Re: [PATCH 4/4] x86/cfi: Add boot time hash randomization

[Kees Cook <keescook@chromium.org>]

On Thu, Oct 27, 2022 at 11:28:16AM +0200, Peter Zijlstra wrote:
> In order to avoid known hashes (from knowing the boot image),
> randomize the CFI hashes with a per-boot random seed.
> 
> Suggested-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  arch/x86/kernel/alternative.c |  120 +++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 108 insertions(+), 12 deletions(-)
> 
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -711,6 +711,24 @@ enum cfi_mode {
>  };
>  
>  static enum cfi_mode cfi_mode __ro_after_init = CFI_DEFAULT;
> +static bool cfi_rand __ro_after_init = true;
> +static u32  cfi_seed __ro_after_init;
> +
> +/*
> + * Re-hash the CFI hash with a boot-time seed while making sure the result is
> + * not a valid ENDBR instruction.
> + */
> +static u32 cfi_rehash(u32 hash)
> +{
> +	hash ^= cfi_seed;
> +	while (unlikely(is_endbr(hash) || is_endbr(-hash))) {
> +		bool lsb = hash & 1;
> +		hash >>= 1;
> +		if (lsb)
> +			hash ^= 0x80200003;
> +	}
> +	return hash;
> +}

I guess this risks hash collision with existing hashes, but meeeh. I'm
glad to have the randomization. :)

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook