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: