net: dsa: mv88e6xxx: avoid unregistering devlink regions which were never registered

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

net: dsa: mv88e6xxx: avoid unregistering devlink regions which were never registered

Russell King reports that a system with mv88e6xxx dereferences a NULL

pointer when unbinding this driver:

https://lore.kernel.org/netdev/[email protected]/

The crash seems to be in devlink_region_destroy(), which is not NULL

tolerant but is given a NULL devlink global region pointer.

At least on some chips, some devlink regions are conditionally registered

since the blamed commit, see mv88e6xxx_setup_devlink_regions_global():

if (cond && !cond(chip))

continue;

These are MV88E6XXX_REGION_STU and MV88E6XXX_REGION_PVT. If the chip

does not have an STU or PVT, it should crash like this.

To fix the issue, avoid unregistering those regions which are NULL, i.e.

were skipped at mv88e6xxx_setup_devlink_regions_global() time.