ACPI: NFIT: Fix input validation of bus-family

Dan reports that smatch thinks userspace can craft an out-of-bound bus
family number. However, nd_cmd_clear_to_send() blocks all non-zero
values of bus-family since only the kernel can initiate these commands.
However, in the speculation path, family is a user controlled array
index value so mask it for speculation safety. Also, since the
nd_cmd_clear_to_send() safety is non-obvious and possibly may change in
the future include input validation as if userspace could get past the
nd_cmd_clear_to_send() gatekeeper.

Link: http://lore.kernel.org/r/20201111113000.GA1237157@mwanda
Reported-by: Dan Carpenter <[e-mail]>
Fixes: 6450ddbd5d8e ("ACPI: NFIT: Define runtime firmware activation commands")
Cc: <[e-mail]>
Signed-off-by: Dan Williams <[e-mail]>

drivers/acpi/nfit/core.c

···
 #include <linux/list_sort.h>
 #include <linux/libnvdimm.h>
 #include <linux/module.h>
+#include <linux/nospec.h>
 #include <linux/mutex.h>
 #include <linux/ndctl.h>
 #include <linux/sysfs.h>
···
 		cmd_mask = nd_desc->cmd_mask;
 		if (cmd == ND_CMD_CALL && call_pkg->nd_family) {
 			family = call_pkg->nd_family;
-			if (!test_bit(family, &nd_desc->bus_family_mask))
+			if (family > NVDIMM_BUS_FAMILY_MAX ||
+			    !test_bit(family, &nd_desc->bus_family_mask))
 				return -EINVAL;
+			family = array_index_nospec(family,
+						    NVDIMM_BUS_FAMILY_MAX + 1);
 			dsm_mask = acpi_desc->family_dsm_mask[family];
 			guid = to_nfit_bus_uuid(family);
 		} else {