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Date:   Tue, 18 Oct 2022 16:47:44 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     x86@kernel.org
Cc:     Kees Cook <keescook@chromium.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>,
        "H.J. Lu" <hjl.tools@gmail.com>
Subject: Re: [PATCH] x86/ibt: Implement FineIBT
Message-ID: <Y068kIjJr/5kJa23@hirez.programming.kicks-ass.net>
References: <Y06rtoE9BsERG9uv@hirez.programming.kicks-ass.net>
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In-Reply-To: <Y06rtoE9BsERG9uv@hirez.programming.kicks-ass.net>
Precedence: bulk

On Tue, Oct 18, 2022 at 03:35:50PM +0200, Peter Zijlstra wrote:
> +asm(	".pushsection .rodata			\n"
> +	"fineibt_caller_start:			\n"
> +	"	movl	$0x12345678, %r10d	\n"
> +	"	sub	$16, %r11		\n"
> +	ASM_NOP4
> +	"fineibt_caller_end:			\n"
> +	".popsection				\n"
> +);

Note: this hard relies on the indirection using %r11 and %r11
being clobbered by the call-abi. That is, there is no expectation on the
value of %r11 after this.

If GCC were to grow kCFI support this needs additional changes; one
option would be to use the 4 byte nop to rewrite it into something like:

	movl	$0x12345678, %r10d
	movl	%[reg], %r11
	sub	$16, %r11

> +static int cfi_rewrite_callers(s32 *start, s32 *end)
> +{
> +	s32 *s;
> +
> +	for (s = start; s < end; s++) {
> +		void *addr = (void *)s + *s;
> +		u32 hash;
> +
> +		addr -= fineibt_caller_size;
> +		hash = decode_caller_hash(addr);
> +		if (hash) {
> +			text_poke_early(addr, fineibt_caller_start, fineibt_caller_size);
> +			WARN_ON(*(u32 *)(addr + fineibt_caller_hash) != 0x12345678);
> +			text_poke_early(addr + fineibt_caller_hash, &hash, 4);
> +		}
> +		/* rely on apply_retpolines() */
> +	}
> +
> +	return 0;
> +}