[PATCH 6.8 135/143] randomize_kstack: Improve entropy diffusion

Greg Kroah-Hartman
2024-04-11T10:13:48Z

From: Kees Cook <keescook@chromium.org>

[ Upstream commit 9c573cd313433f6c1f7236fe64b9b743500c1628 ]

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 <arnd@arndb.de>
Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall")
Link: https://lore.kernel.org/r/20240309202445.work.165-kees@kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Sasha Levin <sashal@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 5d868505a94e4..6d92b68efbf6c 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)
--
2.43.0