From: ebiederm@xmission.com (Eric W. Biederman)
To: Andy Lutomirski <luto@amacapital.net>
Cc: Miklos Szeredi <miklos@szeredi.hu>, "Serge E. Hallyn" <serge@hallyn.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linux-Fsdevel <linux-fsdevel@vger.kernel.org>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Rob Landley <rob@landley.net>,
        Linus Torvalds <torvalds@linux-foundation.org>
References: <CAJfpegsxgnSRUW-E5HM3uT5QfGyUtn_v=i4Ppkkkutp34287AA@mail.gmail.com>
	<87a9kkax0j.fsf@xmission.com>
	<CAJfpeguv+7giYNpAuXE9Ja_9BEwB0-fZBVgRSeVqpzSXgQYZ6Q@mail.gmail.com>
	<8761v7h2pt.fsf@tw-ebiederman.twitter.com>
	<CAJfpegtT7y-1HhbEVAMKkdQugTG_w7G_epGtQHGvQLpcZB5FVA@mail.gmail.com>
	<87li281wx6.fsf_-_@xmission.com> <874n8w1wsz.fsf_-_@xmission.com>
	<20131008155041.GI14242@tucsk.piliscsaba.szeredi.hu>
	<877gdne8pr.fsf@xmission.com> <8761t65kd2.fsf_-_@xmission.com>
	<CALCETrUqXTj7T_bYOgwMiO_kcU=NPD_+ck+Z=JsmXuUJJGshuQ@mail.gmail.com>
Date: Wed, 09 Oct 2013 16:35:56 -0700
In-Reply-To: <CALCETrUqXTj7T_bYOgwMiO_kcU=NPD_+ck+Z=JsmXuUJJGshuQ@mail.gmail.com>
	(Andy Lutomirski's message of "Wed, 9 Oct 2013 13:09:17 -0700")
Message-ID: <87ob6yyq3n.fsf@xmission.com>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.1 (gnu/linux)
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Content-Type: text/plain
Subject: Re: Grrrr fusermount.
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Andy Lutomirski <luto@amacapital.net> writes:

> On Wed, Oct 9, 2013 at 12:12 PM, Eric W. Biederman
> <ebiederm@xmission.com> wrote:
>> ebiederm@xmission.com (Eric W. Biederman) writes:
>>
>>> But I will go through and read the old fusermount code before I get too
>>> much farther just so I understand what I am potentially breaking.
>>
>> Grr.
>>
>> So I have just read the fusermount umount code and the hack that it uses
>> before there was UMOUNT_NOFOLLOW support in the vm.
>>
>> If I walk this path of lazy unmounts and detaching directories, anyone
>> with a new kernel and an old copy of fusermount and a nfs mounted home
>> directory will be able to exploit the fusermount umount symlink race.
>>
>> Unless we can declare that old fusermount binaries are buggy beyond
>> supporting this patchset as it exists is dead.
>
> What's the hack that it does?

The problem is that "umount -l /some/user/supplied/path" can unmount
anything.

Even after checking mtab etc there are races, and replacing "path" with
a symlink will still allow you to unmount anything.

So when not usering UMOUNT_NOFOLLOW fusermount will do:

cd "/some/user/supplied/"

clone(CLONE_NEWNS)
   mount --make-rprivate /
   COUNT=$(cat /proc/mounts | wc -l)
   mount --rbind . /
   Look at new /proc/mounts entries past $COUNT and see if there is an
     entry for "path"
   if $found
	exit 0
   else
        exit 1
wait
if ($? == 0) umount -l "path"

As everything happens in the directory just below the mount point
we are only concerned about things that happen in that directory.

This does mean there is a window between checking that path is
a valid fuse mount point and when that mountpoint is unmounted.

So if someone can unlink/rename the mount point and replace it
with a symlink during that small window it is possible to unmount
anything.

Today the d_mountpoint tests and the requirement distributed filesystems
(aka nfs) lie to the vfs prevent those issues.

The UMOUNT_NOFOLLOW code which appears in fuse 2.9.0 is new enough that
distro's like CentOS6 don't ship it yet.

Eric


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