Re: [PATCH v2 net-next 2/4] net: switchdev: add support for offloading of fdb locked flag

From: Vladimir Oltean <olteanv@gmail.com>
To: Hans Schultz <schultz.hans@gmail.com>
Cc: davem@davemloft.net, kuba@kernel.org, netdev@vger.kernel.org,
	Andrew Lunn <andrew@lunn.ch>,
	Vivien Didelot <vivien.didelot@gmail.com>,
	Florian Fainelli <f.fainelli@gmail.com>,
	Jiri Pirko <jiri@resnulli.us>, Ivan Vecera <ivecera@redhat.com>,
	Roopa Prabhu <roopa@nvidia.com>,
	Nikolay Aleksandrov <razor@blackwall.org>,
	Shuah Khan <shuah@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Ido Schimmel <idosch@nvidia.com>,
	linux-kernel@vger.kernel.org, bridge@lists.linux-foundation.org,
	linux-kselftest@vger.kernel.org
Subject: Re: [PATCH v2 net-next 2/4] net: switchdev: add support for offloading of fdb locked flag
Date: Fri, 25 Mar 2022 16:00:03 +0200	[thread overview]
Message-ID: <20220325140003.a4w4hysqbzmrcxbq@skbuf> (raw)
In-Reply-To: <86fsn6uoqz.fsf@gmail.com>

On Fri, Mar 25, 2022 at 02:48:36PM +0100, Hans Schultz wrote:
> > If you'd cache the locked ATU entry in the mv88e6xxx driver, and you'd
> > notify switchdev only if the entry is new to the cache, then you'd
> > actually still achieve something major. Yes, the bridge FDB will contain
> > locked FDB entries that aren't in the ATU. But that's because your
> > printer has been silent for X seconds. The policy for the printer still
> > hasn't changed, as far as the mv88e6xxx, or bridge, software drivers are
> > concerned. If the unauthorized printer says something again after the
> > locked ATU entry expires, the mv88e6xxx driver will find its MAC SA
> > in the cache of denied addresses, and reload the ATU. What this
> > achieves
> 
> The driver will in this case just trigger a new miss violation and add
> the entry again I think.
> The problem with all this is that a malicious attack that spams the
> switch with random mac addresses will be able to DOS the device as any
> handling of the fdb will be too resource demanding. That is why it is
> needed to remove those fdb entries after a time out, which dynamic
> entries would serve.

An attacker sweeping through the 2^47 source MAC address range is a
problem regardless of the implementations proposed so far, no?
If unlimited growth of the mv88e6xxx locked ATU entry cache is a
concern (which it is), we could limit its size, and when we purge a
cached entry in software is also when we could emit a
SWITCHDEV_FDB_DEL_TO_BRIDGE for it, right?