Re: Missing cache considerations in randstruct performance feature

[Kees Cook <keescook@chromium.org>]

On Sat, Oct 07, 2023 at 12:30:01AM +0200, Lukas Loidolt wrote:
> In my tests, however, the performance version behaves more or less like the
> full version of randstruct.

Can you try this patch?


commit d73a3244700d3c945cedea7e1fb7042243c41e08
Author:     Kees Cook <keescook@chromium.org>
AuthorDate: Fri Oct 6 21:09:28 2023 -0700
Commit:     Kees Cook <keescook@chromium.org>
CommitDate: Fri Oct 6 21:09:28 2023 -0700

    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
    Reported-by: Lukas Loidolt <e1634039@student.tuwien.ac.at>
    Closes: https://lore.kernel.org/all/f3ca77f0-e414-4065-83a5-ae4c4d25545d@student.tuwien.ac.at
    Signed-off-by: Kees Cook <keescook@chromium.org>

diff --git a/scripts/gcc-plugins/randomize_layout_plugin.c b/scripts/gcc-plugins/randomize_layout_plugin.c
index 951b74ba1b24..178831917f01 100644
--- a/scripts/gcc-plugins/randomize_layout_plugin.c
+++ b/scripts/gcc-plugins/randomize_layout_plugin.c
@@ -191,7 +191,7 @@ 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;
@@ -206,11 +206,14 @@ 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 += size_group[x].start;
 			// we could handle this case differently if desired
 			if (DECL_BIT_FIELD_TYPE(newtree[randnum]))
 				continue;

-- 
Kees Cook