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randomize_kstack: Improve entropy diffusion

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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: 39218ff4c6 ("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>
This commit is contained in:
Kees Cook 2024-03-09 12:24:48 -08:00
parent bbda3ba626
commit 9c573cd313
1 changed files with 1 additions and 1 deletions
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include/linux/randomize_kstack.h
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@ -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)