netdev - Re: [PATCH net-next 1/2] bridge: Add MAC Authentication Bypass (MAB) support

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Date:   Thu, 3 Nov 2022 23:18:39 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Ido Schimmel <idosch@...dia.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bridge@...ts.linux-foundation.org" 
        <bridge@...ts.linux-foundation.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "roopa@...dia.com" <roopa@...dia.com>,
        "razor@...ckwall.org" <razor@...ckwall.org>,
        "netdev@...io-technology.com" <netdev@...io-technology.com>,
        "mlxsw@...dia.com" <mlxsw@...dia.com>
Subject: Re: [PATCH net-next 1/2] bridge: Add MAC Authentication Bypass (MAB)
 support

On Tue, Nov 01, 2022 at 09:39:21PM +0200, Ido Schimmel wrote:
> From: "Hans J. Schultz" <netdev@...io-technology.com>
> 
> Hosts that support 802.1X authentication are able to authenticate
> themselves by exchanging EAPOL frames with an authenticator (Ethernet
> bridge, in this case) and an authentication server. Access to the
> network is only granted by the authenticator to successfully
> authenticated hosts.
> 
> The above is implemented in the bridge using the "locked" bridge port
> option. When enabled, link-local frames (e.g., EAPOL) can be locally
> received by the bridge, but all other frames are dropped unless the host
> is authenticated. That is, unless the user space control plane installed
> an FDB entry according to which the source address of the frame is
> located behind the locked ingress port. The entry can be dynamic, in
> which case learning needs to be enabled so that the entry will be
> refreshed by incoming traffic.
> 
> There are deployments in which not all the devices connected to the
> authenticator (the bridge) support 802.1X. Such devices can include
> printers and cameras. One option to support such deployments is to
> unlock the bridge ports connecting these devices, but a slightly more
> secure option is to use MAB. When MAB is enabled, the MAC address of the
> connected device is used as the user name and password for the
> authentication.
> 
> For MAB to work, the user space control plane needs to be notified about
> MAC addresses that are trying to gain access so that they will be
> compared against an allow list. This can be implemented via the regular
> learning process with the sole difference that learned FDB entries are
> installed with a new "locked" flag indicating that the entry cannot be
> used to authenticate the device. The flag cannot be set by user space,
> but user space can clear the flag by replacing the entry, thereby
> authenticating the device.
> 
> Locked FDB entries implement the following semantics with regards to
> roaming, aging and forwarding:
> 
> 1. Roaming: Locked FDB entries can roam to unlocked (authorized) ports,
>    in which case the "locked" flag is cleared. FDB entries cannot roam
>    to locked ports regardless of MAB being enabled or not. Therefore,
>    locked FDB entries are only created if an FDB entry with the given {MAC,
>    VID} does not already exist. This behavior prevents unauthenticated
>    devices from disrupting traffic destined to already authenticated
>    devices.
> 
> 2. Aging: Locked FDB entries age and refresh by incoming traffic like
>    regular entries.
> 
> 3. Forwarding: Locked FDB entries forward traffic like regular entries.
>    If user space detects an unauthorized MAC behind a locked port and
>    wishes to prevent traffic with this MAC DA from reaching the host, it
>    can do so using tc or a different mechanism.

In other words, a user space MAB daemon has a lot of extra work to do.
I'm willing to bet it's going to cut 90% of those corners ;) anyway...

> 
> Enable the above behavior using a new bridge port option called "mab".
> It can only be enabled on a bridge port that is both locked and has
> learning enabled. Locked FDB entries are flushed from the port once MAB
> is disabled. A new option is added because there are pure 802.1X
> deployments that are not interested in notifications about locked FDB
> entries.
> 
> Signed-off-by: Hans J. Schultz <netdev@...io-technology.com>
> Signed-off-by: Ido Schimmel <idosch@...dia.com>
> ---

Reviewed-by: Vladimir Oltean <vladimir.oltean@....com>