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

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Date:   Tue, 25 Jun 2019 11:30:10 +0300
From:   Eyal Birger <eyal.birger@...il.com>
To:     John Hurley <john.hurley@...ronome.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, fw@...len.de,
        jhs@...atatu.com, simon.horman@...ronome.com,
        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

Hi John,

On Mon, 24 Jun 2019 23:13:36 +0100
John Hurley <john.hurley@...ronome.com> wrote:

> TC hooks allow the application of filters and actions to packets at
> both ingress and egress of the network stack. It is possible, with
> poor configuration, that this can produce loops whereby an ingress
> hook calls a mirred egress action that has an egress hook that
> redirects back to the first ingress etc. The TC core classifier
> protects against loops when doing reclassifies but there is no
> protection against a packet looping between multiple hooks and
> recursively calling act_mirred. This can lead to stack overflow
> panics.
> 
> Add a per CPU counter to act_mirred that is incremented for each
> recursive call of the action function when processing a packet. If a
> limit is passed then the packet is dropped and CPU counter reset.
> 
> Note that this patch does not protect against loops in TC datapaths.
> Its aim is to prevent stack overflow kernel panics that can be a
> consequence of such loops.
> 
> Signed-off-by: John Hurley <john.hurley@...ronome.com>
> Reviewed-by: Simon Horman <simon.horman@...ronome.com>
> ---
>  net/sched/act_mirred.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/net/sched/act_mirred.c b/net/sched/act_mirred.c
> index 8c1d736..c3fce36 100644
> --- a/net/sched/act_mirred.c
> +++ b/net/sched/act_mirred.c
> @@ -27,6 +27,9 @@
>  static LIST_HEAD(mirred_list);
>  static DEFINE_SPINLOCK(mirred_list_lock);
>  
> +#define MIRRED_RECURSION_LIMIT    4

Could you increase the limit to maybe 6 or 8? I am aware of cases where
mirred ingress is used for cascading several layers of logical network
interfaces and 4 seems a little limiting.

Thanks,
Eyal.