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Date:   Mon, 22 Feb 2021 14:31:38 -0800
From:   Jakub Kicinski <kuba@kernel.org>
To:     Matthias Schiffer <mschiffer@universe-factory.net>
Cc:     Tom Parkin <tparkin@katalix.com>, netdev@vger.kernel.org,
        "David S. Miller" <davem@davemloft.net>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH net] net: l2tp: reduce log level when passing up invalid
 packets
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On Mon, 22 Feb 2021 17:40:16 +0100 Matthias Schiffer wrote:
> >> This will not be sufficient for my usecase: To stay compatible with older
> >> versions of fastd, I can't set the T flag in the first packet of the
> >> handshake, as it won't be known whether the peer has a new enough fastd
> >> version to understand packets that have this bit set. Luckily, the second
> >> handshake byte is always 0 in fastd's protocol, so these packets fail the
> >> tunnel version check and are passed to userspace regardless.
> >>
> >> I'm aware that this usecase is far outside of the original intentions of the
> >> code and can only be described as a hack, but I still consider this a
> >> regression in the kernel, as it was working fine in the past, without
> >> visible warnings.
> >>  
> > 
> > I'm sorry, but for the reasons stated above I disagree about it being
> > a regression.  
> 
> Hmm, is it common for protocol implementations in the kernel to warn about 
> invalid packets they receive? While L2TP uses connected sockets and thus 
> usually no unrelated packets end up in the socket, a simple UDP port scan 
> originating from the configured remote address/port will trigger the "short 
> packet" warning now (nmap uses a zero-length payload for UDP scans by 
> default). Log spam caused by a malicous party might also be a concern.

Indeed, seems like appropriate counters would be a good fit here? 
The prints are both potentially problematic for security and lossy.