[PATCH] sch_cake: avoid possible divide by zero in cake_enqueue()

[PATCH] sch_cake: avoid possible divide by zero in cake_enqueue()

[Wen Yang]

The variables 'window_interval' is u64 and do_div()
truncates it to 32 bits, which means it can test
non-zero and be truncated to zero for division.
The unit of window_interval is nanoseconds,
so its lower 32-bit is relatively easy to exceed.
Fix this issue by using div64_u64() instead.

Fixes: 7298de9cd725 ("sch_cake: Add ingress mode")
Signed-off-by: Wen Yang <wenyang@linux.alibaba.com>
Cc: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
Cc: Toke Høiland-Jørgensen <toke@redhat.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: cake@lists.bufferbloat.net
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
 net/sched/sch_cake.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
index 6cc3ab1..90ef7cc 100644
--- a/net/sched/sch_cake.c
+++ b/net/sched/sch_cake.c
@@ -1768,7 +1768,7 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 						      q->avg_window_begin));
 			u64 b = q->avg_window_bytes * (u64)NSEC_PER_SEC;
 
-			do_div(b, window_interval);
+			b = div64_u64(b, window_interval);
 			q->avg_peak_bandwidth =
 				cake_ewma(q->avg_peak_bandwidth, b,
 					  b > q->avg_peak_bandwidth ? 2 : 8);
-- 
1.8.3.1

Re: [Cake] [PATCH] sch_cake: avoid possible divide by zero in cake_enqueue()

[Jonathan Morton]

> On 2 Jan, 2020, at 11:21 am, Wen Yang <wenyang@linux.alibaba.com> wrote:
> 
> The variables 'window_interval' is u64 and do_div()
> truncates it to 32 bits, which means it can test
> non-zero and be truncated to zero for division.
> The unit of window_interval is nanoseconds,
> so its lower 32-bit is relatively easy to exceed.
> Fix this issue by using div64_u64() instead.

That might actually explain a few things.  I approve.

Honestly the *correct* fix is for the compiler to implement division in a way that doesn't require substituting it with function calls.  As this shows, it's error-prone to do this manually.

 - Jonathan Morton

Re: [PATCH] sch_cake: avoid possible divide by zero in cake_enqueue()

[Toke Høiland-Jørgensen]

Wen Yang <wenyang@linux.alibaba.com> writes:

> The variables 'window_interval' is u64 and do_div()
> truncates it to 32 bits, which means it can test
> non-zero and be truncated to zero for division.
> The unit of window_interval is nanoseconds,
> so its lower 32-bit is relatively easy to exceed.
> Fix this issue by using div64_u64() instead.
>
> Fixes: 7298de9cd725 ("sch_cake: Add ingress mode")
> Signed-off-by: Wen Yang <wenyang@linux.alibaba.com>
> Cc: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
> Cc: Toke Høiland-Jørgensen <toke@redhat.com>
> Cc: David S. Miller <davem@davemloft.net>
> Cc: Cong Wang <xiyou.wangcong@gmail.com>
> Cc: cake@lists.bufferbloat.net
> Cc: netdev@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org

Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>

Re: [PATCH] sch_cake: avoid possible divide by zero in cake_enqueue()

[David Miller]

From: Wen Yang <wenyang@linux.alibaba.com>
Date: Thu,  2 Jan 2020 17:21:43 +0800

> The variables 'window_interval' is u64 and do_div()
> truncates it to 32 bits, which means it can test
> non-zero and be truncated to zero for division.
> The unit of window_interval is nanoseconds,
> so its lower 32-bit is relatively easy to exceed.
> Fix this issue by using div64_u64() instead.
> 
> Fixes: 7298de9cd725 ("sch_cake: Add ingress mode")
> Signed-off-by: Wen Yang <wenyang@linux.alibaba.com>

Applied and queued up for -stable.