On Sun, Nov 8, 2020 at 9:20 PM Vladimir Oltean <olte...@gmail.com> wrote:
>
> Some DSA switches (and not only) cannot learn source MAC addresses from
> packets injected from the CPU. They only perform hardware address
> learning from inbound traffic.
>
> This can be problematic when we have a bridge spanning some DSA switch
> ports and some non-DSA ports (which we'll call "foreign interfaces" from
> DSA's perspective).
>
> There are 2 classes of problems created by the lack of learning on
> CPU-injected traffic:
> - excessive flooding, due to the fact that DSA treats those addresses as
> unknown
> - the risk of stale routes, which can lead to temporary packet loss
>
> To illustrate the second class, consider the following situation, which
> is common in production equipment (wireless access points, where there
> is a WLAN interface and an Ethernet switch, and these form a single
> bridging domain).
>
> AP 1:
> +------------------------------------------------------------------------+
> | br0 |
> +------------------------------------------------------------------------+
> +------------+ +------------+ +------------+ +------------+ +------------+
> | swp0 | | swp1 | | swp2 | | swp3 | | wlan0 |
> +------------+ +------------+ +------------+ +------------+ +------------+
> | ^ ^
> | | |
> | | |
> | Client A Client B
> |
> |
> |
> +------------+ +------------+ +------------+ +------------+ +------------+
> | swp0 | | swp1 | | swp2 | | swp3 | | wlan0 |
> +------------+ +------------+ +------------+ +------------+ +------------+
> +------------------------------------------------------------------------+
> | br0 |
> +------------------------------------------------------------------------+
> AP 2
>
> - br0 of AP 1 will know that Clients A and B are reachable via wlan0
> - the hardware fdb of a DSA switch driver today is not kept in sync with
> the software entries on other bridge ports, so it will not know that
> clients A and B are reachable via the CPU port UNLESS the hardware
> switch itself performs SA learning from traffic injected from the CPU.
> Nonetheless, a substantial number of switches don't.
> - the hardware fdb of the DSA switch on AP 2 may autonomously learn that
> Client A and B are reachable through swp0. Therefore, the software br0
> of AP 2 also may or may not learn this. In the example we're
> illustrating, some Ethernet traffic has been going on, and br0 from AP
> 2 has indeed learnt that it can reach Client B through swp0.
>
> One of the wireless clients, say Client B, disconnects from AP 1 and
> roams to AP 2. The topology now looks like this:
>
> AP 1:
> +------------------------------------------------------------------------+
> | br0 |
> +------------------------------------------------------------------------+
> +------------+ +------------+ +------------+ +------------+ +------------+
> | swp0 | | swp1 | | swp2 | | swp3 | | wlan0 |
> +------------+ +------------+ +------------+ +------------+ +------------+
> | ^
> | |
> | Client A
> |
> |
> | Client B
> | |
> | v
> +------------+ +------------+ +------------+ +------------+ +------------+
> | swp0 | | swp1 | | swp2 | | swp3 | | wlan0 |
> +------------+ +------------+ +------------+ +------------+ +------------+
> +------------------------------------------------------------------------+
> | br0 |
> +------------------------------------------------------------------------+
> AP 2
>
> - br0 of AP 1 still knows that Client A is reachable via wlan0 (no change)
> - br0 of AP 1 will (possibly) know that Client B has left wlan0. There
> are cases where it might never find out though. Either way, DSA today
> does not process that notification in any way.
> - the hardware FDB of the DSA switch on AP 1 may learn autonomously that
> Client B can be reached via swp0, if it receives any packet with
> Client 1's source MAC address over Ethernet.
> - the hardware FDB of the DSA switch on AP 2 still thinks that Client B
> can be reached via swp0. It does not know that it has roamed to wlan0,
> because it doesn't perform SA learning from the CPU port.
>
> Now Client A contacts Client B.
> AP 1 routes the packet fine towards swp0 and delivers it on the Ethernet
> segment.
> AP 2 sees a frame on swp0 and its fdb says that the destination is swp0.
> Hairpinning is disabled => drop.
