linux-kernel - Re: Grrrr fusermount.

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Date:	Wed, 09 Oct 2013 16:35:56 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Miklos Szeredi <miklos@...redi.hu>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rob Landley <rob@...dley.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Grrrr fusermount.

Andy Lutomirski <luto@...capital.net> writes:

> On Wed, Oct 9, 2013 at 12:12 PM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>> ebiederm@...ssion.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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