Subject: Re: net/unix: sk_socket can disappear when state is unlocked

From: Mark Salyzyn <[email-masked]>
Date: Fri, 22 May 2015 07:51:19 -0700

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 <[email-masked]>
>> ---
>>   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