Getting rid of get_unused_fd() / enable close-on-exec
Subsystems (8)
v6 cover Getting rid of get_unused_fd() / enable close-on-exec
Hi,
Please find the sixth revision of my patchset to remove get_unused_fd() macro in order to help subsystems to use get_unused_fd_flags() (or anon_inode_getfd()) with flags either provided by the userspace or set to O_CLOEXEC by default where appropriate.
Without get_unused_fd() macro, more subsystems are likely to use get_unused_fd_flags() (or anon_inode_getfd()) and be taught to provide an API that let userspace choose the opening flags of the file descriptor.
Not allowing userspace to provide the "open" flags or not using O_CLOEXEC by default should be considered bad practice from security point of view:
in most case O_CLOEXEC must be used to not leak file descriptor across exec().
Not allowing userspace to atomically set close-on-exec flag and not using O_CLOEXEC should be avoided to protect multi-threaded program from race condition when it tried to set close-on-exec flag using fcntl(fd, F_SETFD, FD_CLOEXEC) after opening the file descriptor.
Example:
int fd;
int ret;
fd = open(filename, O_RDONLY);
if (fd < 0) {
perror("open");
return -1;
}
/*
* window opened for another thread to call fork(),
* then the new process can call exec() at any time
* and the file descriptor would be inherited
*/
ret = fcntl(fd, F_SETFD, FD_CLOEXEC)
if (ret < 0) {
perror("fnctl()");
close(fd);
return -1;
}vs.:
int fd;
fd = open(filaneme, O_RDONLY | O_CLOEXEC);
if (fd < 0) {
perror("open");
return -1;
}Using O_CLOEXEC by default when flags are not (eg. cannot be) provided by userspace is the safest bet as it allows userspace to choose, if, when and where the file descriptor is going to be inherited across exec():
userspace is free to call fcntl() to remove FD_CLOEXEC flag in the child process that will call exec().
Unfortunately, O_CLOEXEC cannot be made the default for most existing system calls as it's not the default behavior for POSIX / Unix. Reader interested in this issue could have a look at "Ghosts of Unix past, part 2: Conflated designs" [1] article by Neil Brown.
FAQ:
- Why do one want close-on-exec ?
Setting close-on-exec flag on file descriptor ensure it won't be inherited silently by child, child of child, etc. when executing another program.
If the file descriptor is not closed, some kernel resources can be locked until the last process with the opened file descriptor exit.
If the file descriptor is not closed, this can lead to a security issue, eg. making resources available to a less privileged program allowing information leak and/or deny of service.
- Why do one need atomic close-on-exec ?
Even if it's possible to set close-on-exec flag through call to fcntl() as shown previously, it introduces a race condition in multi-threaded process, where a thread call fork() / exec() while another thread is between call to open() and fcntl().
Additionally, using close-on-exec free the programmer from tracking manually which file descriptor is to close in a child before calling exec():
in a program using multiple third-party libraries, it's difficult to know which file descriptor must be closed.
AFAIK, while there's a atexit(), pthread_atfork(), there's no atexec() userspace function in libc to allow libraries to register a handler in order to close its opened file descriptor before exec().
- Why default to close-on-exec ?
Some kernel interfaces don't allow userspace to pass a O_CLOEXEC-like flag when creating a new file descriptor.
In such cases, if possible (see below), O_CLOEXEC must be made the default so that userspace doesn't have to call fcntl() which, as demonstrated previously, is open to race condition in multi-threaded program.
- How to choose between flag 0 or O_CLOEXEC in call to get_unused_fd_flags() (or anon_inode_getfd()) ?
Short: Will it break existing application ? Will it break kernel ABI ?
If answer is no, use O_CLOEXEC.
If answer is yes, use 0.
Long: If userspace expect to retrieve a file descriptor with plain
old Unix(tm) semantics, O_CLOEXEC must not be the default, as it could break some applications expecting that the file descriptor will be inherited across exec().
But for some subsystems, such as InfiniBand, KVM, VFIO, it makes no sense to have file descriptors inherited across exec() since those are tied to resources that will vanish when a another program will replace the current one by mean of exec(), so it's safe to use O_CLOEXEC in such cases.
