tcp: avoid possible arithmetic overflows

icsk_rto is a 32bit field, and icsk_backoff can reach 15 by default,
or more if some sysctl (eg tcp_retries2) are changed.

Better use 64bit to perform icsk_rto << icsk_backoff operations

As Joe Perches suggested, add a helper for this.

Yuchung spotted the tcp_v4_err() case.

Signed-off-by: Eric Dumazet <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/inet_connection_sock.h

···
 #endif
 }
 
+static inline unsigned long
+inet_csk_rto_backoff(const struct inet_connection_sock *icsk,
+		     unsigned long max_when)
+{
+        u64 when = (u64)icsk->icsk_rto << icsk->icsk_backoff;
+
+        return (unsigned long)min_t(u64, when, max_when);
+}
+
 struct sock *inet_csk_accept(struct sock *sk, int flags, int *err);
 
 struct request_sock *inet_csk_search_req(const struct sock *sk,

net/ipv4/tcp_input.c

···
 		 * This function is not for random using!
 		 */
 	} else {
+		unsigned long when = inet_csk_rto_backoff(icsk, TCP_RTO_MAX);
+
 		inet_csk_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
-					  min(icsk->icsk_rto << icsk->icsk_backoff, TCP_RTO_MAX),
-					  TCP_RTO_MAX);
+					  when, TCP_RTO_MAX);
 	}
 }
 

net/ipv4/tcp_ipv4.c

···
 			break;
 
 		icsk->icsk_backoff--;
-		inet_csk(sk)->icsk_rto = (tp->srtt_us ? __tcp_set_rto(tp) :
-			TCP_TIMEOUT_INIT) << icsk->icsk_backoff;
-		tcp_bound_rto(sk);
+		icsk->icsk_rto = tp->srtt_us ? __tcp_set_rto(tp) :
+					       TCP_TIMEOUT_INIT;
+		icsk->icsk_rto = inet_csk_rto_backoff(icsk, TCP_RTO_MAX);
 
 		skb = tcp_write_queue_head(sk);
 		BUG_ON(!skb);

net/ipv4/tcp_output.c

···
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
+	unsigned long probe_max;
 	int err;
 
 	err = tcp_write_wakeup(sk);
···
 		if (icsk->icsk_backoff < sysctl_tcp_retries2)
 			icsk->icsk_backoff++;
 		icsk->icsk_probes_out++;
-		inet_csk_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
-					  min(icsk->icsk_rto << icsk->icsk_backoff, TCP_RTO_MAX),
-					  TCP_RTO_MAX);
+		probe_max = TCP_RTO_MAX;
 	} else {
 		/* If packet was not sent due to local congestion,
 		 * do not backoff and do not remember icsk_probes_out.
···
 		 */
 		if (!icsk->icsk_probes_out)
 			icsk->icsk_probes_out = 1;
-		inet_csk_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
-					  min(icsk->icsk_rto << icsk->icsk_backoff,
-					      TCP_RESOURCE_PROBE_INTERVAL),
-					  TCP_RTO_MAX);
+		probe_max = TCP_RESOURCE_PROBE_INTERVAL;
 	}
+	inet_csk_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
+				  inet_csk_rto_backoff(icsk, probe_max),
+				  TCP_RTO_MAX);
 }
 
 int tcp_rtx_synack(struct sock *sk, struct request_sock *req)

net/ipv4/tcp_timer.c

···
 
 		retry_until = sysctl_tcp_retries2;
 		if (sock_flag(sk, SOCK_DEAD)) {
-			const int alive = (icsk->icsk_rto < TCP_RTO_MAX);
+			const int alive = icsk->icsk_rto < TCP_RTO_MAX;
 
 			retry_until = tcp_orphan_retries(sk, alive);
 			do_reset = alive ||
···
 	max_probes = sysctl_tcp_retries2;
 
 	if (sock_flag(sk, SOCK_DEAD)) {
-		const int alive = ((icsk->icsk_rto << icsk->icsk_backoff) < TCP_RTO_MAX);
+		const int alive = inet_csk_rto_backoff(icsk, TCP_RTO_MAX) < TCP_RTO_MAX;
 
 		max_probes = tcp_orphan_retries(sk, alive);