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

[Toshiaki Makita <toshiaki.makita1@gmail.com>]

On 2020/07/06 21:29, 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@gmail.com>
> Reported-by: Daniel Borkmann <daniel@iogearbox.net>
> Fixes: d7bf2ebebc2b ("sched: consistently handle layer3 header accesses in the presence of VLANs")
> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> ---
...
> @@ -623,13 +597,12 @@ static inline __be16 __vlan_get_protocol(struct sk_buff *skb, __be16 type,
>   			vlan_depth = ETH_HLEN;
>   		}
>   		do {
> -			struct vlan_hdr *vh;
> +			struct vlan_hdr vhdr, *vh;
>   
> -			if (unlikely(!pskb_may_pull(skb,
> -						    vlan_depth + VLAN_HLEN)))
> +			vh = skb_header_pointer(skb, vlan_depth, sizeof(vhdr), &vhdr);

Some drivers which use vlan_get_protocol to get IP protocol for checksum offload discards
packets when it cannot get the protocol.
I guess for such users this function should try to get protocol even if it is not in skb header?
I'm not sure such a case can happen, but since you care about this, you know real cases where
vlan tag can be in skb frags?

Toshiaki Makita