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

From: Hannes Frederic Sowa <[email-masked]>
Date: Fri, 22 May 2015 11:50:30 +0200

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