Since the commit referenced in the Fixes: tag below the VMBus client driver
is walking the ACPI namespace up from the VMBus ACPI device to the ACPI
namespace root object trying to find Hyper-V MMIO ranges.

However, if it is not able to find them it ends trying to walk resources of
the ACPI namespace root object itself.
This object has all-ones handle, which causes a NULL pointer dereference
in the ACPI code (from dereferencing this pointer with an offset).

This in turn causes an oops on boot with VMBus host implementations that do
not provide Hyper-V MMIO ranges in their VMBus ACPI device or its
ancestors.
The QEMU VMBus implementation is an example of such implementation.

I guess providing these ranges is optional, since all tested Windows
versions seem to be able to use VMBus devices without them.

Fix this by explicitly terminating the lookup at the ACPI namespace root
object.

Note that Linux guests under KVM/QEMU do not use the Hyper-V PV interface
by default - they only do so if the KVM PV interface is missing or
disabled.

Example stack trace of such oops:
[ 3.710827] ? __die+0x1f/0x60
[ 3.715030] ? page_fault_oops+0x159/0x460
[ 3.716008] ? exc_page_fault+0x73/0x170
[ 3.716959] ? asm_exc_page_fault+0x22/0x30
[ 3.717957] ? acpi_ns_lookup+0x7a/0x4b0
[ 3.718898] ? acpi_ns_internalize_name+0x79/0xc0
[ 3.720018] acpi_ns_get_node_unlocked+0xb5/0xe0
[ 3.721120] ? acpi_ns_check_object_type+0xfe/0x200
[ 3.722285] ? acpi_rs_convert_aml_to_resource+0x37/0x6e0
[ 3.723559] ? down_timeout+0x3a/0x60
[ 3.724455] ? acpi_ns_get_node+0x3a/0x60
[ 3.725412] acpi_ns_get_node+0x3a/0x60
[ 3.726335] acpi_ns_evaluate+0x1c3/0x2c0
[ 3.727295] acpi_ut_evaluate_object+0x64/0x1b0
[ 3.728400] acpi_rs_get_method_data+0x2b/0x70
[ 3.729476] ? vmbus_platform_driver_probe+0x1d0/0x1d0 [hv_vmbus]
[ 3.730940] ? vmbus_platform_driver_probe+0x1d0/0x1d0 [hv_vmbus]
[ 3.732411] acpi_walk_resources+0x78/0xd0
[ 3.733398] vmbus_platform_driver_probe+0x9f/0x1d0 [hv_vmbus]
[ 3.734802] platform_probe+0x3d/0x90
[ 3.735684] really_probe+0x19b/0x400
[ 3.736570] ? __device_attach_driver+0x100/0x100
[ 3.737697] __driver_probe_device+0x78/0x160
[ 3.738746] driver_probe_device+0x1f/0x90
[ 3.739743] __driver_attach+0xc2/0x1b0
[ 3.740671] bus_for_each_dev+0x70/0xc0
[ 3.741601] bus_add_driver+0x10e/0x210
[ 3.742527] driver_register+0x55/0xf0
[ 3.744412] ? 0xffffffffc039a000
[ 3.745207] hv_acpi_init+0x3c/0x1000 [hv_vmbus]

Fixes: 7f163a6f ("drivers:hv: Modify hv_vmbus to search for all MMIO ranges available.")
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Link: https://lore.kernel.org/r/fd8e64ceeecfd1d95ff49021080cf699e88dbbde.1691606267.git.maciej.szmigiero@oracle.com