Re: use-after-free in sock_wake_async

[Rainer Weikusat]

Eric Dumazet <edumazet@xxxxxxxxxx> writes:
> On Tue, Nov 24, 2015 at 6:18 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>> Hello,
>>
>> The following program triggers use-after-free in sock_wake_async:

[...]

>> void *thr1(void *arg)
>> {
>>         syscall(SYS_close, r2, 0, 0, 0, 0, 0);
>>         return 0;
>> }
>>
>> void *thr2(void *arg)
>> {
>>         syscall(SYS_write, r3, 0x20003000ul, 0xe7ul, 0, 0, 0);
>>         return 0;
>> }

[...]

>>         pthread_t th[3];
>>         pthread_create(&th[0], 0, thr0, 0);
>>         pthread_create(&th[1], 0, thr1, 0);
>>         pthread_create(&th[2], 0, thr2, 0);
>>         pthread_join(th[0], 0);
>>         pthread_join(th[1], 0);
>>         pthread_join(th[2], 0);
>>         return 0;
>> }

[...]

> Looks like commit 830a1e5c212fb3fdc83b66359c780c3b3a294897 should be reverted ?
>
> commit 830a1e5c212fb3fdc83b66359c780c3b3a294897
> Author: Benjamin LaHaise <benjamin.c.lahaise@xxxxxxxxx>
> Date:   Tue Dec 13 23:22:32 2005 -0800
>
>     [AF_UNIX]: Remove superfluous reference counting in unix_stream_sendmsg
>
>     AF_UNIX stream socket performance on P4 CPUs tends to suffer due to a
>     lot of pipeline flushes from atomic operations.  The patch below
>     removes the sock_hold() and sock_put() in unix_stream_sendmsg().  This
>     should be safe as the socket still holds a reference to its peer which
>     is only released after the file descriptor's final user invokes
>     unix_release_sock().  The only consideration is that we must add a
>     memory barrier before setting the peer initially.
>
>     Signed-off-by: Benjamin LaHaise <benjamin.c.lahaise@xxxxxxxxx>
>     Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>

JFTR: This seems to be unrelated. (As far as I understand this), the
problem is that sk_wake_async accesses sk->sk_socket. That's invoked via
the

other->sk_data_ready(other)

in unix_stream_sendmsg after an

unix_state_unlock(other);

because of this, it can race with the code in unix_release_sock clearing
this pointer (via sock_orphan). The structure this pointer points to is
freed via iput in sock_release (net/socket.c) after the af_unix release
routine returned (it's really one part of a "twin structure" with the
socket inode being the other).

A quick way to test if this was true would be to swap the

unix_state_unlock(other);
other->sk_data_ready(other);

in unix_stream_sendmsg and in case it is, a very 'hacky' fix could be to
put a pointer to the socket inode into the struct unix_sock, do an iget
on that in unix_create1 and a corresponding iput in
unix_sock_destructor.
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