Re: [PATCH net] bridge: mcast: Fix MLD2 Report IPv6 payload length check

[Nikolay Aleksandrov <nikolay@cumulusnetworks.com>]

On 7/5/20 10:49 PM, Linus Lüssing wrote:
> On Sun, Jul 05, 2020 at 10:11:39PM +0300, Nikolay Aleksandrov wrote:
>> On 7/5/20 10:08 PM, Linus Lüssing wrote:
>>> On Sun, Jul 05, 2020 at 09:33:13PM +0300, Nikolay Aleksandrov wrote:
>>>> On 05/07/2020 21:22, Linus Lüssing wrote:
>>>>> Commit e57f61858b7c ("net: bridge: mcast: fix stale nsrcs pointer in
>>>>> igmp3/mld2 report handling") introduced a small bug which would potentially
>>>>> lead to accepting an MLD2 Report with a broken IPv6 header payload length
>>>>> field.
>>>>>
>>>>> The check needs to take into account the 2 bytes for the "Number of
>>>>> Sources" field in the "Multicast Address Record" before reading it.
>>>>> And not the size of a pointer to this field.
>>>>>
>>>>> Fixes: e57f61858b7c ("net: bridge: mcast: fix stale nsrcs pointer in igmp3/mld2 report handling")
>>>>> Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
>>>>> ---
>>>>>    net/bridge/br_multicast.c | 2 +-
>>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>
>>>> I'd rather be more concerned with it rejecting a valid report due to wrong size. The ptr
>>>> size would always be bigger. :)
>>>>
>>>> Thanks!
>>>> Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
>>>
>>> Aiy, you're right, it's the other way round. I'll update the
>>> commit message and send a v2 in a minute, with your Acked-by
>>> included.
>>>
>>
>> By the way, I can't verify at the moment, but I think we can drop that whole
>> hunk altogether since skb_header_pointer() is used and it will simply return
>> an error if there isn't enough data for nsrcs.
>>
> 
> Hm, while unlikely, the IPv6 packet / header payload length might be
> shorter than the frame / skb size.
> 
> And even though it wouldn't crash reading over the IPv6 packet
> length, especially as skb_header_pointer() is used, I think we should
> still avoid reading over the size indicated by the IPv6 header payload
> length field, to stay protocol compliant.
> 
> Does that make sense?
> 

Sure, I just thought the ipv6_mc_may_pull() call after that includes those 2 bytes as well, so
we're covered. That is, it seems to be doing the same check with the full grec size included.