bridge: guard local VLAN-0 FDB helpers against NULL vlan group

In the Linux kernel, the following vulnerability has been resolved:

bridge: guard local VLAN-0 FDB helpers against NULL vlan group

When CONFIG_BRIDGE_VLAN_FILTERING is not set, br_vlan_group() and

nbp_vlan_group() return NULL (br_private.h stub definitions). The

BR_BOOLOPT_FDB_LOCAL_VLAN_0 toggle code is compiled unconditionally and

reaches br_fdb_delete_locals_per_vlan_port() and

br_fdb_insert_locals_per_vlan_port(), where the NULL vlan group pointer

is dereferenced via list_for_each_entry(v, &vg->vlan_list, vlist).

The observed crash is in the delete path, triggered when creating a

bridge with IFLA_BR_MULTI_BOOLOPT containing BR_BOOLOPT_FDB_LOCAL_VLAN_0

via RTM_NEWLINK. The insert helper has the same bug pattern.

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000056: 0000 [#1] KASAN NOPTI

KASAN: null-ptr-deref in range [0x00000000000002b0-0x00000000000002b7]

RIP: 0010:br_fdb_delete_locals_per_vlan+0x2b9/0x310

Call Trace:

br_fdb_toggle_local_vlan_0+0x452/0x4c0

br_toggle_fdb_local_vlan_0+0x31/0x80 net/bridge/br.c:276

br_boolopt_toggle net/bridge/br.c:313

br_boolopt_multi_toggle net/bridge/br.c:364

br_changelink net/bridge/br_netlink.c:1542

br_dev_newlink net/bridge/br_netlink.c:1575

Add NULL checks for the vlan group pointer in both helpers, returning

early when there are no VLANs to iterate. This matches the existing

pattern used by other bridge FDB functions such as br_fdb_add() and

br_fdb_delete().