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

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Tue, 07 Jul 2020 12:57:47 +0200

Toshiaki Makita <[email-masked]> writes:

> 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 <[email-masked]>
>> Reported-by: Daniel Borkmann <[email-masked]>
>> Fixes: d7bf2ebebc2b ("sched: consistently handle layer3 header accesses in the presence of VLANs")
>> Signed-off-by: Toke Høiland-Jørgensen <[email-masked]>
>> ---
> ...
>> @@ -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?

skb_header_pointer() will still succeed in reading the data, it'll just
do so by copying it into the buffer on the stack (vhdr) instead of
moving the SKB data itself around...

-Toke