ACPICA: Avoid subobject buffer overflow when validating RSDP signature

ACPICA commit 6bb72909c1e3d415aee214104a01bc9834b2d4ce

Since the Signature member is accessed through an struct acpi_table_header, the
pointer to it is only to a 4-char array, and so trying to read past the
4th character, as will be done when it is an RSDP, reads beyond the
bounds of the accessed member. On CHERI, and thus Arm's experimental
Morello prototype architecture, pointers are represented as
capabilities, which are unforgeable bounded pointers, providing
always-on fine-grained spatial memory safety. By default, subobject
bounds enforcement is not enabled, only bounds on allocations, but it is
enabled in the cheri_BSD (a port of free_BSD) kernel as intra-object
overflow attacks are common on operating system kernels, and so this
overflow is detected there and traps.

Link: https://github.com/acpica/acpica/commit/6bb72909
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/acpica/tbprint.c

@@ -101,7 +101,8 @@ acpi_tb_print_table_header(acpi_physical_address address,
 		ACPI_INFO(("%-4.4s 0x%8.8X%8.8X %06X",
 			   header->signature, ACPI_FORMAT_UINT64(address),
 			   header->length));
-	} else if (ACPI_VALIDATE_RSDP_SIG(header->signature)) {
+	} else if (ACPI_VALIDATE_RSDP_SIG(ACPI_CAST_PTR(struct acpi_table_rsdp,
+							header)->signature)) {
 
 		/* RSDP has no common fields */