From: Amir Goldstein <amir73il@gmail.com>
To: Christian Brauner <brauner@kernel.org>
Cc: "Christian Göttsche" <cgzones@googlemail.com>,
selinux@vger.kernel.org, "Miklos Szeredi" <mszeredi@redhat.com>,
"Linux API" <linux-api@vger.kernel.org>,
linux-man <linux-man@vger.kernel.org>,
"Alexander Viro" <viro@zeniv.linux.org.uk>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH] f*xattr: allow O_PATH descriptors
Date: Wed, 8 Jun 2022 15:28:52 +0300 [thread overview]
Message-ID: <CAOQ4uxipD6khNUYuZT80WUa0KOMdyyP0ia55uhmeRCLj4NBicg@mail.gmail.com> (raw)
In-Reply-To: <20220608112728.b4xrdppxqmyqmtwf@wittgenstein>
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.
>
> 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.
next prev parent reply other threads:[~2022-06-08 12:29 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-07 15:31 [RFC PATCH] f*xattr: allow O_PATH descriptors Christian Göttsche
2022-06-08 5:13 ` Amir Goldstein
2022-06-08 11:27 ` Christian Brauner
2022-06-08 12:28 ` Amir Goldstein [this message]
2022-06-08 12:48 ` Christian Brauner
2022-06-08 15:12 ` Amir Goldstein
2022-06-09 8:56 ` Christian Brauner
2022-06-18 3:18 ` Aleksa Sarai
2022-06-18 9:11 ` Amir Goldstein
2022-06-18 11:19 ` Christian Göttsche
2022-06-18 15:30 ` Amir Goldstein
2022-06-20 6:07 ` Aleksa Sarai
2022-06-20 7:45 ` Amir Goldstein
2022-06-22 2:57 ` Aleksa Sarai
2022-08-19 18:05 ` Christian Göttsche
2022-08-19 20:27 ` Amir Goldstein
2022-06-08 16:53 ` Andreas Dilger
2022-06-09 4:35 ` Amir Goldstein
2022-06-09 9:14 ` Christian Göttsche
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