Re: [PATCH v4 net-next 3/6] drivers: net: dsa: add locked fdb entry flag to drivers

[netdev@kapio-technology.com]

On 2022-07-24 13:10, Ido Schimmel wrote:
> 
>> In the mv88e6xxx offload implementation, the locked entries eventually
>> age out from time to time, practically giving the true owner of the 
>> MAC
>> address another chance every 5 minutes or so. In the pure software
>> implementation of locked FDB entries I'm not quite sure. It wouldn't
>> make much sense for the behavior to differ significantly though.
> 
> From what I can tell, the same happens in software, but this behavior
> does not really make sense to me. It differs from how other learned
> entries age/roam and can lead to problems such as the one described
> above. It is also not documented anywhere, so I can't tell if it's
> intentional or an oversight. We need to have a good reason for such a
> behavior other than the fact that it appears to conform to the quirks 
> of
> one hardware implementation.
> 
>> 
>> > It seems like the main purpose of these locked entries is to signal to
>> > user space the presence of a certain MAC behind a locked port, but they
>> > should not be able to affect packet forwarding in the bridge, unlike
>> > regular entries.
>> 
>> So essentially what you want is for br_handle_frame_finish() to treat
>> "dst = br_fdb_find_rcu(br, eth_hdr(skb)->h_dest, vid);" as NULL if
>> test_bit(BR_FDB_LOCKED, &dst->flags) is true?
> 
> Yes. It's not clear to me why unauthorized hosts should be given the
> ability to affect packet forwarding in the bridge through these locked
> entries when their primary purpose seems to be notifying user space
> about the presence of the MAC. At the very least this should be
> explained in the commit message, to indicate that some thought went 
> into
> this decision.
> 

I guess you are right that the SW setup locked entries can be used to 
gain uni-directional traffic through a switch, which should really not 
be the case.
In this case I expect the zero-DPV entries to not give this ability, 
which is the correct behaviour with MAB IMHO.

>> 
>> > Regarding a separate knob for MAB, I tend to agree we need it. Otherwise
>> > we cannot control which locked ports are able to populate the FDB with
>> > locked entries. I don't particularly like the fact that we overload an
>> > existing flag ("learning") for that. Any reason not to add an explicit
>> > flag ("mab")? At least with the current implementation, locked entries
>> > cannot roam between locked ports and cannot be refreshed, which differs
>> > from regular learning.
>> 
>> Well, assuming we model the software bridge closer to mv88e6xxx (where
>> locked FDB entries can roam after a certain time), does this change 
>> things?
>> In the software implementation I think it would make sense for them to
>> be able to roam right away (the age-out interval in mv88e6xxx is just 
>> a
>> compromise between responsiveness to roaming and resistance to DoS).
> 
> Exactly. If this is the best that we can do with mv88e6xxx, then so be
> it, but other implementations (software/hardware) do not have the same
> limitations and I don't see a reason to bend them.
> 
> Regarding "learning" vs. "mab" (or something else), the former is a
> well-defined flag available since forever. In 5.18 and 5.19 it can also
> be enabled together with "locked" and packets from an unauthorized host
> (modulo link-local ones) will not populate the FDB. I prefer not to
> change an existing behavior.
> 
> From usability point of view, I think a new flag would be easier to
> explain than explaining that "learning on" behaves like A or B, based 
> on
> whether "locked on" is set. The bridge can also be taught to forbid the
> new flag from being set when "locked" is not set.
> 
> A user space daemon that wants to try 802.1x and fallback to MAB can
> enable both flags or enable "mab" after some timer expires.

With this driver it is not really an option to use +learning for a 
opt-in for MAB. I think locked port should always have -learning before 
locking the port. In fact there is a problem in this implementation with 
MAB if -learning is applied after locking the port, as that will disable 
MAB, but also refresh and all other violation interrupts.
So I guess I need to disable the learning flag to the driver when the 
port is locked, or even from the bridge?