linux-kernel - Re: Use-after-free in ppoll

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Date:	Sun, 22 Nov 2015 15:47:52 +0100
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Rainer Weikusat <rweikusat@...ileactivedefense.com>
Cc:	Jason Baron <jbaron@...mai.com>, Al Viro <viro@...iv.linux.org.uk>,
	David Miller <davem@...emloft.net>,
	LKML <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>,
	netdev <netdev@...r.kernel.org>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: Use-after-free in ppoll

On Sun, Nov 22, 2015 at 3:32 PM, Rainer Weikusat
<rweikusat@...ileactivedefense.com> wrote:
> Dmitry Vyukov <dvyukov@...gle.com> writes:
>> Hello,
>>
>> On commit f2d10565b9bdbb722bd43e6e1a759eeddb9645c8 (Nov 20).
>>
>> The following program triggers use-after-free:
>>
>> // autogenerated by syzkaller (http://github.com/google/syzkaller)
>> #include <syscall.h>
>> #include <string.h>
>> #include <stdint.h>
>> #include <pthread.h>
>>
>> void *thread(void *p)
>> {
>>         syscall(SYS_write, (long)p, 0x2000278ful, 0x1ul, 0, 0, 0);
>>         return 0;
>> }
>
> [...]
>
>
>>         long r1 = syscall(SYS_socketpair, 0x1ul, 0x3ul, 0x0ul,
>
> [...]
>
>>         long r5 = syscall(SYS_close, r2, 0, 0, 0, 0, 0);
>>         pthread_t th;
>>         pthread_create(&th, 0, thread, (void*)(long)r3);
>
> [...]
>
>>         long r21 = syscall(SYS_ppoll, 0x20000ffful, 0x3ul, 0x20000ffcul, 0x20000ffdul, 0x8ul, 0);
>>         return 0;
>> }
>
> That's one of the already known sequences for triggering this issue: The
> close will clear the peer pointer of the closed socket, hence, the 2nd
> sock_poll_wait will be called by unix_dgram_poll. The write will
> execute unix_dgram_sendmsg which detects that the peer is dead and
> disconnects from it, causing the corresponding structures to be freed
> despite they're still used.
>
> NB: I didn't execute this but I spend a fair amount of time with the
> af_unix.c code during the last couple of weeks and consider myself
> "reasonably familiar" with it and that's IMO what should happen here.


I have not read the code. But I just want to point out that all 3
reports are different. For example, in the first one, ppoll both frees
the object and then accesses it. That is, it is not write that frees
the object.
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