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: 39218ff ("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>

Author: kees

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)