Re: [PATCH v2] netem: fix zero division in tabledist

[Stephen Hemminger]

On Wed, 28 Oct 2020 17:07:31 +0000
Aleksandr Nogikh <aleksandrnogikh@xxxxxxxxx> wrote:

> From: Aleksandr Nogikh <nogikh@xxxxxxxxxx>
> 
> Currently it is possible to craft a special netlink RTM_NEWQDISC
> command that can result in jitter being equal to 0x80000000. It is
> enough to set the 32 bit jitter to 0x02000000 (it will later be
> multiplied by 2^6) or just set the 64 bit jitter via
> TCA_NETEM_JITTER64. This causes an overflow during the generation of
> uniformly distributed numbers in tabledist(), which in turn leads to
> division by zero (sigma != 0, but sigma * 2 is 0).
> 
> The related fragment of code needs 32-bit division - see commit
> 9b0ed89 ("netem: remove unnecessary 64 bit modulus"), so switching to
> 64 bit is not an option.
> 
> Fix the issue by keeping the value of jitter within the range that can
> be adequately handled by tabledist() - [0;INT_MAX]. As negative std
> deviation makes no sense, take the absolute value of the passed value
> and cap it at INT_MAX. Inside tabledist(), switch to unsigned 32 bit
> arithmetic in order to prevent overflows.
> 
> Signed-off-by: Aleksandr Nogikh <nogikh@xxxxxxxxxx>
> Reported-by: syzbot+ec762a6342ad0d3c0d8f@xxxxxxxxxxxxxxxxxxxxxxxxx

Acked-by: Stephen Hemminger <stephen@xxxxxxxxxxxxxxxxxx>