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

[Aleksa Sarai]

On 2022-06-08, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
> On Wed, Jun 8, 2022 at 3:48 PM Christian Brauner <brauner@xxxxxxxxxx> wrote:
> >
> > 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@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.
> >
> > 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
> 
> Bug or no bug, this is the UAPI. It is not fixable anymore.
> 
> > 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.)
> 
> Ok. I care mostly about consistent UAPI, so if you want to set the
> rule that modify f*() operations are not allowed to use O_PATH fd,
> I can live with that, although fcntl(2) may be breaking that rule, but
> fine by me.
> It's good to have consistent rules and it's good to add a new UAPI for
> new behavior.
> 
> However...
> 
> >
> > >
> > > >
> > > > 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@xxxxxxxxxx
> 
> ... thinking forward, if this patch is going to be rejected, the patch that
> will follow is *xattrat() syscalls.
> 
> What will you be able to argue then?
> 
> There are several *at() syscalls that modify metadata.
> fchownat(.., AT_EMPTY_PATH) is intentionally designed for this.
> 
> Do you intend to try and block setxattrat()?
> Just try and block setxattrat(.., AT_EMPTY_PATH)?
> those *at() syscalls have real use cases to avoid TOCTOU races.
> Do you propose that applications will have to use fsetxattr() on an open
> file to avert races?
> 
> I completely understand the idea behind upgrade masks
> for limiting f_mode, but I don't know if trying to retroactively
> change semantics of setxattr() in the move to setxattrat()
> is going to be a good idea.

The goal would be that the semantics of fooat(<fd>, AT_EMPTY_PATH) and
foo(/proc/self/fd/<fd>) should always be identical, and the current
semantics of /proc/self/fd/<fd> are too leaky so we shouldn't always
assume that keeping them makes sense (the most obvious example is being
able to do tricks to open /proc/$pid/exe as O_RDWR).

I suspect that the long-term solution would be to have more upgrade
masks so that userspace can opt-in to not allowing any kind of
(metadata) write access through a particular file descriptor. You're
quite right that we have several metadata write AT_EMPTY_PATH APIs, and
so we can't retroactively block /everything/ but we should try to come
up with less leaky rules by default if it won't break userspace.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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