netdev - Re: [syzbot] [net?] WARNING in __ip6_append

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Date: Mon, 18 Sep 2023 15:46:13 +0100
From: David Howells <dhowells@...hat.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: dhowells@...hat.com, Eric Dumazet <edumazet@...gle.com>,
    syzbot <syzbot+62cbf263225ae13ff153@...kaller.appspotmail.com>,
    bpf@...r.kernel.org, davem@...emloft.net, dsahern@...nel.org,
    kuba@...nel.org, linux-kernel@...r.kernel.org,
    netdev@...r.kernel.org, pabeni@...hat.com,
    syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [net?] WARNING in __ip6_append_data

Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:

> 
> An ipv6 packet can carry 64KB of payload, so maxnonfragsize of 65535 + 40
> sounds correct. But payload length passed of 65536 is not (ignoring ipv6
> jumbograms). So that should probably trigger an EINVAL -- if that is indeed
> what the repro does.

The problem is that on entry to __ip6_append_data(), the length includes
transhdrlen.  However, this is a problem if we already have something in the
packet.  At that point, this fails:

			if (WARN_ON_ONCE(copy > msg->msg_iter.count))
				goto error;

because copy includes transhdrlen.

David