For others, like XFS, the file descriptor is retrieved by one program and will be used by a different program, executed as a child. In this case, setting O_CLOEXEC would break existing application which do not expect to have to call fcntl(fd, F_SETFD, 0) to make it available across exec().
If file descriptor created by a subsystem is not tied to the current process resources, it's likely legal to use it in a different process context, thus O_CLOEXEC must not be the default.
If one, as a subsystem maintainer, cannot tell for sure that no userspace program ever rely current behavior, eg. file descriptor being inherited across exec(), then the default flag *must* be kept 0 to not break application.
- This subsystem cannot be turned to use O_CLOEXEC by default:
If O_CLOEXEC cannot be made the default, it would be interesting to think to extend the API to have a (set of) function(s) taking a flag parameter so that userspace can atomically request close-on-exec if it need it (and it should need it).
- Background:
One might want to read "Secure File Descriptor Handling" [2] by Ulrich Drepper who is responsible of adding O_CLOEXEC flag on open(), and flags alike on other syscalls.
One might also want to read PEP-446 "Make newly created file descriptors non-inheritable" [3] by Victor Stinner since it has lot more background information on file descriptor leaking.
One also like to read "Excuse me son, but your code is leaking !!!" [4] by Dan Walsh for advice.
[1] http://lwn.net/Articles/412131/
[2] http://udrepper.livejournal.com/20407.html
[3] http://www.python.org/dev/peps/pep-0446/
[4] http://danwalsh.livejournal.com/53603.html
- Statistics:
In linux-next tag 20140307, they're currently:
- 32 calls to fd_install()
with one call part of anon_inode_getfd()
- 25 calls to get_unused_fd_flags()
with one call part of anon_inode_getfd()
with another part of get_unused_fd() macro
- 11 calls to anon_inode_getfd()
- 9 calls to anon_inode_getfile()
with one call part of anon_inode_getfd()
- 6 calls to get_unused_fd()
Changes from patchset v5 [PATCHSETv5]
- perf: introduce a flag to enable close-on-exec in perf_event_open()
DROPPED: applied upstream, commit a21b0b354d4a.
Changes from patchset v4 [PATCHSETv4]:
- rewrote cover letter following discussion with perf maintainer. Thanks to Peter Zijlstra.
- modified a bit some commit messages.
- events: use get_unused_fd_flags(0) instead of get_unused_fd()
DROPPED: replaced by following patch.
- perf: introduce a flag to enable close-on-exec in perf_event_open()
NEW: instead of hard coding the flags to 0, this patch
allows userspace to specify close-on-exec flag.
- fanotify: use get_unused_fd_flags(0) instead of get_unused_fd()
DROPPED: replaced by following patch.
- fanotify: enable close-on-exec on events' fd when requested in fanotify_init()
NEW: instead of hard coding the flags to 0, this patch
enable close-on-exec if userspace request it.
Changes from patchset v3 [PATCHSETv3]:
- industrialio: use anon_inode_getfd() with O_CLOEXEC flag
DROPPED: applied upstream, commit a646fbf0fd11.
Changes from patchset v2 [PATCHSETv2]:
- android/sw_sync: use get_unused_fd_flags(O_CLOEXEC) instead of get_unused_fd()
DROPPED: applied upstream, commit 45acea57335e.
- android/sync: use get_unused_fd_flags(O_CLOEXEC) instead of get_unused_fd()
DROPPED: applied upstream, commit 9c6cd3b39048.
- vfio: use get_unused_fd_flags(0) instead of get_unused_fd()
DROPPED: applied upstream, commit a5d550703d2c. Additionally subsystem maintainer applied another patch on top to set the flags to O_CLOEXEC, commit 5d042fbdbb2d.
- industrialio: use anon_inode_getfd() with O_CLOEXEC flag
NEW: propose to use O_CLOEXEC as default flag.
Changes from patchset v1 [PATCHSETv1]:
- explicitly added subsystem maintainers as mail recepients.
- infiniband: use get_unused_fd_flags(0) instead of get_unused_fd()
DROPPED: subsystem maintainer applied another patch
using get_unused_fd_flags(O_CLOEXEC) as suggested, commit da183c7af844.
