x86, efi: Check table header length in efi_bgrt_init()

Header length should be validated for all ACPI tables before accessing
any non-header field.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Link: http://lkml.kernel.org/r/509A9E6002000078000A7079@nat28.tlf.novell.com
Acked-by: Matt Fleming <matt.fleming@intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/platform/efi/efi-bgrt.c

@@ -39,6 +39,8 @@ void efi_bgrt_init(void)
 	if (ACPI_FAILURE(status))
 		return;
 
+	if (bgrt_tab->header.length < sizeof(*bgrt_tab))
+		return;
 	if (bgrt_tab->version != 1)
 		return;
 	if (bgrt_tab->image_type != 0 || !bgrt_tab->image_address)