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From:   Eric Dumazet <edumazet@google.com>
Date:   Thu, 21 Sep 2023 13:09:21 +0200
Message-ID: <CANn89i+wUq5R2nFO8eGLp7=8Y5OiJ0fwjR+ES74gk1X4k9r0rw@mail.gmail.com>
Subject: Re: [PATCH net v3] ipv4, ipv6: Fix handling of transhdrlen in __ip{,6}_append_data()
To:     David Howells <dhowells@redhat.com>
Cc:     netdev@vger.kernel.org,
        syzbot+62cbf263225ae13ff153@syzkaller.appspotmail.com,
        Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        David Ahern <dsahern@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Jakub Kicinski <kuba@kernel.org>, bpf@vger.kernel.org,
        syzkaller-bugs@googlegroups.com, linux-kernel@vger.kernel.org
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On Thu, Sep 21, 2023 at 12:41=E2=80=AFPM David Howells <dhowells@redhat.com=
> 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 alrea=
dy
> partially occupied skbuff.  The warning occurs when 'copy' is larger than
> the amount of data in the message iterator.  This is because the requeste=
d
> length includes the transport header length when it shouldn't.  This can =
be
> triggered by, for example:
>
>         sfd =3D 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 buil=
ds
> the UDP packet itself.
>
> Fixes: a32e0eec7042 ("l2tp: introduce L2TPv3 IP encapsulation support for=
 IPv6")
> Reported-by: syzbot+62cbf263225ae13ff153@syzkaller.appspotmail.com
> Link: https://lore.kernel.org/r/0000000000001c12b30605378ce8@google.com/
> Suggested-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Eric Dumazet <edumazet@google.com>
> cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> cc: "David S. Miller" <davem@davemloft.net>
> cc: David Ahern <dsahern@kernel.org>
> cc: Paolo Abeni <pabeni@redhat.com>
> cc: Jakub Kicinski <kuba@kernel.org>
> cc: netdev@vger.kernel.org
> cc: bpf@vger.kernel.org
> cc: syzkaller-bugs@googlegroups.com
> ---

Looks safer indeed, thanks to you and Willem !

Reviewed-by: Eric Dumazet <edumazet@google.com>