tcp: Add TCP_USER_TIMEOUT negative value check

TCP_USER_TIMEOUT is a TCP level socket option that takes an unsigned int. But
patch "tcp: Add TCP_USER_TIMEOUT socket option"(dca43c75) didn't check the negative
values. If a user assign -1 to it, the socket will set successfully and wait
for 4294967295 miliseconds. This patch add a negative value check to avoid
this issue.
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -2681,6 +2681,9 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
 		/* Cap the max timeout in ms TCP will retry/retrans
 		 * before giving up and aborting (ETIMEDOUT) a connection.
 		 */
+		if (val < 0)
+			err = -EINVAL;
+		else
 			icsk->icsk_user_timeout = msecs_to_jiffies(val);
 		break;
 	default: