tcp: cubic: fix overflow error in bictcp_update()

commit 17a6e9f1aa9 ("tcp_cubic: fix clock dependency") added an
overflow error in bictcp_update() in following code :

/* change the unit from HZ to bictcp_HZ */
t = ((tcp_time_stamp + msecs_to_jiffies(ca->delay_min>>3) -
      ca->epoch_start) << BICTCP_HZ) / HZ;

Because msecs_to_jiffies() being unsigned long, compiler does
implicit type promotion.

We really want to constrain (tcp_time_stamp - ca->epoch_start)
to a signed 32bit value, or else 't' has unexpected high values.

This bugs triggers an increase of retransmit rates ~24 days after
boot [1], as the high order bit of tcp_time_stamp flips.

[1] for hosts with HZ=1000

Big thanks to Van Jacobson for spotting this problem.

Diagnosed-by: Van Jacobson <vanj@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 4 deletions

diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index a9077f441cb2..b6b591f0a788 100644
--- a/net/ipv4/tcp_cubic.c
+++ b/net/ipv4/tcp_cubic.c
@@ -206,8 +206,8 @@ static u32 cubic_root(u64 a)
  */
 static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
 {
-        u64 offs;
-        u32 delta, t, bic_target, max_cnt;
+        u32 delta, bic_target, max_cnt;
+        u64 offs, t;
 
         ca->ack_cnt++;  /* count the number of ACKs */
 
@@ -250,9 +250,11 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
          * if the cwnd < 1 million packets !!!
          */
 
+        t = (s32)(tcp_time_stamp - ca->epoch_start);
+        t += msecs_to_jiffies(ca->delay_min >> 3);
         /* change the unit from HZ to bictcp_HZ */
-        t = ((tcp_time_stamp + msecs_to_jiffies(ca->delay_min>>3)
-              - ca->epoch_start) << BICTCP_HZ) / HZ;
+        t <<= BICTCP_HZ;
+        do_div(t, HZ);
 
         if (t < ca->bic_K)              /* t - K */
                 offs = ca->bic_K - t;