[efi] Ensure NUL byte is at lowest address within stack cookie

The NUL byte included within the stack cookie to act as a string
terminator should be placed at the lowest byte address within the
stack cookie, in order to avoid potentially including the stack cookie
value within an accidentally unterminated string.

Suggested-by: Pete Beck <pete.beck@ioactive.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/interface/efi/efi_init.c

@@ -21,6 +21,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
 
 #include <string.h>
 #include <errno.h>
+#include <endian.h>
 #include <ipxe/init.h>
 #include <ipxe/rotate.h>
 #include <ipxe/efi/efi.h>
@@ -128,6 +129,13 @@ efi_stack_cookie ( EFI_HANDLE handle ) {
 	 */
 	cookie <<= 8;
 
+	/* Ensure that the NUL byte is placed at the bottom of the
+	 * stack cookie, to avoid potential disclosure via an
+	 * unterminated string.
+	 */
+	if ( __BYTE_ORDER == __BIG_ENDIAN )
+		cookie >>= 8;
+
 	return cookie;
 }