Re: [PATCH net v3] ipv4, ipv6: Fix handling of transhdrlen in __ip{,6}_append_data()

[Eric Dumazet]

On Thu, Sep 21, 2023 at 12:41 PM David Howells <dhowells@xxxxxxxxxx> wrote:
>
>
> Including the transhdrlen in length is a problem when the packet is
> partially filled (e.g. something like send(MSG_MORE) happened previously)
> when appending to an IPv4 or IPv6 packet as we don't want to repeat the
> transport header or account for it twice.  This can happen under some
> circumstances, such as splicing into an L2TP socket.
>
> The symptom observed is a warning in __ip6_append_data():
>
>     WARNING: CPU: 1 PID: 5042 at net/ipv6/ip6_output.c:1800 __ip6_append_data.isra.0+0x1be8/0x47f0 net/ipv6/ip6_output.c:1800
>
> that occurs when MSG_SPLICE_PAGES is used to append more data to an already
> partially occupied skbuff.  The warning occurs when 'copy' is larger than
> the amount of data in the message iterator.  This is because the requested
> length includes the transport header length when it shouldn't.  This can be
> triggered by, for example:
>
>         sfd = socket(AF_INET6, SOCK_DGRAM, IPPROTO_L2TP);
>         bind(sfd, ...); // ::1
>         connect(sfd, ...); // ::1 port 7
>         send(sfd, buffer, 4100, MSG_MORE);
>         sendfile(sfd, dfd, NULL, 1024);
>
> Fix this by only adding transhdrlen into the length if the write queue is
> empty in l2tp_ip6_sendmsg(), analogously to how UDP does things.
>
> l2tp_ip_sendmsg() looks like it won't suffer from this problem as it builds
> the UDP packet itself.
>
> Fixes: a32e0eec7042 ("l2tp: introduce L2TPv3 IP encapsulation support for IPv6")
> Reported-by: syzbot+62cbf263225ae13ff153@xxxxxxxxxxxxxxxxxxxxxxxxx
> Link: https://lore.kernel.org/r/0000000000001c12b30605378ce8@xxxxxxxxxx/
> Suggested-by: Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx>
> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> cc: Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx>
> cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> cc: David Ahern <dsahern@xxxxxxxxxx>
> cc: Paolo Abeni <pabeni@xxxxxxxxxx>
> cc: Jakub Kicinski <kuba@xxxxxxxxxx>
> cc: netdev@xxxxxxxxxxxxxxx
> cc: bpf@xxxxxxxxxxxxxxx
> cc: syzkaller-bugs@xxxxxxxxxxxxxxxx
> ---

Looks safer indeed, thanks to you and Willem !

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>