Subject: Re: Fwd: Simple kernel attack using socketpair. easy, 100% reproductiblle, works under guest. no way to protect :(

From: Shan Wei <[email-masked]>
Date: Fri, 26 Nov 2010 15:52:43 +0800

Eric Dumazet wrote, at 11/26/2010 02:23 PM:
> Le vendredi 26 novembre 2010 à 12:38 +0800, Shan Wei a écrit :
>> Eric Dumazet wrote, at 11/25/2010 10:11 PM:
>>> Le jeudi 25 novembre 2010 à 13:35 +0500, Марк Коренберг a écrit :
>>>> quick and dirty fix will be not to allow to pass unix socket inside
>>>> unix socket. I think it would not break much applications.
>>>
>>> Really, if it was not needed, net/unix/garbage.c would not exist at
>>> all...
>>>
>>> It is needed by some apps.
>>>
>>>
>>> [PATCH] af_unix: limit recursion level
>>>
>>> Its easy to eat all kernel memory and trigger NMI watchdog, using an
>>> exploit program that queues unix sockets on top of others.
>>>
>>> lkml ref : http://lkml.org/lkml/2010/11/25/8
>>>
>>> This mechanism is used in applications, one choice we have is to have a
>>> recursion limit.
>>>
>>> Other limits might be needed as well (if we queue other types of files),
>>> since the passfd mechanism is currently limited by socket receive queue
>>> sizes only.
>>>
>>> Add a recursion_level to unix socket, allowing up to 4 levels.
>>>
>>> Each time we send an unix socket through sendfd mechanism, we copy its
>>> recursion level (plus one) to receiver. This recursion level is cleared
>>> when socket receive queue is emptied.
>>>
>>> Reported-by: Марк Коренберг <[email-masked]>
>>> Signed-off-by: Eric Dumazet <[email-masked]>
>>
>> This problem is same as that reported with title "Unix socket local DOS (OOM)", right?
>> After applied this patch, this program can be killed now. but still eat 100% cpu. 
>>
> 
> Not the same problem, but a different one. 
> 
> In this case, we queue files on top of another and never give a chance
> to free them, unless the program dies (and full memory eaten)
> 
> And yes, its eating 100% cpu, since it has no sleep inside, like
> 
> for (;;) ;

Got it. Thanks.

Have a out of topic question. 
There is some difficulty for me to understand this issue. :-(
why can't we kill this program?

When send fd[0] to ff[0] socket, fd[0] is in flight and will be add reference value.
Athough we close fd[0], their references is still exist.

The reason that can't be killed is about the references or about the latest sockets
created by socketpair() but never be freeed.

-- 
Best Regards
-----
Shan Wei