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

From: Pablo Neira Ayuso <[email-masked]>
Date: Mon, 6 Feb 2017 14:04:43 +0100

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 <[email-masked]>
> ---
>  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.