Re: [PATCH] mnt: Prevent pivot_root from creating a loop in the mount tree

[Andy Lutomirski <luto@amacapital.net>]

On Tue, Oct 14, 2014 at 12:57 PM, Eric W. Biederman
<ebiederm@xmission.com> wrote:
> Andy Lutomirski <luto@amacapital.net> writes:
>
>> On Wed, Oct 8, 2014 at 10:42 AM, Eric W. Biederman
>> <ebiederm@xmission.com> wrote:
>>>
>>> Andy Lutomirski recently demonstrated that when chroot is used to set
>>> the root path below the path for the new ``root'' passed to pivot_root
>>> the pivot_root system call succeeds and leaks mounts.
>>
>> As part of my security fix what-happened-to-it quest: what happened to
>> this fix?
>
> On my part I am in the middle of a move right now and I don't have time
> to push it to Linus.
>
> But I will mention quickly that the fix below addresses your question of
> how should pivot_root behave if chrooted because I continue to allow the
> cases that actually work. (Not that I think anyone cares but this is
> what we have).

FWIW, the patch is:

Reviewed-by: Andy Lutomirski <luto@amacapital.net>

Linus, Al, can one of you take this?  Eric, are you okay with that?

Also, this needs Cc: stable@vger.kernel.org, I believe.  (No point in
limiting it to 3.8 and newer -- this is a bug bug, not just a security
bug, and based on my extremely limited understanding of how Docker and
lxc work, this could be exploitable even without user namespaces.  Of
course, containers set up like that are giant security problems
regardless.)

--Andy

>
>> --Andy
>>
>>>
>>> In examining the code I see that starting with a new root that is
>>> below the current root in the mount tree will result in a loop in the
>>> mount tree after the mounts are detached and then reattached to one
>>> another.  Resulting in all kinds of ugliness including a leak of that
>>> mounts involved in the leak of the mount loop.
>>>
>>> Prevent this problem by ensuring that the new mount is reachable from
>>> the current root of the mount tree.
>>>
>>> Reported-by: Andy Lutomirski <luto@amacapital.net>
>>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
>>> ---
>>>  fs/namespace.c | 3 +++
>>>  1 file changed, 3 insertions(+)
>>>
>>> diff --git a/fs/namespace.c b/fs/namespace.c
>>> index b3bdda8b5a01..7b776285832e 100644
>>> --- a/fs/namespace.c
>>> +++ b/fs/namespace.c
>>> @@ -2830,6 +2830,9 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
>>>         /* make sure we can reach put_old from new_root */
>>>         if (!is_path_reachable(old_mnt, old.dentry, &new))
>>>                 goto out4;
>>> +       /* make certain new is below the root */
>>> +       if (!is_path_reachable(new_mnt, new.dentry, &root))
>>> +               goto out4;
>>>         root_mp->m_count++; /* pin it so it won't go away */
>>>         lock_mount_hash();
>>>         detach_mnt(new_mnt, &parent_path);
>>> --
>>> 1.9.1
>>>



-- 
Andy Lutomirski
AMA Capital Management, LLC