On Sat, Feb 04, 2017 at 11:47:31PM +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.
>
> Signed-off-by: Alban Browaeys <alban.browaeys@xxxxxxxxx>
> ---
> net/netfilter/xt_hashlimit.c | 18 ++++++++----------
> 1 file changed, 8 insertions(+), 10 deletions(-)
>
> diff --git a/net/netfilter/xt_hashlimit.c b/net/netfilter/xt_hashlimit.c
> index 10063408141d..df75ad643eef 100644
> --- a/net/netfilter/xt_hashlimit.c
> +++ b/net/netfilter/xt_hashlimit.c
> @@ -463,21 +463,19 @@ static u32 xt_hashlimit_len_to_chunks(u32 len)
> /* Precision saver. */
> static u64 user2credits(u64 user, int revision)
> {
> + /* Avoid overflow: divide the constant operands first */
> if (revision == 1) {
> - /* If multiplying would overflow... */
> - if (user > 0xFFFFFFFF / (HZ*CREDITS_PER_JIFFY_v1))
> - /* Divide first. */
> - return div64_u64(user, XT_HASHLIMIT_SCALE)
> - * HZ * CREDITS_PER_JIFFY_v1;
> + return div64_u64(user, div64_u64(XT_HASHLIMIT_SCALE,
> + HZ * CREDITS_PER_JIFFY_v1));
>
> - return div64_u64(user * HZ * CREDITS_PER_JIFFY_v1,
> + return user * div64_u64(HZ * CREDITS_PER_JIFFY_v1,
> XT_HASHLIMIT_SCALE);
I see two return statements here, the one coming later is
shadowed, this can't be right.
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