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: <20220717134610.k3nw6mam256yxj37@skbuf>
Date:   Sun, 17 Jul 2022 16:46:10 +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

Hi Hans,

On Fri, Jul 01, 2022 at 06:07:10PM +0200, Hans S wrote:
> On Fri, Jul 1, 2022 at 3:51 PM Ido Schimmel <idosch@...dia.com> wrote:
> >
> > On Fri, Jul 01, 2022 at 09:47:24AM +0200, Hans S wrote:
> > > One question though... wouldn't it be an issue that the mentioned
> > > option setting is bridge wide, while the patch applies a per-port
> > > effect?
> >
> > Why would it be an issue? To me, the bigger issue is changing the
> > semantics of "locked" in 5.20 compared to previous kernels.
> >
> > What is even the use case for enabling learning when the port is locked?
> > In current kernels, only SAs from link local traffic will be learned,
> > but with this patch, nothing will be learned. So why enable learning in
> > the first place? As an administrator, I mark a port as "locked" so that
> > only traffic with SAs that I configured will be allowed. Enabling
> > learning when the port is locked seems to defeat the purpose?
> >
> > It would be helpful to explain why mv88e6xxx needs to have learning
> > enabled in the first place. IIUC, the software bridge can function
> > correctly with learning disabled. It might be better to solve this in
> > mv88e6xxx so that user space would not need to enable learning on the SW
> > bridge and then work around issues caused by it such as learning from
> > link local traffic.
> 
> There is several issues when learning is turned off with the mv88e6xxx driver:
> 
> Mac-Auth requires learning turned on, otherwise there will be no miss
> violation interrupts afair.
> Refreshing of ATU entries does not work with learning turn off, as the
> PAV is set to zero when learning is turned off.
> This then further eliminates the use of the HoldAt1 feature and
> age-out interrupts.
> 
> With dynamic ATU entries (an upcoming patch set), an authorized unit
> gets a dynamic ATU entry, and if it goes quiet for 5 minutes, it's
> entry will age out and thus get removed.
> That also solves the port relocation issue as if a device relocates to
> another port it will be able to get access again after 5 minutes.

I think the discussion derailed at this exact point, when you responded
that "Mac-Auth requires learning turned on".

What precise feature do you describe when you say "Mac-Auth"? Do you
mean 802.1X MAC-based authentication in general (where data plane
packets on a locked port are dropped unless their MAC SA is in the FDB,
and populating the FDB is *entirely* up to user space, there aren't any
locked FDB entries on locked ports), or MAC authentication *bypass*
(where the kernel auto-adds locked FDB entries on locked ports)?

I *think* it's just the bypass that requires learning in mv88e6xxx.
But the bypass (the feature where the kernel auto-adds locked FDB
entries on locked ports) doesn't exist in net-next.

Here, what happens is that a locked port learns the MAC SA from the
traffic it didn't drop, i.e. link-local. In other words, the bridge
behaves as expected and instructed: +locked +learning will cause just
that. It's the administrator's fault for not disabling learning.
It's also the mv88e6xxx driver's fault for not validating the "locked" +
"learning" brport flag *combination* until it properly supports "+locked
+learning" (the feature you are currently working on).

I'm still confused why we don't just say that "+locked -learning" means
plain 802.1X, "+locked +learning" means MAB where we learn locked FDB entries.