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SubjectRe: net/unix: sk_socket can disappear when state is unlocked
On 05/22/2015 02:50 AM, Hannes Frederic Sowa wrote:
> On Do, 2015-05-21 at 09:25 -0700, Mark Salyzyn wrote:
>> got a rare NULL pointer dereference in clear_bit
>>
>> Signed-off-by: Mark Salyzyn <salyzyn@android.com>
>> ---
>> net/unix/af_unix.c | 5 +++++
>> 1 file changed, 5 insertions(+)
>>
>> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
>> index 5266ea7..37a8925 100644
>> --- a/net/unix/af_unix.c
>> +++ b/net/unix/af_unix.c
>> @@ -1880,6 +1880,11 @@ static long unix_stream_data_wait(struct sock *sk, long timeo,
>> unix_state_unlock(sk);
>> timeo = freezable_schedule_timeout(timeo);
>> unix_state_lock(sk);
>> +
>> + /* sk_socket may have been killed while unlocked */
>> + if (!sk->sk_socket)
>> + break;
>> +
>> clear_bit(SOCK_ASYNC_WAITDATA, &sk->sk_socket->flags);
>> }
>>
> Canonical way is to test for sock_flag(sk, SOCK_DEAD). Also it does not
> seem like we are returning an error to user space but are still looping
> to try to dequeue skbs from sk_receive_queue, which is concurrently
> emptied by unix_release (maybe, without holding unix_state_lock).
>
> Bye,
> Hannes
>
I will send an updated patch shortly.

It may be acceptable given the expectation that sk_set_socket(sk, NULL)
occurs after SOCK_DEAD flag is set since we would not be here during the
socket initialization/connection phases. As such, for all phases (and I
re-iterate, we can only be here if in connected state), it is not a
generic guarantee of sk_socket != NULL. But I only saw one apparent
example (in net/decnet/dn_nsp_in.c) of using sock_flag(sk, SOCK_DEAD) as
protection against a possible deference NULL access with sk_socket, and
many KISS examples of checking sk_socket for NULL to protect against thus.

Thanks for making me look though, it appears that I missed the same
problem in net/caif/caif_socket.c and will add it!

Sincerely -- Mark Salyzyn


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