netdev - Re: [PATCH net-next 00/27] Remove VLAN CFI bit abuse

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]

Message-ID: <20161214020017.c6u3l4dttkojscq2@rere.qmqm.pl>
Date:   Wed, 14 Dec 2016 03:00:18 +0100
From:   Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net-next 00/27] Remove VLAN CFI bit abuse

On Tue, Dec 13, 2016 at 05:16:26PM -0800, Stephen Hemminger wrote:
> On Tue, 13 Dec 2016 01:12:32 +0100 (CET)
> Michał Mirosław <mirq-linux@...e.qmqm.pl> wrote:
> > This series removes an abuse of VLAN CFI bit in Linux networking stack.
> > Currently Linux always clears the bit on outgoing traffic and presents
> > it cleared to userspace (even via AF_PACKET/tcpdump when hw-accelerated).
> > 
> > This uses a new vlan_present bit in struct skbuff, and removes an assumption
> > that vlan_proto != 0 when VLAN tag is present.
> > 
> > As I can't test most of the driver changes, please look at them carefully.
> > 
> > The series is supposed to be bisect-friendly and that requires temporary
> > insertion of #define VLAN_TAG_PRESENT in BPF code to be able to split
> > JIT changes per architecture.
> 
> I wonder if CFI can every validly be non-zero in the modern world, on Hyper-V.
> There are no token ring devices and that seems to be the only use case where CFI would
> be non-zero. Unless someone is planning to reuse it a a protocol bit which seems
> like a really bad idea.
> 
> Maybe the right thing is to keep hard coded as zero and not start adding
> more untestable code conditions.
> 
> My recommendation would be get rid of VLAN_TAG_PRESENT, but don't preserve
> CFI bit.

According to Wikipedia page [1] on 802.1Q, CFI bit got already changed
to DEI (Drop eligible indicator) in 2011 revision of the IEEE standard.

I can't verify this, though.

Best Regards,
Michał Mirosław

[1] https://en.wikipedia.org/wiki/IEEE_802.1Q#Frame_format