randstruct: Fix gcc-plugin performance mode to stay in group

The performance mode of the gcc-plugin randstruct was shuffling struct
members outside of the cache-line groups. Limit the range to the
specified group indexes.

Cc: linux-hardening@vger.kernel.org
Cc: stable@vger.kernel.org
Reported-by: Lukas Loidolt <e1634039@student.tuwien.ac.at>
Closes: https://lore.kernel.org/all/f3ca77f0-e414-4065-83a5-ae4c4d25545d@student.tuwien.ac.at
Fixes: 313dd1b ("gcc-plugins: Add the randstruct plugin")
Signed-off-by: Kees Cook <keescook@chromium.org>

Author: kees

scripts/gcc-plugins/randomize_layout_plugin.c

@@ -191,12 +191,14 @@ static void partition_struct(tree *fields, unsigned long length, struct partitio
 
 static void performance_shuffle(tree *newtree, unsigned long length, ranctx *prng_state)
 {
-	unsigned long i, x;
+	unsigned long i, x, index;
 	struct partition_group size_group[length];
 	unsigned long num_groups = 0;
 	unsigned long randnum;
 
 	partition_struct(newtree, length, (struct partition_group *)&size_group, &num_groups);
+
+	/* FIXME: this group shuffle is currently a no-op. */
 	for (i = num_groups - 1; i > 0; i--) {
 		struct partition_group tmp;
 		randnum = ranval(prng_state) % (i + 1);
@@ -206,11 +208,14 @@ static void performance_shuffle(tree *newtree, unsigned long length, ranctx *prn
 	}
 
 	for (x = 0; x < num_groups; x++) {
-		for (i = size_group[x].start + size_group[x].length - 1; i > size_group[x].start; i--) {
+		for (index = size_group[x].length - 1; index > 0; index--) {
 			tree tmp;
+
+			i = size_group[x].start + index;
 			if (DECL_BIT_FIELD_TYPE(newtree[i]))
 				continue;
-			randnum = ranval(prng_state) % (i + 1);
+			randnum = ranval(prng_state) % (index + 1);
+			randnum += size_group[x].start;
 			// we could handle this case differently if desired
 			if (DECL_BIT_FIELD_TYPE(newtree[randnum]))
 				continue;