netdev - Re: [PATCH net] sctp: do sanity checks before migrating the asoc

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Date:	Wed, 3 Feb 2016 17:13:25 +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>, linux-sctp@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: [PATCH net] sctp: do sanity checks before migrating the asoc

On Tue, Jan 19, 2016 at 9:08 PM, Marcelo Ricardo Leitner
<marcelo.leitner@...il.com> wrote:
> Em 19-01-2016 17:55, Vlad Yasevich escreveu:
>>
>> On 01/19/2016 02:31 PM, Marcelo Ricardo Leitner wrote:
>>>
>>> Em 19-01-2016 16:37, Vlad Yasevich escreveu:
>>>>
>>>> On 01/19/2016 10:59 AM, Marcelo Ricardo Leitner wrote:
>>>>>
>>>>> Yes, not thrilled here either about connect-to-self.
>>>>>
>>>>> But there is a big difference on how both works. For rx we can just
>>>>> look for wanted skbs
>>>>> in rx queue, as they aren't going anywhere, but for tx I don't think we
>>>>> can easily block
>>>>> sctp_wfree() call because that may be happening on another CPU (or am I
>>>>> mistaken here?
>>>>> sctp still doesn't have RFS but even irqbalance could affect this
>>>>> AFAICT) and more than
>>>>> one skb may be in transit at a time.
>>>>
>>>>
>>>> The way it's done now, we wouldn't have to block sctp_wfree.  Chunks are
>>>> released under
>>>> lock when they are acked, so we are OK here.  The tx completions will
>>>> just put 1 byte back
>>>> to the socket associated with the tx'ed skb, and that should still be ok
>>>> as
>>>> sctp_packet_release_owner will call sk_free().
>>>
>>>
>>> Please let me rephrase it. I'm actually worried about the asoc->base.sk
>>> part of the story
>>> and how it's fetched in sctp_wfree(). I think we can update that sk
>>> pointer after
>>> sock_wfree() has fetched it but not used it yet, possibly leading to
>>> accounting it twice,
>>> one during migration and one on sock_wfree.
>>> In sock_wfree() it will update some sk stats like sk->sk_wmem_alloc,
>>> among others.
>>
>>
>> sctp_wfree() is only used on skbs that were created as sctp chunks to be
>> transmitted.
>> Right now, these skbs aren't actually submitted to the IP or to nic to be
>> transmitted.
>> They are queued at the association level (either in transports or in the
>> outqueue).
>> They are only freed during ACK processing.
>>
>> The ACK processing happens under a socket lock and thus asoc->base.sk can
>> not move.
>>
>> The migration process also happens under a socket lock.  As a result,
>> during migration
>> we are guaranteed the chunk queues remain consistent and that
>> asoc->base.sk linkage
>> remains consistent.  In fact, if you look at the sctp_sock_migrate, we
>> lock both
>> sockets when we reassign the assoc->base.sk so we know both sockets are
>> properly locked.
>>
>> So, I am not sure that what you are worried about can happen.  Please feel
>> free to
>> double-check the above of course.
>
>
> Ohh, right. That makes sense. I'll rework the patch. Thanks Vlad.


Hi Marcelo,

Any updates on this? I still see the leak.