netdev - Re: [PATCH net-next v1 1/1] net: bridge: ensure that link-local traffic cannot unlock a locked port

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Message-ID: <20220721114540.ovm22rtnwqs77nfb@skbuf>
Date:   Thu, 21 Jul 2022 14:45:40 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Hans S <schultz.hans@...il.com>
Cc:     Ido Schimmel <idosch@...dia.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>, Jiri Pirko <jiri@...nulli.us>,
        Ivan Vecera <ivecera@...hat.com>,
        Roopa Prabhu <roopa@...dia.com>,
        Nikolay Aleksandrov <razor@...ckwall.org>,
        Shuah Khan <shuah@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Hans Schultz <schultz.hans+netdev@...il.com>,
        linux-kernel@...r.kernel.org, bridge@...ts.linux-foundation.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH net-next v1 1/1] net: bridge: ensure that link-local
 traffic cannot unlock a locked port

On Sun, Jul 17, 2022 at 09:20:57PM +0200, Hans S wrote:
> On Sun, Jul 17, 2022 at 8:38 PM Vladimir Oltean <olteanv@...il.com> wrote:
> >
> > On Sun, Jul 17, 2022 at 06:22:57PM +0200, Hans S wrote:
> > > On Sun, Jul 17, 2022 at 4:03 PM Vladimir Oltean <olteanv@...il.com> wrote:
> > >
> > > Yes, it creates an FDB entry in the bridge without the locked flag
> > > set, and sends an ADD_TO_DEVICE notice with it.
> > > And furthermore link-local packets include of course EAPOL packets, so
> > > that's why +learning is a problem.
> >
> > So if we fix that, and make the dynamically learned FDB entry be locked
> > because the port is locked (and offload them correctly in mv88e6xxx),
> > what would be the problem, exactly? The +learning is what would allow
> > these locked FDB entries to be created, and would allow the MAB to work.
> > User space may still decide to not authorize this address, and it will
> > remain locked.
> 
> The alternative is to have -learning and let the driver only enable
> the PAV to admit the interrupts, which is what this implementation
> does.
> The plus side of this is that having EAPOL packets triggering locked
> entries from the bridge side is not really so nice IMHO. In a
> situation with 802.1X and MAB on the same port, there will then not be
> any triggering of MAB when initiating the 802.1X session, which I
> think is the best option. It then also lessens the confusion between
> hostapd and the daemon that handles MAB sessions.

Why is it "not really so nice" to "trigger MAB" (in fact only to learn a
locked entry on a locked port) when initiating the 802.1X session?
You can disable link-local learning via the bridge option if you're
really bothered by that. When you have MAB enabled on an 802.1X port,
I think it's reasonable to expect that there will be some locked entries
which user space won't ever unlock via MAB. If those entries happen to
be created as a side effect of the normal EAPOL authentication process,
I don't exactly see where is the functional problem. This shouldn't
block EAPOL from proceeding any further, because this protocol uses
link-local packets which are classified as control traffic, and that
isn't subject to FDB lookups but rather always trapped to CPU, so locked
or not, it should still be received.

I'm only pointing out the obvious here, we need an opt in for MAB, and
the implemented behavior I've seen here kind of points to mapping this
to "+learning +locked", where the learning process creates locked FDB entries.