linux-kernel - [PATCH] randomize_kstack: Improve entropy diffusion

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Message-Id: <20240309202445.work.165-kees@kernel.org>
Date: Sat,  9 Mar 2024 12:24:48 -0800
From: Kees Cook <keescook@...omium.org>
To: Jeremy Linton <jeremy.linton@....com>
Cc: Kees Cook <keescook@...omium.org>,
	Arnd Bergmann <arnd@...db.de>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	linux-hardening@...r.kernel.org,
	Elena Reshetova <elena.reshetova@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] 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@...db.de>
Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall")
Signed-off-by: Kees Cook <keescook@...omium.org>
---
Cc: Jeremy Linton <jeremy.linton@....com>
Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: linux-hardening@...r.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