Re: use-after-free in sock_wake_async

[Eric Dumazet]

On Tue, Nov 24, 2015 at 3:34 PM, Rainer Weikusat
<rweikusat@xxxxxxxxxxxxxxxxxxxxxxx> wrote:
> 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.

This is interesting, but is not the problem or/and the fix.

We are supposed to own a reference on the 'other' socket or make sure
it cannot disappear under us.

Otherwise, no matter what you do, it is racy to even access other->any_field

In particular, you can trap doing unix_state_lock(other), way before
the code you want to change.

Please do not propose hacky things like iget or anything inode
related, this is clearly af_unix bug.
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