- android/sw_sync: use get_unused_fd_flags(0) instead of get_unused_fd()
MODIFIED: use get_unused_fd_flags(O_CLOEXEC) as suggested.
- android/sync: use get_unused_fd_flags(0) instead of get_unused_fd()
MODIFIED: use get_unused_fd_flags(O_CLOEXEC) as suggested.
- xfs: use get_unused_fd_flags(0) instead of get_unused_fd()
DROPPED: applied asis by subsystem maintainer, commit 862a62937e76.
- sctp: use get_unused_fd_flags(0) instead of get_unused_fd()
DROPPED: applied asis by subsystem maintainer, commit 8a59bd3e9b29.
Links:
[PATCHSETv5]
http://lkml.kernel.org/r/[email protected]
[PATCHSETv4]
http://lkml.kernel.org/r/[email protected]
[PATCHSETv3]
http://lkml.kernel.org/r/[email protected]
[PATCHSETv2]
http://lkml.kernel.org/r/[email protected]
[PATCHSETv1]
http://lkml.kernel.org/r/[email protected]
Yann Droneaud (6):
ia64: use get_unused_fd_flags(0) instead of get_unused_fd() ppc/cell: use get_unused_fd_flags(0) instead of get_unused_fd()
binfmt_misc: use get_unused_fd_flags(0) instead of get_unused_fd()
file: use get_unused_fd_flags(0) instead of get_unused_fd()
fanotify: enable close-on-exec on events' fd when requested in
fanotify_init()
file: remove macro get_unused_fd()
arch/ia64/kernel/perfmon.c | 2 +- arch/powerpc/platforms/cell/spufs/inode.c | 4 ++-- fs/binfmt_misc.c | 2 +- fs/file.c | 2 +- fs/notify/fanotify/fanotify_user.c | 2 +- include/linux/file.h | 1 - 6 files changed, 6 insertions(+), 7 deletions(-)
Signature · 1 line
1.8.5.3
PATCH v6 1/6 ia64: use get_unused_fd_flags(0) instead of get_unused_fd()
Macro get_unused_fd() is used to allocate a file descriptor with default flags. Those default flags (0) can be "unsafe":O_CLOEXEC must be used by default to not leak file descriptor across exec().
Instead of macro get_unused_fd(), function get_unused_fd_flags() (or anon_inode_getfd()) should be used with flags given by userspace. If not possible, flags should be set to O_CLOEXEC to provide userspace with a default safe behavor.
In a further patch, get_unused_fd() will be removed so that new code start using get_unused_fd_flags() (or anon_inode_getfd()) with correct flags.
This patch replaces calls to get_unused_fd() with equivalent call to get_unused_fd_flags(0) to preserve current behavor for existing code.
The hard coded flag value (0) should be reviewed on a per-subsystem basis, and, if possible, set to O_CLOEXEC.
--- arch/ia64/kernel/perfmon.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
arch/ia64/kernel/perfmon.c +1 -1
@@ -2662,7 +2662,7 @@ pfm_context_create(pfm_context_t *ctx, void *arg, int count, struct pt_regs *reg ret = -ENOMEM ; - fd =get_unused_fd (); + fd =get_unused_fd_flags (0 );if (fd <0 )return fd;
Signature · 1 line
1.8.5.3
PATCH v6 2/6 ppc/cell: use get_unused_fd_flags(0) instead of get_unused_fd()
In reply to Yann DroneaudMacro get_unused_fd() is used to allocate a file descriptor with default flags. Those default flags (0) can be "unsafe":O_CLOEXEC must be used by default to not leak file descriptor across exec().
Instead of macro get_unused_fd(), function get_unused_fd_flags() (or anon_inode_getfd()) should be used with flags given by userspace. If not possible, flags should be set to O_CLOEXEC to provide userspace with a default safe behavor.
In a further patch, get_unused_fd() will be removed so that new code start using get_unused_fd_flags() (or anon_inode_getfd()) with correct flags.
This patch replaces calls to get_unused_fd() with equivalent call to get_unused_fd_flags(0) to preserve current behavor for existing code.
