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

[Nikolay Aleksandrov <nikolay@cumulusnetworks.com>]

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>

> diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
> index 83490bf73a13..4c4a93abde68 100644
> --- a/net/bridge/br_multicast.c
> +++ b/net/bridge/br_multicast.c
> @@ -1000,21 +1000,21 @@ static int br_ip6_multicast_mld2_report(struct net_bridge *br,
>  	num = ntohs(icmp6h->icmp6_dataun.un_data16[1]);
>  	len = skb_transport_offset(skb) + sizeof(*icmp6h);
>  
>  	for (i = 0; i < num; i++) {
>  		__be16 *_nsrcs, __nsrcs;
>  		u16 nsrcs;
>  
>  		nsrcs_offset = len + offsetof(struct mld2_grec, grec_nsrcs);
>  
>  		if (skb_transport_offset(skb) + ipv6_transport_len(skb) <
> -		    nsrcs_offset + sizeof(_nsrcs))
> +		    nsrcs_offset + sizeof(__nsrcs))
>  			return -EINVAL;
>  
>  		_nsrcs = skb_header_pointer(skb, nsrcs_offset,
>  					    sizeof(__nsrcs), &__nsrcs);
>  		if (!_nsrcs)
>  			return -EINVAL;
>  
>  		nsrcs = ntohs(*_nsrcs);
>  		grec_len = struct_size(grec, grec_src, nsrcs);
>  
>