randomize_kstack: Improve entropy diffusion

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>

diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
index 5d868505a94e43fe4f6124915e90dc814c269278..6d92b68efbf6c3afe86b9f6c22a8759ce51e7a28 100644 (file)
--- 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)