[PATCH] tcp: cubic: fix overflow error in bictcp_update()

[PATCH] tcp: cubic: fix overflow error in bictcp_update()

[Eric Dumazet]

From: Eric Dumazet <edumazet@google.com>

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>
---
 net/ipv4/tcp_cubic.c |   10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index a9077f4..b6b591f 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;

Re: [PATCH] tcp: cubic: fix overflow error in bictcp_update()

[Neal Cardwell]

On Mon, Aug 5, 2013 at 8:10 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> From: Eric Dumazet <edumazet@google.com>
>
> 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>

Also a nice catch!

neal

Re: [PATCH] tcp: cubic: fix overflow error in bictcp_update()

[David Miller]

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Mon, 05 Aug 2013 17:10:15 -0700

> From: Eric Dumazet <edumazet@google.com>
> 
> 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>

Applied and queued up for -stable.