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

Vladimir Oltean
2022-03-25T14:00:21Z

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?