The hard coded flag value (0) should be reviewed on a per-subsystem basis, and, if possible, set to O_CLOEXEC.
--- arch/powerpc/platforms/cell/spufs/inode.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
arch/powerpc/platforms/cell/spufs/inode.c +2 -2
@@ -301,7 +301,7 @@ static int spufs_context_open(struct path *path)int ret;struct file *filp; - ret =get_unused_fd (); + ret =get_unused_fd_flags (0 );if (ret <0 )return ret;
@@ -518,7 +518,7 @@ static int spufs_gang_open(struct path *path)int ret;struct file *filp; - ret =get_unused_fd (); + ret =get_unused_fd_flags (0 );if (ret <0 )return ret;
Signature · 1 line
1.8.5.3
PATCH v6 3/6 binfmt_misc: use get_unused_fd_flags(0) instead of get_unused_fd()
In reply to Yann DroneaudMacro get_unused_fd() is used to allocate a file descriptor with default flags. Those default flags (0) can be "unsafe":O_CLOEXEC must be used by default to not leak file descriptor across exec().
Instead of macro get_unused_fd(), function get_unused_fd_flags() (or anon_inode_getfd()) should be used with flags given by userspace. If not possible, flags should be set to O_CLOEXEC to provide userspace with a default safe behavor.
In a further patch, get_unused_fd() will be removed so that new code start using get_unused_fd_flags() (or anon_inode_getfd()) with correct flags.
This patch replaces calls to get_unused_fd() with equivalent call to get_unused_fd_flags(0) to preserve current behavor for existing code.
The hard coded flag value (0) should be reviewed on a per-subsystem basis, and, if possible, set to O_CLOEXEC.
--- fs/binfmt_misc.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
fs/binfmt_misc.c +1 -1
@@ -138,7 +138,7 @@ static int load_misc_binary(struct linux_binprm *bprm)/* if the binary should be opened on behalf of the * interpreter than keep it open and assign descriptor * to it */ - fd_binary =get_unused_fd (); + fd_binary =get_unused_fd_flags (0 );if (fd_binary <0 ) { retval = fd_binary; goto _ret;
Signature · 1 line
1.8.5.3
PATCH v6 4/6 file: use get_unused_fd_flags(0) instead of get_unused_fd()
In reply to Yann DroneaudMacro get_unused_fd() is used to allocate a file descriptor with default flags. Those default flags (0) can be "unsafe":O_CLOEXEC must be made the default to not leak file descriptor across exec().
In a further patch, get_unused_fd() will be removed so that new code start using get_unused_fd_flags() (or anon_inode_getfd()) with correct flags.
This patch replaces calls to get_unused_fd() with equivalent call to get_unused_fd_flags(0).
--- fs/file.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
fs/file.c +1 -1
@@ -857,7 +857,7 @@ SYSCALL_DEFINE1(dup, unsigned int, fildes)struct file *file =fget_raw (fildes);if (file) { - ret =get_unused_fd (); + ret =get_unused_fd_flags (0 );if (ret >=0 )fd_install (ret, file);else
Signature · 1 line
1.8.5.3
PATCH v6 5/6 fanotify: enable close-on-exec on events' fd when requested in fanotify_init()
In reply to Yann DroneaudAccording to commit 80af258867648, file descriptor created as part of file access notification events inherit flags from the event_f_flags argument passed to syscall fanotify_init(2).
So while it is legal for userspace to call fanotify_init() with O_CLOEXEC as part of its second argument, O_CLOEXEC is currently silently ignored.
Indeed event_f_flags are only given to dentry_open(), which only seems to care about O_ACCMODE and O_PATH in do_dentry_open(), O_DIRECT in open_check_o_direct() and O_LARGEFILE in generic_file_open().
More, there's no effective check on event_f_flags value that would catch unknown / unsupported values, unlike the one on f_flags argument of the syscall (see FAN_ALL_INIT_FLAGS in include/uapi/linux/fanotify.h).
Reading article "Botching up ioctls"[1] by Daniel Vetter might make one feel uncomfortable when trying to add extension to an API that doesn't check for unrecognized values.
But it seems logical to set close-on-exec flag on the file descriptor if userspace is allowed to request it with O_CLOEXEC.
