On Tue, Nov 24, 2015 at 3:34 PM, Rainer Weikusat
<rweiku...@mobileactivedefense.com> wrote:
> Eric Dumazet <eduma...@google.com> writes:
>> On Tue, Nov 24, 2015 at 6:18 AM, Dmitry Vyukov <dvyu...@google.com> 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.laha...@intel.com>
>> 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.laha...@intel.com>
>> Signed-off-by: David S. Miller <da...@davemloft.net>
>
> 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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