Re: [RFC net-next v2 1/1] net: sched: protect against loops in TC filter hooks

[Florian Westphal <fw@strlen.de>]

John Hurley <john.hurley@netronome.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. This can lead to stack overflow panics among other
> things.
> 
> Previous versions of the kernel (<4.2) had a TTL count in the tc_verd skb
> member that protected against loops. This was removed and the tc_verd
> variable replaced by bit fields.
> 
> Extend the TC fields in the skb with an additional 2 bits to store the TC
> hop count. This should use existing allocated memory in the skb.
> 
> Add the checking and setting of the new hop count to the act_mirred file
> given that it is the source of the loops. This means that the code
> additions are not in the main datapath.
> 
> v1->v2
> - change from per cpu counter to per skb tracking (Jamal)
> - move check/update from fast path to act_mirred (Daniel)
> 
> Signed-off-by: John Hurley <john.hurley@netronome.com>
> ---
>  include/linux/skbuff.h | 2 ++
>  net/sched/act_mirred.c | 9 +++++++++
>  2 files changed, 11 insertions(+)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 2ee5e63..f0dbc5b 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -645,6 +645,7 @@ typedef unsigned char *sk_buff_data_t;
>   *	@tc_at_ingress: used within tc_classify to distinguish in/egress
>   *	@tc_redirected: packet was redirected by a tc action
>   *	@tc_from_ingress: if tc_redirected, tc_at_ingress at time of redirect
> + *	@tc_hop_count: hop counter to prevent packet loops
>   *	@peeked: this packet has been seen already, so stats have been
>   *		done for it, don't do them again
>   *	@nf_trace: netfilter packet trace flag
> @@ -827,6 +828,7 @@ struct sk_buff {
>  	__u8			tc_at_ingress:1;
>  	__u8			tc_redirected:1;
>  	__u8			tc_from_ingress:1;
> +	__u8			tc_hop_count:2;

I dislike this, why can't we just use a pcpu counter?

The only problem is with recursion/nesting; whenever we
hit something that queues the skb for later we're safe.

We can't catch loops in real (physical) setups either,
e.g. bridge looping back on itself.