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Message-ID: <CAF=yD-+kRXmwuKHVrUUL6oBPhWiPKucm_5-Y+YM==9Bp3DQiGQ@mail.gmail.com>
Date: Tue, 19 Sep 2023 16:32:41 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: David Howells <dhowells@...hat.com>
Cc: netdev@...r.kernel.org,
syzbot+62cbf263225ae13ff153@...kaller.appspotmail.com,
Eric Dumazet <edumazet@...gle.com>, "David S. Miller" <davem@...emloft.net>,
David Ahern <dsahern@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Jakub Kicinski <kuba@...nel.org>,
bpf@...r.kernel.org, syzkaller-bugs@...glegroups.com,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] ipv4, ipv6: Fix handling of transhdrlen in __ip{,6}_append_data()
On Tue, Sep 19, 2023 at 12:12 PM David Howells <dhowells@...hat.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 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 pushing the addition of transhdrlen into length down into
> __ip_append_data() and __ip6_append_data(), making it conditional on the
> write queue being empty (otherwise we just clear transhdrlen).
I'm afraid that we might start to dig an ever deeping hole.
The proposed fix is non-trivial, and changes not just the new path
that observes the issue (MSG_SPLICE_PAGES), but also the other more
common paths that exercise __ip6_append_data.
There is significant risk to introduce an unintended side effect
requiring a follow-up fix. Because this function is notoriously
complex, multiplexing a lot of behavior: with and without transport
headers, edge cases like fragmentation, MSG_MORE, absence of
scatter-gather, ....
Does the issue discovered only affect MSG_SPLICE_PAGES or can it
affect other paths too? If the first, it possible to create a more
targeted fix that can trivially be seen to not affect code prior to
introduction of splice pages?
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