netdev - Re: [PATCH net] udp: Fix __ip_append_data()'s handling of MSG_SPLICE

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Date: Tue, 01 Aug 2023 17:11:49 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: David Howells <dhowells@...hat.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: dhowells@...hat.com, 
 netdev@...r.kernel.org, 
 syzbot+f527b971b4bdc8e79f9e@...kaller.appspotmail.com, 
 bpf@...r.kernel.org, 
 brauner@...nel.org, 
 davem@...emloft.net, 
 dsahern@...nel.org, 
 edumazet@...gle.com, 
 kuba@...nel.org, 
 pabeni@...hat.com, 
 axboe@...nel.dk, 
 viro@...iv.linux.org.uk, 
 linux-fsdevel@...r.kernel.org, 
 syzkaller-bugs@...glegroups.com, 
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] udp: Fix __ip_append_data()'s handling of
 MSG_SPLICE_PAGES

David Howells wrote:
> Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:
> 
> > __ip6_append_data probably needs the same.
> 
> Now that's interesting.  __ip6_append_data() has a check for this and returns
> -EINVAL in this case:
> 
> 		copy = datalen - transhdrlen - fraggap - pagedlen;
> 		if (copy < 0) {
> 			err = -EINVAL;
> 			goto error;
> 		}
> 
> but should I bypass that check for MSG_SPLICE_PAGES?  It hits the check when
> it should be able to get past it.  The code seems to go back to prehistoric
> times, so I'm not sure why it's there.

Argh, saved by inconsistency between the two stacks.

I don't immediately understand the race that caused this code to move,
in commit 232cd35d0804 ("ipv6: fix out of bound writes in __ip6_append_data()").
Maybe a race with a mtu update?

Technically there is no Fixes tag to apply, so this would not be a fix
for net.

If we want equivalent behavior, a patch removing this branch is probably
best sent to net-next, in a way that works from the start.