Messages in this thread | | | Date | Fri, 28 Oct 2022 10:42:52 -0700 | From | Kees Cook <> | Subject | Re: [PATCH 4/4] x86/cfi: Add boot time hash randomization |
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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
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