netdev - Re: use-after-free in sctp_do

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Date:	Tue, 8 Dec 2015 20:22:03 +0100
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc:	Vlad Yasevich <vyasevich@...il.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

On Tue, Dec 8, 2015 at 6:40 PM, Marcelo Ricardo Leitner
<marcelo.leitner@...il.com> wrote:
> On Tue, Dec 08, 2015 at 06:30:51PM +0100, Dmitry Vyukov wrote:
>> On Mon, Dec 7, 2015 at 9:52 PM, Marcelo Ricardo Leitner
>> <marcelo.leitner@...il.com> wrote:
>> > 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.
>>
>>
>> So I am waiting for a new patch, right?
>> Can you please combine all changes into a single patch (as far as I
>> understand the previous one must be applied on top of the first one)?
>
> The patches were combined already, but this last pick by Vlad is just
> not yet patched. It's not necessary for your testing and I didn't want
> to interrupt it in case you were already testing it.
>
> You can use my last patch here, from 2 emails ago, the one which
> contains this line:
> -       case SCTP_DISPOSITION_ABORT:


You are right. I missed that they are combined. Testing with it now.
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