linux-kernel - Re: [PATCH v2 5/9] x86/cfi: Add "debug" option to "cfi=" bootparam

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Message-ID: <20250904070035.GW4067720@noisy.programming.kicks-ass.net>
Date: Thu, 4 Sep 2025 09:00:35 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Kees Cook <kees@...nel.org>
Cc: Nathan Chancellor <nathan@...nel.org>,
	Vegard Nossum <vegard.nossum@...cle.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Jeff Johnson <jeff.johnson@....qualcomm.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	David Woodhouse <dwmw2@...radead.org>,
	"Russell King (Oracle)" <rmk+kernel@...linux.org.uk>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Marco Elver <elver@...gle.com>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	Ramon de C Valle <rcvalle@...gle.com>,
	Jonathan Corbet <corbet@....net>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Nicolas Schier <nicolas.schier@...ux.dev>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev, linux-doc@...r.kernel.org,
	linux-kbuild@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	x86@...nel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2 5/9] x86/cfi: Add "debug" option to "cfi=" bootparam

On Wed, Sep 03, 2025 at 08:46:44PM -0700, Kees Cook wrote:

> @@ -1734,6 +1737,8 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
>  	 * rewrite them. This disables all CFI. If this succeeds but any of the
>  	 * later stages fails, we're without CFI.
>  	 */
> +	if (cfi_debug)
> +		pr_info("CFI: disabling all indirect call checking\n");
>  	ret = cfi_disable_callers(start_retpoline, end_retpoline);
>  	if (ret)
>  		goto err;
> @@ -1744,14 +1749,23 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
>  			cfi_bpf_hash = cfi_rehash(cfi_bpf_hash);
>  			cfi_bpf_subprog_hash = cfi_rehash(cfi_bpf_subprog_hash);
>  		}
> +		if (cfi_debug)
> +			pr_info("CFI: cfi_seed: 0x%08x\n", cfi_seed);
>  
> +		if (cfi_debug)
> +			pr_info("CFI: rehashing all preambles\n");

So this repeated if() bugs the heck out of me ... :-)

What would you prefer, this:

		if (cfi_debug) {
			pr_info("CFI: cfi_seed: 0x%08x\n", cfi_seed);
			pr_info("CFI: rehashing all preambles\n");
		}

or something like:

#define pr_cfi_debug(X...) if (cfi_debug) pr_info(X)

		pr_cfi_debug("CFI: cfi_seed: 0x%08x\n", cfi_seed);
		pr_cfi_debug("CFI: rehashing all preambles\n");

?

>  		ret = cfi_rand_preamble(start_cfi, end_cfi);
>  		if (ret)
>  			goto err;
>  
> +		if (cfi_debug)
> +			pr_info("CFI: rehashing all indirect calls\n");
>  		ret = cfi_rand_callers(start_retpoline, end_retpoline);
>  		if (ret)
>  			goto err;
> +	} else {
> +		if (cfi_debug)
> +			pr_info("CFI: rehashing disabled\n");
>  	}
>  
>  	switch (cfi_mode) {
> @@ -1761,6 +1775,8 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
>  		return;
>  
>  	case CFI_KCFI:
> +		if (cfi_debug)
> +			pr_info("CFI: enabling all indirect call checking\n");

This should be "CFI: re-enabling all..." I suppose, to better match the
earlier "CFI: disabling all ..." message.

>  		ret = cfi_enable_callers(start_retpoline, end_retpoline);
>  		if (ret)
>  			goto err;
> @@ -1771,17 +1787,23 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
>  		return;
>  
>  	case CFI_FINEIBT:
> +		if (cfi_debug)
> +			pr_info("CFI: adding FineIBT to all preambles\n");
>  		/* place the FineIBT preamble at func()-16 */
>  		ret = cfi_rewrite_preamble(start_cfi, end_cfi);
>  		if (ret)
>  			goto err;
>  
>  		/* rewrite the callers to target func()-16 */
> +		if (cfi_debug)
> +			pr_info("CFI: rewriting indirect call sites to use FineIBT\n");
>  		ret = cfi_rewrite_callers(start_retpoline, end_retpoline);
>  		if (ret)
>  			goto err;
>  
>  		/* now that nobody targets func()+0, remove ENDBR there */
> +		if (cfi_debug)
> +			pr_info("CFI: removing old endbr insns\n");
>  		cfi_rewrite_endbr(start_cfi, end_cfi);
>  
>  		if (builtin) {
> @@ -2324,6 +2346,7 @@ void __init alternative_instructions(void)
>  
>  	__apply_fineibt(__retpoline_sites, __retpoline_sites_end,
>  			__cfi_sites, __cfi_sites_end, true);
> +	cfi_debug = false;
>  
>  	/*
>  	 * Rewrite the retpolines, must be done before alternatives since