This is a note to let you know that I've just added the patch titled
randomize_kstack: Improve entropy diffusion
to the 5.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
randomize_kstack-improve-entropy-diffusion.patch
and it can be found in the queue-5.15 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 05af706e7f55a69d24697a7359c600cab4909418
Author: Kees Cook <keescook@xxxxxxxxxxxx>
Date: Sat Mar 9 12:24:48 2024 -0800
randomize_kstack: Improve entropy diffusion
[ 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@xxxxxxxx>
Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall")
Link: https://lore.kernel.org/r/20240309202445.work.165-kees@xxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
index d373f1bcbf7ca..5d52d15faee0c 100644
--- a/include/linux/randomize_kstack.h
+++ b/include/linux/randomize_kstack.h
@@ -58,7 +58,7 @@ 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)
