tcp: avoid integer overflows in tcp_rcv_space_adjust()

BugLink: https://bugs.launchpad.net/bugs/1776177

commit 607065ba upstream.

When using large tcp_rmem[2] values (I did tests with 500 MB),
I noticed overflows while computing rcvwin.

Lets fix this before the following patch.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Wei Wang <weiwan@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[Backport: sysctl_tcp_rmem is not Namespace-ify'd in older kernels]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

include/linux/tcp.h

@@ -324,7 +324,7 @@ struct tcp_sock {
 
 /* Receiver queue space */
 	struct {
-		int	space;
+		u32	space;
 		u32	seq;
 		u32	time;
 	} rcvq_space;

net/ipv4/tcp_input.c

@@ -557,8 +557,8 @@ static inline void tcp_rcv_rtt_measure_ts(struct sock *sk,
 void tcp_rcv_space_adjust(struct sock *sk)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
+	u32 copied;
 	int time;
-	int copied;
 
 	time = tcp_time_stamp - tp->rcvq_space.time;
 	if (time < (tp->rcv_rtt_est.rtt >> 3) || tp->rcv_rtt_est.rtt == 0)
@@ -580,12 +580,13 @@ void tcp_rcv_space_adjust(struct sock *sk)
 
 	if (sysctl_tcp_moderate_rcvbuf &&
 	    !(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) {
-		int rcvwin, rcvmem, rcvbuf;
+		int rcvmem, rcvbuf;
+		u64 rcvwin;
 
 		/* minimal window to cope with packet losses, assuming
 		 * steady state. Add some cushion because of small variations.
 		 */
-		rcvwin = (copied << 1) + 16 * tp->advmss;
+		rcvwin = ((u64)copied << 1) + 16 * tp->advmss;
 
 		/* If rate increased by 25%,
 		 *	assume slow start, rcvwin = 3 * copied
@@ -605,7 +606,8 @@ void tcp_rcv_space_adjust(struct sock *sk)
 		while (tcp_win_from_space(rcvmem) < tp->advmss)
 			rcvmem += 128;
 
-		rcvbuf = min(rcvwin / tp->advmss * rcvmem, sysctl_tcp_rmem[2]);
+		do_div(rcvwin, tp->advmss);
+		rcvbuf = min_t(u64, rcvwin * rcvmem, sysctl_tcp_rmem[2]);
 		if (rcvbuf > sk->sk_rcvbuf) {
 			sk->sk_rcvbuf = rcvbuf;