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randomize_kstack: Improve entropy diffusion
[ Upstream commit9c573cd313
] 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> Signed-off-by: Sasha Levin <sashal@kernel.org>
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@ -80,7 +80,7 @@ DECLARE_PER_CPU(u32, kstack_offset);
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if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
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&randomize_kstack_offset)) { \
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u32 offset = raw_cpu_read(kstack_offset); \
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offset ^= (rand); \
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offset = ror32(offset, 5) ^ (rand); \
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raw_cpu_write(kstack_offset, offset); \
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} \
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} while (0)
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