Verify signature before verifying sbat levels

Verifying the validity of a files signature can protect from
an attacker creating a file that exploits a potential issue
in the sbat validation. If the signature is not checked first,
an attacker can just create a file with a valid .sbat section
and can still attack the signature validation.

Signed-off-by: Jan Setje-Eilers <Jan.SetjeEilers@oracle.com>

shim.c

@@ -753,11 +753,11 @@ verify_buffer (char *data, int datasize,
 {
 	EFI_STATUS efi_status;
 
-	efi_status = verify_buffer_sbat(data, datasize, context);
+	efi_status = verify_buffer_authenticode(data, datasize, context, sha256hash, sha1hash);
 	if (EFI_ERROR(efi_status))
 		return efi_status;
 
-	return verify_buffer_authenticode(data, datasize, context, sha256hash, sha1hash);
+	return verify_buffer_sbat(data, datasize, context);
 }
 
 static int