On 07.10.23 06:12, Kees Cook wrote:
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 <keesc...@chromium.org>
AuthorDate: Fri Oct 6 21:09:28 2023 -0700
Commit: Kees Cook <keesc...@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 <e1634...@student.tuwien.ac.at>
Closes:
https://lore.kernel.org/all/f3ca77f0-e414-4065-83a5-ae4c4d255...@student.tuwien.ac.at
Signed-off-by: Kees Cook <keesc...@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
I think, this is still missing a change in the randnum calculation to use index
instead of i.
Without that, randnum can be larger than the length of newtree, which crashes
kernel compilation for me.
diff --git a/scripts/gcc-plugins/randomize_layout_plugin.c
b/scripts/gcc-plugins/randomize_layout_plugin.c
index 178831917f01..4b4627e3f2ce 100644
--- a/scripts/gcc-plugins/randomize_layout_plugin.c
+++ b/scripts/gcc-plugins/randomize_layout_plugin.c
@@ -212,7 +212,7 @@ static void performance_shuffle(tree *newtree, unsigned
long length, ranctx *prn
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]))
The patch seems to work after that though. For the previous example, I now get
the following layout:
func1 (offset: 0)
func3 (offset: 8)
func4 (offset: 16)
func6 (offset: 24)
func7 (offset: 32)
func8 (offset: 40)
func5 (offset: 48)
func2 (offset: 56)
func10 (offset: 64)
func9 (offset: 72)
Regarding the shuffling of groups/partitions (rather than just the
randomization of structure members within each partition), I'm not sure if that
was intended at some point, but it might be worth looking into.
I'd assume it would improve randomization without sacrificing performance, and
it's also what the clang implementation of randstruct does.
Lukas Loidolt
