tcp: limit sk_rcvlowat by the maximum receive buffer

The user-provided value to setsockopt(SO_RCVLOWAT) can be
larger than the maximum possible receive buffer. Such values
mute POLLIN signals on the socket which can stall progress
on the socket.

Limit the user-provided value to half of the maximum receive
buffer, i.e., half of sk_rcvbuf when the receive buffer size
is set by the user, or otherwise half of sysctl_tcp_rmem[2].

Fixes: d1361840f8c5 ("tcp: fix SO_RCVLOWAT and RCVBUF autotuning")
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -1694,6 +1694,13 @@ EXPORT_SYMBOL(tcp_peek_len);
 /* Make sure sk_rcvbuf is big enough to satisfy SO_RCVLOWAT hint */
 int tcp_set_rcvlowat(struct sock *sk, int val)
 {
+	int cap;
+
+	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK)
+		cap = sk->sk_rcvbuf >> 1;
+	else
+		cap = sock_net(sk)->ipv4.sysctl_tcp_rmem[2] >> 1;
+	val = min(val, cap);
 	sk->sk_rcvlowat = val ? : 1;
 
 	/* Check if we need to signal EPOLLIN right now */
@@ -1702,12 +1709,7 @@ int tcp_set_rcvlowat(struct sock *sk, int val)
 	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK)
 		return 0;
 
-	/* val comes from user space and might be close to INT_MAX */
 	val <<= 1;
-	if (val < 0)
-		val = INT_MAX;
-
-	val = min(val, sock_net(sk)->ipv4.sysctl_tcp_rmem[2]);
 	if (val > sk->sk_rcvbuf) {
 		sk->sk_rcvbuf = val;
 		tcp_sk(sk)->window_clamp = tcp_win_from_space(sk, val);