netdev - Re: use-after-free in sctp_do

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Date:	Mon, 7 Dec 2015 18:52:11 -0200
From:	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To:	Vlad Yasevich <vyasevich@...il.com>
Cc:	Dmitry Vyukov <dvyukov@...gle.com>,
	netdev <netdev@...r.kernel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	syzkaller <syzkaller@...glegroups.com>,
	linux-sctp@...r.kernel.org, Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: use-after-free in sctp_do_sm

Em 07-12-2015 18:37, Vlad Yasevich escreveu:
> On 12/07/2015 02:50 PM, Marcelo Ricardo Leitner wrote:
>> On Mon, Dec 07, 2015 at 02:33:52PM -0500, Vlad Yasevich wrote:
>>> On 12/07/2015 01:52 PM, Marcelo Ricardo Leitner wrote:
>>>> Vlad, I reviewed the places on which it returns SCTP_DISPOSITION_ABORT,
>>>> and if I didn't miss something in there all of them either issue
>>>> SCTP_CMD_ASSOC_FAILED or SCTP_CMD_INIT_FAILED before returning it, thus
>>>> delaying DELETE_TCB and with that the asoc free.
>>>
>>> They delay it from the perspective of the command interpreter since the command
>>> to delete the TCB happens a little later, but status code  is checked after all
>>> commands are processed and command processing doesn't change it.  So the 'status'
>>> code would still be SCTP_DISPOSITION_ABORT after DELETE_TCB command was processed.
>>> So, I think we may still have an use-after-free issue here.
>>
>> Gotcha! That's pretty much it then. From that point of view now, there
>> shouldn't be a case that it returns _ABORT without freeing the asoc in
>> the same loop. (more below)
>>
>>>> There is one place,
>>>> though, that may not do it that way, it's sctp_sf_abort_violation(), but
>>>> then that code only runs if asoc is already NULL by then.
>>>
>>> I don't believe so.  The violation state function can run with a non-NULL association
>>> if we are encountering protocol violations after the association is established.
>>
>> Yup, that's correct. I just tried to reference one case on which it
>> would return _ABORT without issuing any of those _FAILEDs before doing
>> so (meaning the association could still be valid) but that in that case,
>> the asoc was already NULL.
>
> I think it is possible to hit the 'discard:' tag in that function while still
> having a valid association.  That happens when ABORT chunk is required to be
> authenticated.  This that case, instead of generating an ABORT and terminating the
> current association, we just drop the packet, but still report an _ABORT disposition code.
>
> This probably need to change if we are going to catch the _ABORT disposition and
> clear the asoc pointer.

Oups. Nice one. I'll switch it to SCTP_DISPOSITION_DISCARD if it hits 
that if() then. Thanks Vlad.

   Marcelo
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