Re: Bug in MACSec - stops passing traffic after approx 5TB

[Josh Coombs <jcoombs@staff.gwi.net>]

I've got wpa_supplicant working with macsec on Fedora, my test bed has
shuffled 16 billion packets so far without interruption.  I am a bit
concerned that I've just pushed the resource exhaustion issue down the
road though, looking at the output of ip macsec show I see four SAs
for TX and RX, it appears to negotiate a new pair every 3 to 3.5
billion packets.  It doesn't appear to be ripping down old SAs.  What
happens when available SA slots run out?

Joshua Coombs
GWI

office 207-494-2140
www.gwi.net

On Mon, Oct 15, 2018 at 11:45 AM Josh Coombs <jcoombs@staff.gwi.net> wrote:
>
> And confirmed, starting with a high packet number results in a very
> short testbed run, 296 packets and then nothing, just as you surmised.
> Sorry for raising the alarm falsely.  Looks like I need to roll my own
> build of wpa_supplicant as the ubuntu builds don't include the macsec
> driver, haven't tested Gentoo's ebuilds yet to see if they do.
>
> Josh Coombs
>
> On Sun, Oct 14, 2018 at 4:52 PM Josh Coombs <jcoombs@staff.gwi.net> wrote:
> >
> > On Sun, Oct 14, 2018 at 4:24 PM Sabrina Dubroca <sd@queasysnail.net> wrote:
> > >
> > > 2018-10-14, 10:59:31 -0400, Josh Coombs wrote:
> > > > I initially mistook this for a traffic control issue, but after
> > > > stripping the test beds down to just the MACSec component, I can still
> > > > replicate the issue.  After approximately 5TB of transfer / 4 billion
> > > > packets over a MACSec link it stops passing traffic.
> > >
> > > I think you're just hitting packet number exhaustion. After 2^32
> > > packets, the packet number would wrap to 0 and start being reused,
> > > which breaks the crypto used by macsec. Before this point, you have to
> > > add a new SA, and tell the macsec device to switch to it.
> >
> > I had not considered that, I naively thought as long as I didn't
> > specify a replay window, it'd roll the PN over on it's own and life
> > would be good.  I'll test that theory tomorrow, should be easy to
> > prove out.
> >
> > > That's why you should be using wpa_supplicant. It will monitor the
> > > growth of the packet number, and handle the rekey for you.
> >
> > Thank you for the heads up, I'll read up on this as well.
> >
> > Josh C