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

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Date:   Mon, 6 Jul 2020 12:13:00 +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 11:18:36PM +0300, Nikolay Aleksandrov wrote:
> > > 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.
> 

Ah, okay, that's what you mean! You're right, technically the
ipv6_mc_may_pull() later would cover it, too. And it should work,
even if nsrcs were outside of the IPv6 packet and had a bogus
value. (I think.)

My brain linearly parsing the parser code would probably get a bit
confused initially, as it'd look like a bit like a bug. But the
current check for nsrcs might look confusing, too (q.e.d.).

So as you prefer, I'd be okay with both leaving that check for
consistency or removing it for brevity.