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

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

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.

Toshiaki Makita