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

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Date:   Sun, 5 Jul 2020 21:49:15 +0200
From:   Linus Lüssing <linus.luessing@...3.blue>
To:     Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
Cc:     netdev@...r.kernel.org, Roopa Prabhu <roopa@...ulusnetworks.com>,
        Martin Weinelt <martin@...uxlounge.net>,
        "David S . Miller" <davem@...emloft.net>,
        bridge@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] bridge: mcast: Fix MLD2 Report IPv6 payload length
 check

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@...3.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@...ulusnetworks.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?