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

From: Christian Brauner <brauner@kernel.org>
To: "Christian Göttsche" <cgzones@googlemail.com>
Cc: selinux@vger.kernel.org, Miklos Szeredi <mszeredi@redhat.com>,
	linux-api@vger.kernel.org, linux-man@vger.kernel.org,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH] f*xattr: allow O_PATH descriptors
Date: Wed, 8 Jun 2022 13:27:28 +0200	[thread overview]
Message-ID: <20220608112728.b4xrdppxqmyqmtwf@wittgenstein> (raw)
In-Reply-To: <20220607153139.35588-1-cgzones@googlemail.com>

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.

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.