Re: [PATCH net] net: l2tp: reduce log level when passing up invalid packets

On  Mon, Feb 22, 2021 at 14:31:38 -0800, Jakub Kicinski wrote:

On Mon, 22 Feb 2021 17:40:16 +0100 Matthias Schiffer wrote:

quoted

quoted

quoted

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.

Yes, I agree with this argument.