Re: [RFC PATCH] f*xattr: allow O_PATH descriptors

On Wed, Jun 8, 2022 at 2:57 PM Christian Brauner [off-list ref] wrote:

On Tue, Jun 07, 2022 at 05:31:39PM +0200, Christian Göttsche wrote:

quoted

From: Miklos Szeredi <redacted>

Support file descriptors obtained via O_PATH for extended attribute
operations.

Extended attributes are for example used by SELinux for the security
context of file objects. To avoid time-of-check-time-of-use issues while
setting those contexts it is advisable to pin the file in question and
operate on a file descriptor instead of the path name. This can be
emulated in userspace via /proc/self/fd/NN [1] but requires a procfs,
which might not be mounted e.g. inside of chroots, see[2].

[1]: https://github.com/SELinuxProject/selinux/commit/7e979b56fd2cee28f647376a7233d2ac2d12ca50
[2]: https://github.com/SELinuxProject/selinux/commit/de285252a1801397306032e070793889c9466845

Original patch by Miklos Szeredi [off-list ref]
https://patchwork.kernel.org/project/linux-fsdevel/patch/20200505095915.11275-6-mszeredi@redhat.com/

quoted

While this carries a minute risk of someone relying on the property of
xattr syscalls rejecting O_PATH descriptors, it saves the trouble of
introducing another set of syscalls.

Only file->f_path and file->f_inode are accessed in these functions.

Current versions return EBADF, hence easy to detect the presense of
this feature and fall back in case it's missing.

CC: linux-api@vger.kernel.org
CC: linux-man@vger.kernel.org
Signed-off-by: Christian Göttsche <redacted>
---

I'd be somewhat fine with getxattr and listxattr but I'm worried that
setxattr/removexattr waters down O_PATH semantics even more. I don't
want O_PATH fds to be useable for operations which are semantically
equivalent to a write.

It is not really semantically equivalent to a write if it works on a
O_RDONLY fd already.

In sensitive environments such as service management/container runtimes
we often send O_PATH fds around precisely because it is restricted what
they can be used for. I'd prefer to not to plug at this string.

But unless I am mistaken, path_setxattr() and syscall_fsetxattr()
are almost identical w.r.t permission checks and everything else.

So this change introduces nothing new that a user in said environment
cannot already accomplish with setxattr().

Besides, as the commit message said, doing setxattr() on an O_PATH
fd is already possible with setxattr("/proc/self/$fd"), so whatever security
hole you are trying to prevent is already wide open.

In effect, I think containing setxattr() can only be accomplished with LSM.

Thanks,
Amir.