netdev - Re: [PATCH net-next 2/2] net: sched: protect against stack overflow in TC act

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Message-ID: <20190625091507.pwtingx6yk4ltmbo@breakpoint.cc>
Date:   Tue, 25 Jun 2019 11:15:07 +0200
From:   Florian Westphal <fw@...len.de>
To:     John Hurley <john.hurley@...ronome.com>
Cc:     Eyal Birger <eyal.birger@...il.com>,
        Linux Netdev List <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Florian Westphal <fw@...len.de>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Simon Horman <simon.horman@...ronome.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        oss-drivers@...ronome.com, shmulik@...anetworks.com
Subject: Re: [PATCH net-next 2/2] net: sched: protect against stack overflow
 in TC act_mirred

John Hurley <john.hurley@...ronome.com> wrote:
> Hi Eyal,
> The value of 4 is basically a revert to what it was on older kernels
> when TC had a TTL value in the skb:
> https://elixir.bootlin.com/linux/v3.19.8/source/include/uapi/linux/pkt_cls.h#L97

IIRC this TTL value was not used ever.

> I also found with my testing that a value greater than 4 was sailing
> close to the edge.
> With a larger value (on my system anyway), I could still trigger a
> stack overflow here.
> I'm not sure on the history of why a value of 4 was selected here but
> it seems to fall into line with my findings.
> Is there a hard requirement for >4 recursive calls here?

One alternative would be to (instead of dropping the skb), to
decrement the ttl and use netif_rx() instead.