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 16:56:12 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Tanner Love <tannerlove.kernel@...il.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemb@...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 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.