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From: Aleksa Sarai <cyphar@cyphar.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
	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: Sat, 11 Jan 2020 08:07:19 +1100	[thread overview]
Message-ID: <20200110210719.ktg3l2kwjrdutlh6@yavin> (raw)
In-Reply-To: <CAHk-=wgQ3yOBuK8mxpnntD8cfX-+10ba81f86BYg8MhvwpvOMg@mail.gmail.com>

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On 2020-01-07, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Tue, Jan 7, 2020 at 7:13 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> > Another interesting question is whether we want O_PATH open
> > to trigger automounts.
> 
> It does sound like they shouldn't, but as you say:
> 
> >     The thing is, we do *NOT* trigger them
> > (or traverse mountpoints) at the starting point of lookups.
> > I believe it's a mistake (and mine, at that), but I doubt that
> > there's anything that can be done about it at that point.
> > It's a user-visible behaviour [..]
> 
> Hmm. I wonder how set in stone that is. We may have two decades of
> history of not doing it at start point of lookups, but we do *not*
> have two decades of history of O_PATH.
> 
> So what I think we agree would be sane behavior would be for O_PATH
> opens to not trigger automounts (unless there's a slash at the end,
> whatever), but _do_ add the mount-point traversal to the beginning of
> lookups.
> 
> But only do it for the actual O_PATH fd case, not the cwd/root/non-O_PATH case.
> 
> That way we maintain original behavior: if somebody overmounts your
> cwd, you still see the pre-mount directory on lookups, because your
> cwd is "under" the mount.
> 
> But if you open a file with O_PATH, and somebody does a mount
> _afterwards_, the openat() will see that later mount and/or do the
> automount.
> 
> Don't you think that would be the more sane/obvious semantics of how
> O_PATH should work?

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.

Don't get me wrong, I want to fix this issue *properly* by adding some
new kernel features that allow us to avoid worrying about
mounts-over-magiclinks -- but on old kernels (which libpathrs cares
about) I would be worried about changes like this being backported
resulting in it being not possible to implement the hardening I
mentioned up-thread.

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

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  parent reply	other threads:[~2020-01-10 21:07 UTC|newest]

Thread overview: 92+ 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
2019-12-30  7:29     ` Aleksa Sarai
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 [this message]
2020-01-14  4:57                           ` Al Viro
2020-01-14  5:12                             ` Al Viro
2020-01-14 20:01                             ` Aleksa Sarai
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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