The kstack_offset variable was really only ever using the low bits for
kernel stack offset entropy. Add a ror32() to increase bit diffusion.
Suggested-by: Arnd Bergmann <a...@arndb.de>
Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each
syscall")
Signed-off-by: Kees Cook <keesc...@chromium.org>
---
Cc: Jeremy Linton <jeremy.lin...@arm.com>
Cc: "Gustavo A. R. Silva" <gustavo...@kernel.org>
Cc: linux-hardening@vger.kernel.org
---
include/linux/randomize_kstack.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
index 5d868505a94e..6d92b68efbf6 100644
--- a/include/linux/randomize_kstack.h
+++ b/include/linux/randomize_kstack.h
@@ -80,7 +80,7 @@ DECLARE_PER_CPU(u32, kstack_offset);
if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
&randomize_kstack_offset)) { \
u32 offset = raw_cpu_read(kstack_offset); \
- offset ^= (rand); \
+ offset = ror32(offset, 5) ^ (rand); \
raw_cpu_write(kstack_offset, offset); \
} \
} while (0)
--
2.34.1
