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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Mon, 22 Feb 2021 14:31:38 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Matthias Schiffer <mschiffer@...verse-factory.net>
Cc:     Tom Parkin <tparkin@...alix.com>, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net: l2tp: reduce log level when passing up invalid
 packets

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.