linux-kernel - Re: net/sctp: use-after-free in __sctp

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Message-ID: <CAAeHK+xxcV8XV1Pc2_eBrQWTDSp3ooc1LLX81jhCTCvk7hhaVg@mail.gmail.com>
Date:   Thu, 3 Nov 2016 18:11:01 +0100
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc:     Vlad Yasevich <vyasevich@...il.com>,
        Neil Horman <nhorman@...driver.com>,
        "David S. Miller" <davem@...emloft.net>,
        linux-sctp@...r.kernel.org, netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>,
        Kostya Serebryany <kcc@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: net/sctp: use-after-free in __sctp_connect

On Wed, Nov 2, 2016 at 11:42 PM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
> On Wed, Oct 19, 2016 at 6:57 PM, Marcelo Ricardo Leitner
> <marcelo.leitner@...il.com> wrote:
>> On Wed, Oct 19, 2016 at 02:25:24PM +0200, Andrey Konovalov wrote:
>>> Hi,
>>>
>>> I've got the following error report while running the syzkaller fuzzer:
>>>
>>> ==================================================================
>>> BUG: KASAN: use-after-free in __sctp_connect+0xabe/0xbf0 at addr
>>> ffff88006b1dc610
>>
>> Seems this is the same that Dmitry Vyukov had reported back in Jan 13th.
>> So far I couldn't identify the reason.
>> "Good" to know it's still there, thanks for reporting it.

Hi Marcelo,

So I've looked at the code.
As far as I understand, the problem is a race condition between
setsockopt(SCTP_SOCKOPT_CONNECTX) and shutdown on an sctp socket.
setsockopt() calls sctp_wait_for_connect(), which exits the for loop
on the sk->sk_shutdown & RCV_SHUTDOWN if clause, and then frees asoc
with sctp_association_put() and returns err = 0.
Then __sctp_connect() checks that err == 0 and reads asoc->assoc_id
from the freed asoc.