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

[Christian Brauner <brauner@kernel.org>]

On Wed, Jun 08, 2022 at 03:28:52PM +0300, Amir Goldstein wrote:
> On Wed, Jun 8, 2022 at 2:57 PM Christian Brauner <brauner@kernel.org> wrote:
> >
> > On Tue, Jun 07, 2022 at 05:31:39PM +0200, Christian Göttsche wrote:
> > > From: Miklos Szeredi <mszeredi@redhat.com>
> > >
> > > 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@redhat.com>
> > > https://patchwork.kernel.org/project/linux-fsdevel/patch/20200505095915.11275-6-mszeredi@redhat.com/
> > >
> > > > 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 <cgzones@googlemail.com>
> > > ---
> >
> > 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.

The fact that it works on a O_RDONLY fd has always been weird. And is
probably a bug. If you look at xattr_permission() you can see that it
checks for MAY_WRITE for set operations... setxattr() writes to disk for
real filesystems. I don't know how much closer to a write this can get.

In general, one semantic aberration doesn't justify piling another one
on top.

(And one thing that speaks for O_RDONLY is at least that it actually
opens the file wheres O_PATH doesn't.)

> 
> >
> > 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.

That is very much a something that we're trying to restrict for this
exact reason and is one of the main motivator for upgrade mask in
openat2(). If I want to send a O_PATH around I want it to not be
upgradable. Aleksa is working on upgrade masks with openat2() (see [1]
and part of the original patchset in [2]. O_PATH semantics don't need to
become weird.

[1]: https://lore.kernel.org/all/20220526130355.fo6gzbst455fxywy@senku
[2]: https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20190728010207.9781-8-cyphar@cyphar.com