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Date:   Tue, 18 Oct 2022 22:05:26 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     x86@kernel.org, Sami Tolvanen <samitolvanen@google.com>,
        Joao Moreira <joao@overdrivepizza.com>,
        linux-kernel@vger.kernel.org, Mark Rutland <mark.rutland@arm.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [PATCH] x86/ibt: Implement FineIBT
Message-ID: <202210182200.50680AE@keescook>
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 <Y08FhjK3fKsfRAaw@hirez.programming.kicks-ass.net>
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On Tue, Oct 18, 2022 at 09:59:02PM +0200, Peter Zijlstra wrote:
> On Tue, Oct 18, 2022 at 11:09:13AM -0700, Kees Cook wrote:
> 
> > An improvement for kCFI would be to mutate all the hashes both at build
> > time (perhaps using the same seed infrastructure that randstruct depends
> > on for sharing a seed across compilation units), and at boot time, so
> > an actual .text content exposure is needed to find the target hash value.
> 
> What's the purpose of the build time randomization?

I was just considering options if run-time was too onerous.

> Find here the boot
> time randomization (on top of my other patch).

Which it's clearly not. :P Nice!

> [...]
>  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;

This is saved because we need to fix up modules, yes? I look forward
to fine-grain randomization of the .data section. ;)

> [...]
> +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...

Otherwise, yes, this looks lovely. Thank you!

-- 
Kees Cook