dccp: put dccp_qpolicy_full() and dccp_qpolicy_push() in the same lock

[ Upstream commit a41b17f ]

In the case of sk->dccps_qpolicy == DCCPQ_POLICY_PRIO, dccp_qpolicy_full
will drop a skb when qpolicy is full. And the lock in dccp_sendmsg is
released before sock_alloc_send_skb and then relocked after
sock_alloc_send_skb. The following conditions may lead dccp_qpolicy_push
to add skb to an already full sk_write_queue:

thread1--->lock
thread1--->dccp_qpolicy_full: queue is full. drop a skb
thread1--->unlock
thread2--->lock
thread2--->dccp_qpolicy_full: queue is not full. no need to drop.
thread2--->unlock
thread1--->lock
thread1--->dccp_qpolicy_push: add a skb. queue is full.
thread1--->unlock
thread2--->lock
thread2--->dccp_qpolicy_push: add a skb!
thread2--->unlock

Fix this by moving dccp_qpolicy_full.

Fixes: b1308dc ("[DCCP]: Set TX Queue Length Bounds via Sysctl")
Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
Link: https://lore.kernel.org/r/20220729110027.40569-1-hbh25y@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

net/dccp/proto.c

@@ -747,11 +747,6 @@ int dccp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 
 	lock_sock(sk);
 
-	if (dccp_qpolicy_full(sk)) {
-		rc = -EAGAIN;
-		goto out_release;
-	}
-
 	timeo = sock_sndtimeo(sk, noblock);
 
 	/*
@@ -770,6 +765,11 @@ int dccp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	if (skb == NULL)
 		goto out_release;
 
+	if (dccp_qpolicy_full(sk)) {
+		rc = -EAGAIN;
+		goto out_discard;
+	}
+
 	if (sk->sk_state == DCCP_CLOSED) {
 		rc = -ENOTCONN;
 		goto out_discard;