net: bridge: validate the NUD_PERMANENT bit when adding an extern_learn FDB entry

Currently it is possible to add broken extern_learn FDB entries to the
bridge in two ways:

1. Entries pointing towards the bridge device that are not local/permanent:

ip link add br0 type bridge
bridge fdb add 00:01:02:03:04:05 dev br0 self extern_learn static

2. Entries pointing towards the bridge device or towards a port that
are marked as local/permanent, however the bridge does not process the
'permanent' bit in any way, therefore they are recorded as though they
aren't permanent:

ip link add br0 type bridge
bridge fdb add 00:01:02:03:04:05 dev br0 self extern_learn permanent

Since commit 52e4bec1 ("net: bridge: switchdev: treat local FDBs the
same as entries towards the bridge"), these incorrect FDB entries can
even trigger NULL pointer dereferences inside the kernel.

This is because that commit made the assumption that all FDB entries
that are not local/permanent have a valid destination port. For context,
local / permanent FDB entries either have fdb->dst == NULL, and these
point towards the bridge device and are therefore local and not to be
used for forwarding, or have fdb->dst == a net_bridge_port structure
(but are to be treated in the same way, i.e. not for forwarding).

That assumption _is_ correct as long as things are working correctly in
the bridge driver, i.e. we cannot logically have fdb->dst == NULL under
any circumstance for FDB entries that are not local. However, the
extern_learn code path where FDB entries are managed by a user space
controller show that it is possible for the bridge kernel driver to
misinterpret the NUD flags of an entry transmitted by user space, and
end up having fdb->dst == NULL while not being a local entry. This is
invalid and should be rejected.

Before, the two commands listed above both crashed the kernel in this
check from br_switchdev_fdb_notify:

	struct net_device *dev = info.is_local ? br->dev : dst->dev;

info.is_local == false, dst == NULL.

After this patch, the invalid entry added by the first command is
rejected:

ip link add br0 type bridge && bridge fdb add 00:01:02:03:04:05 dev br0 self extern_learn static; ip link del br0
Error: bridge: FDB entry towards bridge must be permanent.

and the valid entry added by the second command is properly treated as a
local address and does not crash br_switchdev_fdb_notify anymore:

ip link add br0 type bridge && bridge fdb add 00:01:02:03:04:05 dev br0 self extern_learn permanent; ip link del br0

Fixes: eb100e0e

 ("net: bridge: allow to add externally learned entries from user-space")
Reported-by:  <syzbot+9ba1174359adba5a5b7c@syzkaller.appspotmail.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Link: https://lore.kernel.org/r/20210801231730.7493-1-vladimir.oltean@nxp.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>