From: Aleksa Sarai <cyphar@cyphar.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
David Howells <dhowells@redhat.com>,
Eric Biederman <ebiederm@xmission.com>,
stable <stable@vger.kernel.org>,
Christian Brauner <christian.brauner@ubuntu.com>,
Serge Hallyn <serge@hallyn.com>,
dev@opencontainers.org,
Linux Containers <containers@lists.linux-foundation.org>,
Linux API <linux-api@vger.kernel.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Ian Kent <raven@themaw.net>
Subject: Re: [PATCH RFC 0/1] mount: universally disallow mounting over symlinks
Date: Wed, 15 Jan 2020 07:01:50 +1100 [thread overview]
Message-ID: <20200114200150.ryld4npoblns2ybe@yavin> (raw)
In-Reply-To: <20200114045733.GW8904@ZenIV.linux.org.uk>
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On 2020-01-14, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Sat, Jan 11, 2020 at 08:07:19AM +1100, Aleksa Sarai wrote:
>
> > If I'm understanding this proposal correctly, this would be a problem
> > for the libpathrs use-case -- if this is done then there's no way to
> > avoid a TOCTOU with someone mounting and the userspace program checking
> > whether something is a mountpoint (unless you have Linux >5.6 and
> > RESOLVE_NO_XDEV). Today, you can (in theory) do it with MNT_EXPIRE:
> >
> > 1. Open the candidate directory.
> > 2. umount2(MNT_EXPIRE) the fd.
> > * -EINVAL means it wasn't a mountpoint when we got the fd, and the
> > fd is a stable handle to the underlying directory.
> > * -EAGAIN or -EBUSY means that it was a mountpoint or became a
> > mountpoint after the fd was opened (we don't care about that, but
> > fail-safe is better here).
> > 3. Use the fd from (1) for all operations.
>
> ... except that foo/../bar *WILL* cross into the covering mount, on any
> kernel that supports ...at(2) at all, so I would be very cautious about
> any kind "hardening" claims in that case.
In the use-case I have, we would have full control over what the path
being opened is (and thus you wouldn't open "foo/../bar"). But I agree
that generally the MNT_EXPIRE solution is really non-ideal anyway.
Not to mention that we're still screwed when it comes to using
magic-links (because if someone bind-mounts a magic-link over a
magic-link there's absolutely no race-free way to be sure that we're
traversing the right magic-link -- for that we'll need to have a
different solution).
> I'm not sure about Linus' proposal - it looks rather convoluted and we
> get a hard to describe twist of semantics in an area (procfs symlinks
> vs. mount traversal) on top of everything else in there...
Yeah, I agree.
> 1) do you see any problems on your testcases with the current #fixes?
> That's commit 7a955b7363b8 as branch tip.
I will take a quick look later today, but I'm currently at a conference.
> 2) do you have any updates you would like to fold into stuff in
> #work.openat2? Right now I have a local variant of #work.namei (with
> fairly cosmetical change compared to vfs.git one) that merges clean
> with #work.openat2; I would like to do any updates/fold-ins/etc.
> of #work.openat2 *before* doing a merge and continuing to work on
> top of the merge results...
Yes, there were two patches I sent a while ago[1]. I can re-send them if
you like. The second patch switches open_how->mode to a u64, but I'm
still on the fence about whether that makes sense to do...
[1]: https://lore.kernel.org/lkml/20191219105533.12508-1-cyphar@cyphar.com/
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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next prev parent reply other threads:[~2020-01-14 20:02 UTC|newest]
Thread overview: 91+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-30 5:20 [PATCH RFC 0/1] mount: universally disallow mounting over symlinks Aleksa Sarai
2019-12-30 5:20 ` [PATCH RFC 1/1] " Aleksa Sarai
2019-12-30 7:34 ` Linus Torvalds
2019-12-30 8:28 ` Aleksa Sarai
2020-01-08 4:39 ` Andy Lutomirski
2019-12-30 5:44 ` [PATCH RFC 0/1] " Al Viro
2019-12-30 5:49 ` Aleksa Sarai
[not found] ` <20191230072959.62kcojxpthhdwmfa@yavin.dot.cyphar.com>
2019-12-30 7:53 ` Linus Torvalds
2019-12-30 8:32 ` Aleksa Sarai
2020-01-02 8:58 ` David Laight
2020-01-02 9:09 ` Aleksa Sarai
2020-01-01 0:43 ` Al Viro
2020-01-01 0:54 ` Al Viro
2020-01-01 3:08 ` Al Viro
2020-01-01 14:44 ` Aleksa Sarai