>
> This problem comes from the fact that these switches have a 'blind spot'
> for addresses coming from software bridging. The generic solution is not
> to assume that hardware learning can be enabled somehow, but to listen
> to more bridge learning events. It turns out that the bridge driver does
> learn in software from all inbound frames, in __br_handle_local_finish.
> A proper SWITCHDEV_FDB_ADD_TO_DEVICE notification is emitted for the
> addresses serviced by the bridge on 'foreign' interfaces. The problem is
> that DSA currently only cares about SWITCHDEV_FDB_ADD_TO_DEVICE events
> received on its own interfaces, such as static FDB entries.
>
> Luckily we can change that, and DSA can listen to all switchdev FDB
> add/del events in the system and figure out if those events were emitted
> by a bridge that spans at least one of DSA's own ports. In case that is
> true, DSA will also offload that address towards its own CPU port, in
> the eventuality that there might be bridge clients attached to the DSA
> switch who want to talk to the station connected to the foreign
> interface.
>
> Reported-by: DENG Qingfang <dqf...@gmail.com>
> Signed-off-by: Vladimir Oltean <olte...@gmail.com>
> ---
> net/dsa/slave.c | 51 ++++++++++++++++++++++++++++++++++++++++++++-----
> 1 file changed, 46 insertions(+), 5 deletions(-)
>
> diff --git a/net/dsa/slave.c b/net/dsa/slave.c
> index b34da39722c7..5b3b07a39105 100644
> --- a/net/dsa/slave.c
> +++ b/net/dsa/slave.c
> @@ -2120,6 +2120,28 @@ static void dsa_slave_switchdev_event_work(struct
> work_struct *work)
> dev_put(dp->slave);
> }
>
> +static int dsa_lower_dev_walk(struct net_device *lower_dev,
> + struct netdev_nested_priv *priv)
> +{
> + if (dsa_slave_dev_check(lower_dev)) {
> + priv->data = netdev_priv(lower_dev);
> + return 1;
> + }
> +
> + return 0;
> +}
> +
> +struct dsa_slave_priv *dsa_slave_dev_lower_find(struct net_device *dev)
> +{
> + struct netdev_nested_priv priv = {
> + .data = NULL,
> + };
> +
> + netdev_walk_all_lower_dev_rcu(dev, dsa_lower_dev_walk, &priv);
> +
> + return priv.data;
> +}
> +
> /* Called under rcu_read_lock() */
> static int dsa_slave_switchdev_event(struct notifier_block *unused,
> unsigned long event, void *ptr)
> @@ -2140,13 +2162,32 @@ static int dsa_slave_switchdev_event(struct
> notifier_block *unused,
> case SWITCHDEV_FDB_DEL_TO_DEVICE:
> fdb_info = ptr;
>
> - if (!dsa_slave_dev_check(dev))
> - return NOTIFY_DONE;
> + if (dsa_slave_dev_check(dev)) {
> + if (!fdb_info->added_by_user)
> + return NOTIFY_OK;
> +
> + dp = dsa_slave_to_port(dev);
> + } else {
> + /* Snoop addresses learnt on foreign interfaces
> + * bridged with us, for switches that don't
> + * automatically learn SA from CPU-injected traffic
> + */
Can it be turned off for switches that support SA learning from CPU?
> + struct net_device *br_dev;
> + struct dsa_slave_priv *p;
>
> - if (!fdb_info->added_by_user)
> - return NOTIFY_OK;
> + br_dev = netdev_master_upper_dev_get_rcu(dev);
> + if (!br_dev)
> + return NOTIFY_DONE;
>
> - dp = dsa_slave_to_port(dev);
> + if (!netif_is_bridge_master(br_dev))
> + return NOTIFY_DONE;
> +
> + p = dsa_slave_dev_lower_find(br_dev);
> + if (!p)
> + return NOTIFY_DONE;
> +
> + dp = p->dp->cpu_dp;
> + }
>
> switchdev_work = kzalloc(sizeof(*switchdev_work), GFP_ATOMIC);
> if (!switchdev_work)
> --
> 2.25.1
>
