biederman" <[email protected]>

---
 man2/setns.2 |   41 +++++++++++++++++++++++++++++++++--------
 1 files changed, 33 insertions(+), 8 deletions(-)

man2/setns.2 +15 -0

index 6aa01e1..63b04dc 100644
@@ -48,6 +48,18 @@ must refer to a network namespace.
 .BR CLONE_NEWUTS
 .I fd
 must refer to a UTS namespace.
+.TP
+.BR CLONE_NEWPID
+.I fd
+must refer to a PID namespace.
+.TP
+.BR CLONE_NEWUSER
+.I fd
+must refer to a user namespace.
+.TP
+.BR CLONE_NEWNS
+.I fd
+must refer to a mount namespace.
 .PP
 Specifying
 .I nstype

@@ -63,6 +75,25 @@ and wants to ensure that the namespace is of a particular type.
 .IR fd
 if the file descriptor was opened by another process and, for example,
 passed to the caller via a UNIX domain socket.)
+
+The pid namespace is a little different.  Reassociating the calling
+thread with a pid namespace only changes the pid namespace that the

Quoted 8 lines

man2/setns.2 and wants to ensure that the namespace is of a particular type. · line 86

Quoted 6 lines

man2/setns.2 +6 -0

+child processes will be created in.
+
+Changing the pid namespace for child processes is only allowed if the
+pid namespace specified by
+.IR fd
+is a child pid namespace of the pid namespace of the current thread.

I assume "current thread" above should be "calling thread", right?

What I mean in "current" from a kernel perspective.

It should be just "caller".

Threads must share a pid namespace so mentioning threads seems wrong.

Quoted 6 lines

man2/setns.2 and wants to ensure that the namespace is of a particular type.

Quoted 4 lines

man2/setns.2 +4 -0

+
+A multi-threaded process may not change user namespace with setns.  A
+process may not reassociate the thread with the current user
+namespace.

What do you mean by "the current user nsamesapce"?

fd = open("/proc/self/ns/user");
setns(fd) -> -EINVAL.

So from a userspace perspective I mean "the callers user namespace".

Quoted 3 lines

Quoted 3 lines

The process reassociating itself with a user namespace +must have CAP_SYS_ADMIN privileges in the target user namespace.

Quoted 5 lines

man2/setns.2

Quoted 2 lines

man2/setns.2 +2 -0

+A process may not be reassociated with a new mount namespace if it is
+multi-threaded

I tried to verify the precdeing two lines from the kernel source, but did not work out where this check is made. Where is it?

kernel/user_namespace.c:userns_install()
fs/namespace.c:mntns_install()

A couple of the security checks have been pushed down into a per namespace context, because the exact check that makes sense depends on the namespace.

Quoted 5 lines

Quoted 2 lines

or it does not possess both CAP_SYS_CHROOT privileges +and CAP_SYS_ADMIN rights over the target mount namespace.

Could you please expand/clarify the preceding two lines. As they stand, I don't really understand them.

Ugh. The text is slightly wrong.

The code is:

        if (!ns_capable(mnt_ns->user_ns, CAP_SYS_ADMIN) ||
            !nsown_capable(CAP_SYS_CHROOT) ||
            !nsown_capable(CAP_SYS_ADMIN))
                return -EPERM;

Basically you aren't allowed change your mount namespace into a mount namespace that doesn't see you as the all powerful root able to mount and unmount filesystems.

You aren't allowed to change your mount namespace unless you possesses CAP_SYS_CHROOT and CAP_SYS_ADMIN.

Hi Eric,

Quoted 61 lines · Eric W. Biederman

Quoted 3 lines · Michael Kerrisk (man-pages)

Hi Eric,

Some questions below.

A quick note. Getting the permission checks correct has been a little more interesting that I would have preferred.

I had to add a nsown_capable(CAP_SYS_ADMIN) check to all of the setns() install methods except the user namespace. Not a change in pre 3.8 behavior but a change to my patch, and possibly a documentation change below.