2020-01-01 23:40 ` Al Viro
2020-01-02 3:59 ` Aleksa Sarai
2020-01-03 1:49 ` Al Viro
2020-01-04 4:46 ` Ian Kent
2020-01-08 3:13 ` Al Viro
2020-01-08 3:54 ` Linus Torvalds
2020-01-08 21:34 ` Al Viro
2020-01-10 0:08 ` Linus Torvalds
2020-01-10 4:15 ` Al Viro
2020-01-10 5:03 ` Linus Torvalds
2020-01-10 6:20 ` Ian Kent
2020-01-12 21:33 ` Al Viro
2020-01-13 2:59 ` Ian Kent
2020-01-14 0:25 ` Ian Kent
2020-01-14 4:39 ` Al Viro
2020-01-14 5:01 ` Ian Kent
2020-01-14 5:59 ` Ian Kent
2020-01-10 21:07 ` Aleksa Sarai
2020-01-14 4:57 ` Al Viro
2020-01-14 5:12 ` Al Viro
2020-01-14 20:01 ` Aleksa Sarai [this message]
2020-01-15 14:25 ` Al Viro
2020-01-15 14:29 ` Aleksa Sarai
2020-01-15 14:34 ` Aleksa Sarai
2020-01-15 14:48 ` Al Viro
2020-01-18 12:07 ` [PATCH v3 0/2] openat2: minor uapi cleanups Aleksa Sarai
2020-01-18 12:07 ` [PATCH v3 1/2] open: introduce openat2(2) syscall Aleksa Sarai
2020-01-18 12:08 ` [PATCH v3 2/2] selftests: add openat2(2) selftests Aleksa Sarai
2020-01-18 15:28 ` [PATCH v3 0/2] openat2: minor uapi cleanups Al Viro
2020-01-18 18:09 ` Al Viro
2020-01-18 23:03 ` Aleksa Sarai
2020-01-19 1:12 ` Al Viro
2020-01-15 13:57 ` [PATCH RFC 0/1] mount: universally disallow mounting over symlinks Aleksa Sarai
2020-01-19 3:14 ` [RFC][PATCHSET][CFT] pathwalk cleanups and fixes Al Viro
2020-01-19 3:17 ` [PATCH 01/17] do_add_mount(): lift lock_mount/unlock_mount into callers Al Viro
2020-01-19 3:17 ` [PATCH 02/17] fix automount/automount race properly Al Viro
2020-01-30 14:34 ` Christian Brauner
2020-01-19 3:17 ` [PATCH 03/17] follow_automount(): get rid of dead^Wstillborn code Al Viro
2020-01-30 14:38 ` Christian Brauner
2020-01-19 3:17 ` [PATCH 04/17] follow_automount() doesn't need the entire nameidata Al Viro
2020-01-30 14:45 ` Christian Brauner
2020-01-30 15:38 ` Al Viro
2020-01-30 15:55 ` Al Viro
2020-01-19 3:17 ` [PATCH 05/17] make build_open_flags() treat O_CREAT | O_EXCL as implying O_NOFOLLOW Al Viro
2020-01-19 3:17 ` [PATCH 06/17] handle_mounts(): start building a sane wrapper for follow_managed() Al Viro
2020-01-19 3:17 ` [PATCH 07/17] atomic_open(): saner calling conventions (return dentry on success) Al Viro
2020-01-19 3:17 ` [PATCH 08/17] lookup_open(): " Al Viro
2020-01-19 3:17 ` [PATCH 09/17] do_last(): collapse the call of path_to_nameidata() Al Viro
2020-01-19 3:17 ` [PATCH 10/17] handle_mounts(): pass dentry in, turn path into a pure out argument Al Viro
2020-01-19 3:17 ` [PATCH 11/17] lookup_fast(): consolidate the RCU success case Al Viro
2020-01-19 3:17 ` [PATCH 12/17] teach handle_mounts() to handle RCU mode Al Viro
2020-01-19 3:17 ` [PATCH 13/17] lookup_fast(): take mount traversal into callers Al Viro
2020-01-19 3:17 ` [PATCH 14/17] new step_into() flag: WALK_NOFOLLOW Al Viro
2020-01-19 3:17 ` [PATCH 15/17] fold handle_mounts() into step_into() Al Viro
2020-01-19 3:17 ` [PATCH 16/17] LOOKUP_MOUNTPOINT: fold path_mountpointat() into path_lookupat() Al Viro
2020-01-19 3:17 ` [PATCH 17/17] expand the only remaining call of path_lookup_conditional() Al Viro
2020-01-19 3:17 ` [PATCH 1/9] merging pick_link() with get_link(), part 1 Al Viro
2020-01-19 3:17 ` [PATCH 2/9] merging pick_link() with get_link(), part 2 Al Viro
2020-01-19 3:17 ` [PATCH 3/9] merging pick_link() with get_link(), part 3 Al Viro
2020-01-19 3:17 ` [PATCH 4/9] merging pick_link() with get_link(), part 4 Al Viro
2020-01-19 3:17 ` [PATCH 5/9] merging pick_link() with get_link(), part 5 Al Viro
2020-01-19 3:17 ` [PATCH 6/9] merging pick_link() with get_link(), part 6 Al Viro
2020-01-19 3:17 ` [PATCH 7/9] finally fold get_link() into pick_link() Al Viro
2020-01-19 3:17 ` [PATCH 8/9] massage __follow_mount_rcu() a bit Al Viro
2020-01-19 3:17 ` [PATCH 9/9] new helper: traverse_mounts() Al Viro
2020-01-30 14:13 ` [PATCH 01/17] do_add_mount(): lift lock_mount/unlock_mount into callers Christian Brauner
2020-01-19 14:33 ` [RFC][PATCHSET][CFT] pathwalk cleanups and fixes Ian Kent
2020-01-10 23:19 ` [PATCH RFC 0/1] mount: universally disallow mounting over symlinks Al Viro
2020-01-13 1:48 ` Ian Kent
2020-01-13 3:54 ` Al Viro
2020-01-13 6:00 ` Ian Kent
2020-01-13 6:03 ` Ian Kent
2020-01-13 13:30 ` Al Viro
2020-01-14 7:25 ` Ian Kent
2020-01-14 12:17 ` Ian Kent
2020-01-04 5:52 ` Andy Lutomirski
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