Re: [PATCH v2] netfilter: xt_hashlimit: Fix integer divide round to zero.

From: Pablo Neira Ayuso
Date: Tue Feb 21 2017 - 07:49:16 EST

On Mon, Feb 06, 2017 at 11:50:33PM +0100, Alban Browaeys wrote:
> Diving the divider by the multiplier before applying to the input.
> When this would "divide by zero", divide the multiplier by the divider
> first then multiply the input by this value.
> Currently user2creds outputs zero when input value is bigger than the
> number of slices and lower than scale.
> This as then user input is applied an integer divide operation to
> a number greater than itself (scale).
> That rounds up to zero, then we mulitply zero by the credits slice size.
> iptables -t filter -I INPUT --protocol tcp --match hashlimit
> --hashlimit 40/second --hashlimit-burst 20 --hashlimit-mode srcip
> --hashlimit-name syn-flood --jump RETURN
> thus trigger the overflow detection code:
> xt_hashlimit: overflow, try lower: 25000/20
> (25000 as hashlimit avd and 20 the burst)
> Here:
> 134217 slices of (HZ * CREDITS_PER_JIFFY) size.
> 500000 is user input value
> 1000000 is XT_HASHLIMIT_SCALE_v2
> gives: 0 as user2creds output
> Setting burst to "1" typically solve the issue ...
> but setting it to "40" does too !
> This is on 32bit arch calling into revision 2 of hashlimit.

Applied to nf.git, thanks.