bpf: Fix sk_psock refcnt leak when receiving message

tcp_bpf_recvmsg() invokes sk_psock_get(), which returns a reference of
the specified sk_psock object to "psock" with increased refcnt.

When tcp_bpf_recvmsg() returns, local variable "psock" becomes invalid,
so the refcount should be decreased to keep refcount balanced.

The reference counting issue happens in several exception handling paths
of tcp_bpf_recvmsg(). When those error scenarios occur such as "flags"
includes MSG_ERRQUEUE, the function forgets to decrease the refcnt
increased by sk_psock_get(), causing a refcnt leak.

Fix this issue by calling sk_psock_put() or pulling up the error queue
read handling when those error scenarios occur.

Fixes: e7a5f1f1 ("bpf/sockmap: Read psock ingress_msg before sk_receive_queue")
Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/1587872115-42805-1-git-send-email-xiyuyang19@fudan.edu.cn

net/ipv4/tcp_bpf.c

@@ -262,14 +262,17 @@ static int tcp_bpf_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
 	struct sk_psock *psock;
 	int copied, ret;
 
+	if (unlikely(flags & MSG_ERRQUEUE))
+		return inet_recv_error(sk, msg, len, addr_len);
+
 	psock = sk_psock_get(sk);
 	if (unlikely(!psock))
 		return tcp_recvmsg(sk, msg, len, nonblock, flags, addr_len);
-	if (unlikely(flags & MSG_ERRQUEUE))
-		return inet_recv_error(sk, msg, len, addr_len);
 	if (!skb_queue_empty(&sk->sk_receive_queue) &&
-	    sk_psock_queue_empty(psock))
+	    sk_psock_queue_empty(psock)) {
+		sk_psock_put(sk, psock);
 		return tcp_recvmsg(sk, msg, len, nonblock, flags, addr_len);
+	}
 	lock_sock(sk);
 msg_bytes_ready:
 	copied = __tcp_bpf_recvmsg(sk, psock, msg, len, flags);