llc: Fix race condition in llc_ui_recvmsg

There is a race on sk_receive_queue between llc_ui_recvmsg and
sock_queue_rcv_skb.

Our current solution is to protect skb_eat in llc_ui_recvmsg
with the queue spinlock.
Signed-off-by: Radu Iliescu <riliescu@ixiacom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/llc/af_llc.c

@@ -713,6 +713,7 @@ static int llc_ui_recvmsg(struct kiocb *iocb, struct socket *sock,
 	struct sk_buff *skb = NULL;
 	struct sock *sk = sock->sk;
 	struct llc_sock *llc = llc_sk(sk);
+	unsigned long cpu_flags;
 	size_t copied = 0;
 	u32 peek_seq = 0;
 	u32 *seq;
@@ -838,7 +839,9 @@ static int llc_ui_recvmsg(struct kiocb *iocb, struct socket *sock,
 			goto copy_uaddr;
 
 		if (!(flags & MSG_PEEK)) {
+			spin_lock_irqsave(&sk->sk_receive_queue.lock, cpu_flags);
 			sk_eat_skb(sk, skb, 0);
+			spin_unlock_irqrestore(&sk->sk_receive_queue.lock, cpu_flags);
 			*seq = 0;
 		}
 
@@ -859,7 +862,9 @@ static int llc_ui_recvmsg(struct kiocb *iocb, struct socket *sock,
 		llc_cmsg_rcv(msg, skb);
 
 	if (!(flags & MSG_PEEK)) {
+			spin_lock_irqsave(&sk->sk_receive_queue.lock, cpu_flags);
 			sk_eat_skb(sk, skb, 0);
+			spin_unlock_irqrestore(&sk->sk_receive_queue.lock, cpu_flags);
 			*seq = 0;
 	}