tcp: Overflow bug in Vegas

From: Lachlan Andrew <lachlan.andrew@gmail.com>

There is an overflow bug in net/ipv4/tcp_vegas.c for large BDPs
(e.g. 400Mbit/s, 400ms).  The multiplication (old_wnd *
vegas->baseRTT) << V_PARAM_SHIFT overflows a u32.

[ Fix tcp_veno.c too, it has similar calculations. -DaveM ]

Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 4 deletions

diff --git a/net/ipv4/tcp_vegas.c b/net/ipv4/tcp_vegas.c
index be24d6ee34bd..0e1a8c91f78e 100644
--- a/net/ipv4/tcp_vegas.c
+++ b/net/ipv4/tcp_vegas.c
@@ -229,7 +229,8 @@ static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
                          */
                         tcp_reno_cong_avoid(sk, ack, in_flight);
                 } else {
-                        u32 rtt, target_cwnd, diff;
+                        u32 rtt, diff;
+                        u64 target_cwnd;
 
                         /* We have enough RTT samples, so, using the Vegas
                          * algorithm, we determine if we should increase or
@@ -252,8 +253,9 @@ static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
                          * We keep it as a fixed point number with
                          * V_PARAM_SHIFT bits to the right of the binary point.
                          */
-                        target_cwnd = ((old_wnd * vegas->baseRTT)
-                                       << V_PARAM_SHIFT) / rtt;
+                        target_cwnd = ((u64)old_wnd * vegas->baseRTT);
+                        target_cwnd <<= V_PARAM_SHIFT;
+                        do_div(target_cwnd, rtt);
 
                         /* Calculate the difference between the window we had,
                          * and the window we would like to have. This quantity
@@ -279,7 +281,7 @@ static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
                                  * utilization.
                                  */
                                 tp->snd_cwnd = min(tp->snd_cwnd,
-                                                   (target_cwnd >>
+                                                   ((u32)target_cwnd >>
                                                     V_PARAM_SHIFT)+1);
 
                         } else if (tp->snd_cwnd <= tp->snd_ssthresh) {