Quoted 38 lines

Quoted 36 lines · Eric W. Biederman

Signed-off-by "Eric W. Biederman" <[email protected]>

---
 man2/setns.2 |   41 +++++++++++++++++++++++++++++++++--------
 1 files changed, 33 insertions(+), 8 deletions(-)

man2/setns.2 +15 -0

index 6aa01e1..63b04dc 100644
@@ -48,6 +48,18 @@ must refer to a network namespace.
 .BR CLONE_NEWUTS
 .I fd
 must refer to a UTS namespace.
+.TP
+.BR CLONE_NEWPID
+.I fd
+must refer to a PID namespace.
+.TP
+.BR CLONE_NEWUSER
+.I fd
+must refer to a user namespace.
+.TP
+.BR CLONE_NEWNS
+.I fd
+must refer to a mount namespace.
 .PP
 Specifying
 .I nstype

@@ -63,6 +75,25 @@ and wants to ensure that the namespace is of a particular type.
 .IR fd
 if the file descriptor was opened by another process and, for example,
 passed to the caller via a UNIX domain socket.)
+
+The pid namespace is a little different.  Reassociating the calling
+thread with a pid namespace only changes the pid namespace that the

Quoted 8 lines

man2/setns.2 and wants to ensure that the namespace is of a particular type. · line 86

Quoted 6 lines

man2/setns.2 +6 -0

+child processes will be created in.
+
+Changing the pid namespace for child processes is only allowed if the
+pid namespace specified by
+.IR fd
+is a child pid namespace of the pid namespace of the current thread.

I assume "current thread" above should be "calling thread", right?

Quoted 3 lines

What I mean in "current" from a kernel perspective.

It should be just "caller".

Okay. Changed.

Quoted 17 lines

Threads must share a pid namespace so mentioning threads seems wrong.

Quoted 6 lines

man2/setns.2 and wants to ensure that the namespace is of a particular type. · line 90

Quoted 4 lines

man2/setns.2 +4 -0

+
+A multi-threaded process may not change user namespace with setns.  A
+process may not reassociate the thread with the current user
+namespace.

What do you mean by "the current user nsamesapce"?

fd = open("/proc/self/ns/user");
setns(fd) -> -EINVAL.

So from a userspace perspective I mean "the callers user namespace".

Quoted 3 lines

Quoted 3 lines

The process reassociating itself with a user namespace +must have CAP_SYS_ADMIN privileges in the target user namespace.

Quoted 8 lines

Quoted 5 lines

man2/setns.2

Quoted 2 lines

man2/setns.2 +2 -0

+A process may not be reassociated with a new mount namespace if it is
+multi-threaded

I tried to verify the precdeing two lines from the kernel source, but did not work out where this check is made. Where is it?

kernel/user_namespace.c:userns_install()
fs/namespace.c:mntns_install()

Thanks.

Quoted 18 lines

A couple of the security checks have been pushed down into a per namespace context, because the exact check that makes sense depends on the namespace.

Quoted 5 lines

Quoted 2 lines

or it does not possess both CAP_SYS_CHROOT privileges +and CAP_SYS_ADMIN rights over the target mount namespace.

Could you please expand/clarify the preceding two lines. As they stand, I don't really understand them.

Ugh. The text is slightly wrong.

The code is:

        if (!ns_capable(mnt_ns->user_ns, CAP_SYS_ADMIN) ||
            !nsown_capable(CAP_SYS_CHROOT) ||
            !nsown_capable(CAP_SYS_ADMIN))
                return -EPERM;

Quoted 6 lines

Basically you aren't allowed change your mount namespace into a mount namespace that doesn't see you as the all powerful root able to mount and unmount filesystems.

You aren't allowed to change your mount namespace unless you possesses CAP_SYS_CHROOT and CAP_SYS_ADMIN.

Okay -- reworded.

So, I've done some more reworking of the text, which now reads as folows. Could you please check this (and see my questions below).

CLONE_NEWPID behaves somewhat differently from the other nstype values: reassociating the calling thread with a PID namespace only changes the PID namespace that child processes of the caller will be created in; it does not change the PID namespace of the caller itself.

I reworked the preceding piece a lot. Is it correct still?

Reassoci‐
ating with a PID namespace is only allowed if the PID namespace specified by fd is a descendant (child, grand‐ child, etc.)

Is the preceding sentence correct? (You talked only of children in your original patch, but I believe it's more general than that.)

PID namespace of the PID namespace of the caller.

A multi-threaded process may not change user namespace with setns(). A process may not reassociate the thread with the caller's user namespace.

What does the last sentence above *mean*? I don't understand it.

A process reassociat‐
ing itself with a user namespace must have CAP_SYS_ADMIN privileges in the target user namespace.

A process may not be reassociated with a new mount names‐ pace if it is multi-threaded. Changing the mount names‐ pace requires that the caller possess both CAP_SYS_CHROOT and CAP_SYS_ADMIN capabilities.

Re the last sentence: are those capabilities required in (1) the target namespace, or (2) the source namespace, or (3) both? I suspect (1), but please confirm.

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

Quoted 8 lines

PID namespace of the PID namespace of the caller.

