Re: [RFC PATCH] net: bridge: call br_multicast_del_port before the port leaves

[Nikolay Aleksandrov <nikolay@nvidia.com>]

On Thu, 2020-10-15 at 20:33 +0300, Vladimir Oltean wrote:
> Switchdev drivers often have different VLAN semantics than the bridge.
> For example, consider this:
> 
> ip link add br0 type bridge
> ip link set swp0 master br0
> bridge mdb add dev br0 port swp0 grp 01:02:03:04:05:06 permanent
> ip link del br0
> [   26.085816] mscc_felix 0000:00:00.5 swp0: failed (err=-2) to del object (id=2)
> 
> This is because the mscc_ocelot driver, when VLAN awareness is disabled,
> classifies all traffic to the port-based VLAN (pvid). The pvid is 0 when
> the port is standalone, and it is inherited from the bridge default pvid
> (which is 1 by default, but it may take other values) when it joins the
> VLAN-unaware bridge, and then the pvid resets to 0 when the port leaves
> the bridge again.
> 
> Now because the mscc_ocelot switch classifies all traffic to its private
> pvid, it needs to translate between the vid that the mdb comes with, and
> the vid that will actually be programmed into hardware. The bridge uses
> the vid of 0 in VLAN-unaware mode, while the hardware uses the pvid
> inherited from the bridge, that's the difference.
> 
> So what will happen is:
> 
> Step 1 (addition):
> br_mdb_notify(RTM_NEWMDB)
> -> ocelot_port_mdb_add(mdb->addr=01:02:03:04:05:06, mdb->vid=0)
>    -> mdb->vid is remapped from 0 to 1 and installed into ocelot->multicast
> 
> Step 2 (removal):
> del_nbp
> -> netdev_upper_dev_unlink(dev, br->dev)
>    -> ocelot_port_bridge_leave
>       -> ocelot_port_set_pvid(ocelot, port, 0)
> -> br_multicast_del_port is called and the switchdev notifier is
>    deferred for some time later
>    -> ocelot_port_mdb_del(mdb->addr=01:02:03:04:05:06, mdb->vid=0)
>       -> mdb->vid is remapped from 0 to 0, the port pvid (!!!)
>       -> the remapped mdb (addr=01:02:03:04:05:06, vid=0) is not found
>          inside the ocelot->multicast list, and -ENOENT is returned
> 
> So the problem is that mscc_ocelot assumes that the port is removed
> _after_ the multicast entries have been deleted. And this is not an
> unreasonable assumption, presumably it isn't the only switchdev that
> needs to remap the vid. So we can reorder the teardown path in order
> for that assumption to hold true.
> 
> Since br_mdb_notify() issues a SWITCHDEV_F_DEFER operation, we must move
> the call not only before netdev_upper_dev_unlink(), but in fact before
> switchdev_deferred_process().
> 
> Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
> ---
>  net/bridge/br_if.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 

It can potentially use after free, multicast resources (per-cpu stats) are freed
in br_multicast_del_port() and can be used due to a race with port state
sync on other CPUs since the handler can still process packets. That has a
chance of happening if vlans are not used.

Interesting that br_stp_disable_port() calls br_multicast_disable_port() which
flushes all non-permanent mdb entries, so I'm guessing you have problem only
with permanent ones? Perhaps we can flush them all before. Either by passing an
argument to br_stp_disable_port() that we're deleting the port which will be
passed down to br_multicast_disable_port() or by calling an additional helper to
flush all which can be re-used by both disable_port() and stop_multicast()
calls. Adding an argument to br_stp_disable_port() to be passed down sounds
cleaner to me. What do you think?

Cheers,
 Nik