From: Stephen Hemminger <stephen@networkplumber.org>
To: Aleksandr Nogikh <aleksandrnogikh@gmail.com>
Cc: jhs@mojatatu.com, xiyou.wangcong@gmail.com, jiri@resnulli.us,
davem@davemloft.net, kuba@kernel.org, andreyknvl@google.com,
dvyukov@google.com, elver@google.com, rdunlap@infradead.org,
dave.taht@gmail.com, edumazet@google.com, netdev@vger.kernel.org,
linux-kernel@vger.kernel.org,
Aleksandr Nogikh <nogikh@google.com>,
syzbot+ec762a6342ad0d3c0d8f@syzkaller.appspotmail.com
Subject: Re: [PATCH v2] netem: fix zero division in tabledist
Date: Wed, 28 Oct 2020 11:19:59 -0700 [thread overview]
Message-ID: <20201028111959.6ed6d2c2@hermes.local> (raw)
In-Reply-To: <20201028170731.1383332-1-aleksandrnogikh@gmail.com>
On Wed, 28 Oct 2020 17:07:31 +0000
Aleksandr Nogikh <aleksandrnogikh@gmail.com> wrote:
> From: Aleksandr Nogikh <nogikh@google.com>
>
> 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@google.com>
> Reported-by: syzbot+ec762a6342ad0d3c0d8f@syzkaller.appspotmail.com
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
next prev parent reply other threads:[~2020-10-28 22:18 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-28 17:07 [PATCH v2] netem: fix zero division in tabledist Aleksandr Nogikh
2020-10-28 18:19 ` Stephen Hemminger [this message]
2020-10-29 18:46 ` Jakub Kicinski
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