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

On 2022-06-08, Amir Goldstein [off-list ref] wrote:

On Wed, Jun 8, 2022 at 3:48 PM Christian Brauner [off-list ref] wrote:

quoted

On Wed, Jun 08, 2022 at 03:28:52PM +0300, Amir Goldstein wrote:

quoted

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

quoted

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.

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.

quoted

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...

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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 (local)
[2]: https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20190728010207.9781-8-cyphar@cyphar.com

... 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/>