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

[Amir Goldstein]

On Wed, Jun 8, 2022 at 2:57 PM Christian Brauner <brauner@xxxxxxxxxx> wrote:
>
> On Tue, Jun 07, 2022 at 05:31:39PM +0200, Christian Göttsche wrote:
> > From: Miklos Szeredi <mszeredi@xxxxxxxxxx>
> >
> > 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 <mszeredi@xxxxxxxxxx>
> > https://patchwork.kernel.org/project/linux-fsdevel/patch/20200505095915.11275-6-mszeredi@xxxxxxxxxx/
> >
> > > 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@xxxxxxxxxxxxxxx
> > CC: linux-man@xxxxxxxxxxxxxxx
> > Signed-off-by: Christian Göttsche <cgzones@xxxxxxxxxxxxxx>
> > ---
>
> 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.