Re: Use-after-free in ppoll

Dmitry Vyukov
2015-11-22T14:48:16Z

On Sun, Nov 22, 2015 at 3:32 PM, Rainer Weikusat
<rweikusat@mobileactivedefense.com> wrote:
> Dmitry Vyukov <dvyukov@google.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.