netdev - Re: [PATCH net] vlan: consolidate VLAN parsing code and limit max parsing depth

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Message-ID: <4f7b2b71-8b2a-3aea-637d-52b148af1802@iogearbox.net>
Date:   Mon, 6 Jul 2020 22:01:58 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        davem@...emloft.net
Cc:     netdev@...r.kernel.org, cake@...ts.bufferbloat.net,
        Davide Caratti <dcaratti@...hat.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Toshiaki Makita <toshiaki.makita1@...il.com>
Subject: Re: [PATCH net] vlan: consolidate VLAN parsing code and limit max
 parsing depth

On 7/6/20 2:29 PM, Toke Høiland-Jørgensen wrote:
> Toshiaki pointed out that we now have two very similar functions to extract
> the L3 protocol number in the presence of VLAN tags. And Daniel pointed out
> that the unbounded parsing loop makes it possible for maliciously crafted
> packets to loop through potentially hundreds of tags.
> 
> Fix both of these issues by consolidating the two parsing functions and
> limiting the VLAN tag parsing to an arbitrarily-chosen, but hopefully
> conservative, max depth of 32 tags. As part of this, switch over
> __vlan_get_protocol() to use skb_header_pointer() instead of
> pskb_may_pull(), to avoid the possible side effects of the latter and keep
> the skb pointer 'const' through all the parsing functions.
> 
> Reported-by: Toshiaki Makita <toshiaki.makita1@...il.com>
> Reported-by: Daniel Borkmann <daniel@...earbox.net>
> Fixes: d7bf2ebebc2b ("sched: consistently handle layer3 header accesses in the presence of VLANs")
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---
>   include/linux/if_vlan.h | 57 ++++++++++++++++-------------------------
>   1 file changed, 22 insertions(+), 35 deletions(-)
> 
> diff --git a/include/linux/if_vlan.h b/include/linux/if_vlan.h
> index 427a5b8597c2..855d16192e6a 100644
> --- a/include/linux/if_vlan.h
> +++ b/include/linux/if_vlan.h
> @@ -25,6 +25,8 @@
>   #define VLAN_ETH_DATA_LEN	1500	/* Max. octets in payload	 */
>   #define VLAN_ETH_FRAME_LEN	1518	/* Max. octets in frame sans FCS */
>   
> +#define VLAN_MAX_DEPTH	32		/* Max. number of nested VLAN tags parsed */
> +

Any insight on limits of nesting wrt QinQ, maybe from spec side? Why not 8 as max, for
example (I'd probably even consider a depth like this as utterly broken setup ..)?

Thanks,
Daniel