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SubjectRe: [PATCH] x86/ibt: Implement FineIBT
On Tue, Oct 18, 2022 at 10:05:26PM -0700, Kees Cook wrote:

> > +static int cfi_rand_preamble(s32 *start, s32 *end)
> > +{
> > + s32 *s;
> > +
> > + for (s = start; s < end; s++) {
> > + void *addr = (void *)s + *s;
> > + u32 hash;
> > +
> > + hash = decode_preamble_hash(addr);
> > + if (WARN(!hash, "no CFI hash found at: %pS %px %*ph\n",
> > + addr, addr, 5, addr))
> > + return -EINVAL;
> > +
> > + hash ^= cfi_seed;
> > + text_poke_early(addr + 1, &hash, 4);
> > + }
> > +
> > + return 0;
> > +}
>
> The one glitch here is that the resulting hash needs to not contain
> an endbr...

Oh right,.. duh. How about something like:

static u32 cfi_rehash(u32 hash)
{
hash ^= cfi_hash;
while (unlikely(is_endbr(hash))) {
bool lsb = hash & 1;
hash >>= 1;
if (lsb)
hash ^= 0x80200003;
}
return hash;
}

Which seems properly over-engineered :-)

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