Patch "randomize_kstack: Improve entropy diffusion" has been added to the 6.1-stable tree

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This is a note to let you know that I've just added the patch titled

    randomize_kstack: Improve entropy diffusion

to the 6.1-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-6.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit f7e96a69608e416522f52dde91d2c6bfd606e304
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 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)




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