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

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Toshiaki Makita <toshiaki.makita1@gmail.com> writes:

> On 2020/07/07 7:44, Toke Høiland-Jørgensen wrote:
>> Daniel Borkmann <daniel@iogearbox.net> writes:
>>> 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@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>
>>>> ---
>>>>    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?
>> 
>> Don't think so. Wikipedia says this:
>> 
>>   802.1ad is upward compatible with 802.1Q. Although 802.1ad is limited
>>   to two tags, there is no ceiling on the standard limiting a single
>>   frame to more than two tags, allowing for growth in the protocol. In
>>   practice Service Provider topologies often anticipate and utilize
>>   frames having more than two tags.
>> 
>>> Why not 8 as max, for example (I'd probably even consider a depth like
>>> this as utterly broken setup ..)?
>> 
>> I originally went with 8, but chickened out after seeing how many places
>> call the parsing function. While I do agree that eight tags is... somewhat
>> excessive... I was trying to make absolutely sure no one would hit this
>> limit in normal use. See also https://xkcd.com/1172/ :)
>
> Considering that XMIT_RECURSION_LIMIT is 8, I also think 8 is sufficient.

Alright, fair enough, I'll send a v2 with a limit of 8 :)

-Toke