In fact, according to some lookup on http://codesearch.debian.net/ and various search engine, there's already some userspace code requesting it:
- in systemd's readahead[2]:
fanotify_fd = fanotify_init(FAN_CLOEXEC|FAN_NONBLOCK, O_RDONLY|O_LARGEFILE|O_CLOEXEC|O_NOATIME);
- in clsync[3]:
#define FANOTIFY_EVFLAGS (O_LARGEFILE|O_RDONLY|O_CLOEXEC)
int fanotify_d = fanotify_init(FANOTIFY_FLAGS, FANOTIFY_EVFLAGS);- in examples [4] from "Filesystem monitoring in the Linux kernel" article[5] by Aleksander Morgado:
if ((fanotify_fd = fanotify_init (FAN_CLOEXEC,
O_RDONLY | O_CLOEXEC | O_LARGEFILE)) < 0)
Lookup also returned some wrong usage of the syscall:
- in Gonk HAL from Mozilla Firefox OS sources[6]:
mFd = fanotify_init(FAN_CLASS_NOTIF, FAN_CLOEXEC);
Adding support for O_CLOEXEC in fanotify won't magically enable it for Gonk since FAN_CLOEXEC is defined as 0x1, which is likely equal to O_WRONLY when used in open flag context. In the other hand, it won't hurt it either.
So this patch replaces call to macro get_unused_fd() by a call to function get_unused_fd_flags() with event_f_flags value as argument. This way O_CLOEXEC flag in the second argument of fanotify_init(2) syscall is interpreted so that close-on-exec get enabled.
[1] http://blog.ffwll.ch/2013/11/botching-up-ioctls.html
[2] http://cgit.freedesktop.org/systemd/systemd/tree/src/readahead/readahead-collect.c?id=v208#n294
[3] https://github.com/xaionaro/clsync/blob/v0.2.1/sync.c#L1631
https://github.com/xaionaro/clsync/blob/v0.2.1/configuration.h#L38
[4] http://www.lanedo.com/~aleksander/fanotify/fanotify-example.c
[5] http://www.lanedo.com/2013/filesystem-monitoring-linux-kernel/
[6] http://hg.mozilla.org/mozilla-central/file/325c74addeba/hal/gonk/GonkDiskSpaceWatcher.cpp#l167
--- fs/notify/fanotify/fanotify_user.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
fs/notify/fanotify/fanotify_user.c +1 -1
@@ -65,7 +65,7 @@ static int create_fd(struct fsnotify_group *group,pr_debug ("%s: group=%p event=%p\n" , __func__, group, event); - client_fd =get_unused_fd (); + client_fd =get_unused_fd_flags (group->fanotify_data.f_flags);if (client_fd <0 )return client_fd;
Signature · 1 line
1.8.5.3
PATCH v6 6/6 file: remove macro get_unused_fd()
In reply to Yann DroneaudMacro get_unused_fd() allocates a file descriptor without enabling
close-on-exec: it calls function get_unused_fd_flags() without O_CLOEXEC flag.
This can be seen as an unsafe default: in most case close-on-exec should be enabled to not leak file descriptor across exec().
This patch removes get_unused_fd() instead of updating it to use O_CLOEXEC so that out of tree modules won't be affect by a runtime behavor change which might introduce other kind of bug. It's better to catch the change at build time, making it easier to fix.
Removing the macro will also promote use of get_unused_fd_flags() (or anon_inode_getfd()) with flags provided by userspace. Or, if flags cannot be given by userspace, with flags set to O_CLOEXEC set by default.
--- include/linux/file.h | 1 - 1 file changed, 1 deletion(-)
include/linux/file.h +0 -1
@@ -63,7 +63,6 @@ extern void set_close_on_exec(unsigned int fd, int flag);extern bool get_close_on_exec (unsigned int fd);extern void put_filp (struct file *);extern int get_unused_fd_flags (unsigned flags); -#define get_unused_fd () get_unused_fd_flags(0)extern void put_unused_fd (unsigned int fd);extern void fd_install (unsigned int fd,struct file *file);
Signature · 1 line
1.8.5.3