A multi-threaded process may not change user namespace with setns(). A process may not reassociate the thread with the caller's user namespace.

What does the last sentence above *mean*? I don't understand it.

So the set of checks are:

        /* Don't allow gaining capabilities by reentering
         * the same user namespace.
         */
        if (user_ns == current_user_ns())
                return -EINVAL;

        /* Threaded processes may not enter a different user namespace */
        if (atomic_read(&current->mm->mm_users) > 1)
                return -EINVAL;

        if (!ns_capable(user_ns, CAP_SYS_ADMIN))
                return -EPERM;

Rereading it looks like I was going fast and suffered from dropping important words.

A multi-threaded process may not change it's user namespace with setns().

aka if you have threads setns for a user namespace will fail.

A process may not change the user namespace to the caller's user namespace via setns. This is important because changing to a user namespace via setns implies gaining all caps, and you should not be able to gain all caps over your current user namespace.

Hopefully that clears it up.

Quoted 9 lines

A process reassociat‐
ing itself with a user namespace must have CAP_SYS_ADMIN privileges in the target user namespace.

A process may not be reassociated with a new mount names‐ pace if it is multi-threaded. Changing the mount names‐ pace requires that the caller possess both CAP_SYS_CHROOT and CAP_SYS_ADMIN capabilities.

Quoted 3 lines

Re the last sentence: are those capabilities required in (1) the target namespace, or (2) the source namespace, or (3) both? I suspect (1), but please confirm.

CAP_SYS_ADMIN is required in the current user namespace. CAP_SYS_ADMIN is required over the target mount namesapce.

CAP_SYS_CHROOT is required in the current user namespace.

Okay. See below.

So, let's take one more pass. How does the following look:

A multi-threaded process may not change user namespace with
setns(). It is not permitted to use setns() to reenter the
caller's current user namespace. This prevents a caller that has dropped capabilities from regaining those capabilities via a call to setns() A process reassociating itself with a user namespace must have CAP_SYS_ADMIN privileges in the target user namespace.

A process may not be reassociated with a new mount namespace if
it is multi-threaded. Changing the mount namespace requires
that the caller possess both CAP_SYS_CHROOT and CAP_SYS_ADMIN capabilities in its own user namespace and CAP_SYS_ADMIN in the target mount namespace.

Okay. See below.

So, let's take one more pass. How does the following look:

A multi-threaded process may not change user namespace with
setns(). It is not permitted to use setns() to reenter the
caller's current user namespace. This prevents a caller that has dropped capabilities from regaining those capabilities via a call to setns() A process reassociating itself with a user namespace must have CAP_SYS_ADMIN privileges in the target user namespace.

A process may not be reassociated with a new mount namespace if
it is multi-threaded. Changing the mount namespace requires
that the caller possess both CAP_SYS_CHROOT and CAP_SYS_ADMIN capabilities in its own user namespace and CAP_SYS_ADMIN in the target mount namespace.

Signed-off-by "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/[email protected]>

---
 man2/setns.2 |   41 +++++++++++++++++++++++++++++++++--------
 1 files changed, 33 insertions(+), 8 deletions(-)

man2/setns.2 +12 -0

index 6aa01e1..63b04dc 100644
@@ -48,6 +48,18 @@ must refer to a network namespace.
 .BR CLONE_NEWUTS
 .I fd
 must refer to a UTS namespace.
+.TP
+.BR CLONE_NEWPID
+.I fd
+must refer to a PID namespace.
+.TP
+.BR CLONE_NEWUSER
+.I fd
+must refer to a user namespace.
+.TP
+.BR CLONE_NEWNS
+.I fd
+must refer to a mount namespace.
 .PP
 Specifying
 .I nstype

@@ -63,6 +75,25 @@ and wants to ensure that the namespace is of a particular type.
 .IR fd
 if the file descriptor was opened by another process and, for example,

or it does not possess both CAP_SYS_CHROOT privileges +and CAP_SYS_ADMIN rights over the target mount namespace.

.SH RETURN VALUE
On success,
.IR setns ()

man2/setns.2 +2 -1

@@ -94,7 +125,8 @@ for this operation.
 The
 .BR setns ()
 system call first appeared in Linux in kernel 3.0;
-library support was added to glibc in version 2.14.
+library support was added to glibc in version 2.14;
+Support for PID, user and mount namespaces first appeard in Linux in kernel 3.8.
 .SH CONFORMING TO
 The
 .BR setns ()

@@ -106,13 +138,6 @@ a new thread is created using
 can be changed using
 .BR setns ().
 .SH BUGS