netdev - Re: [PATCH net-next v3 2/3] virtio_net: add optional flow dissection in virtio_net_hdr_to

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Date:   Thu, 3 Jun 2021 20:44:51 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Tanner Love <tannerlove.kernel@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Petar Penkov <ppenkov@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Tanner Love <tannerlove@...gle.com>,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH net-next v3 2/3] virtio_net: add optional flow dissection
 in virtio_net_hdr_to_skb

On Thu, Jun 3, 2021 at 7:56 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Tue, Jun 01, 2021 at 06:18:39PM -0400, Tanner Love wrote:
> > From: Tanner Love <tannerlove@...gle.com>
> >
> > Syzkaller bugs have resulted from loose specification of
> > virtio_net_hdr[1]. Enable execution of a BPF flow dissector program
> > in virtio_net_hdr_to_skb to validate the vnet header and drop bad
> > input.
> >
> > The existing behavior of accepting these vnet headers is part of the
> > ABI.
>
> So ?
> It's ok to fix ABI when it's broken.
> The whole feature is a way to workaround broken ABI with additional
> BPF based validation.
> It's certainly a novel idea.
> I've never seen BPF being used to fix the kernel bugs.
> But I think the better way forward is to admit that vnet ABI is broken
> and fix it in the kernel with proper validation.
> BPF-based validation is a band-aid. The out of the box kernel will
> stay broken and syzbot will continue to crash it.

The intention is not to use this to avoid kernel fixes.

There are three parts:

1. is the ABI broken and can we simply drop certain weird packets?
2. will we accept an extra packet parsing stage in
virtio_net_hdr_to_skb to find all the culprits?
3. do we want to add yet another parser in C when this is exactly what
the BPF flow dissector does well?

The virtio_net_hdr interface is more permissive than I think it was
intended. But weird edge cases like protocol 0 do occur. So I believe
it is simply too late to drop everything that is not obviously
correct.

More problematic is that some of the gray area cases are non-trivial
and require protocol parsing. Do the packet headers match the gso
type? Are subtle variants like IPv6/TCP with extension headers
supported or not?

We have previously tried to add unconditional protocol parsing in
virtio_net_hdr_to_skb, but received pushback because this will cause
performance regressions, e.g., for vm-to-vm forwarding.

Combined, that's why I think BPF flow dissection is the right approach
here. It is optional, can be as pedantic as the admin wants / workload
allows (e.g., dropping UFO altogether) and ensures protocol parsing
itself is robust. Even if not enabled continuously, offers a quick way
to patch a vulnerability when it is discovered. This is the same
reasoning of the existing BPF flow dissector.

It is not intended as an alternative to subsequently fixing a bug in
the kernel path.