linux-hardening - Re: [PATCH v2 2/7] kcfi: Add core Kernel Control Flow Integrity infrastructure

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Message-ID: <20250911072935.GW3245006@noisy.programming.kicks-ass.net>
Date: Thu, 11 Sep 2025 09:29:35 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Kees Cook <kees@...nel.org>
Cc: Qing Zhao <qing.zhao@...cle.com>, Andrew Pinski <pinskia@...il.com>,
	Richard Biener <rguenther@...e.de>,
	Joseph Myers <josmyers@...hat.com>, Jan Hubicka <hubicka@....cz>,
	Richard Earnshaw <richard.earnshaw@....com>,
	Richard Sandiford <richard.sandiford@....com>,
	Marcus Shawcroft <marcus.shawcroft@....com>,
	Kyrylo Tkachov <kyrylo.tkachov@....com>,
	Kito Cheng <kito.cheng@...il.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Andrew Waterman <andrew@...ive.com>,
	Jim Wilson <jim.wilson.gcc@...il.com>,
	Dan Li <ashimida.1990@...il.com>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Ramon de C Valle <rcvalle@...gle.com>,
	Joao Moreira <joao@...rdrivepizza.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Bill Wendling <morbo@...gle.com>,
	"gcc-patches@....gnu.org" <gcc-patches@....gnu.org>,
	"linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH v2 2/7] kcfi: Add core Kernel Control Flow Integrity
 infrastructure

On Wed, Sep 10, 2025 at 08:05:11PM -0700, Kees Cook wrote:

> > > +/* Check if a function needs KCFI preamble generation.
> > > +   ALL functions get preambles when -fsanitize=kcfi is enabled, regardless
> > > +   of no_sanitize("kcfi") attribute.  */
> > 
> > Why no_sanitize(“kcfi”) is not considered here?
> 
> no_sanitize(“kcfi”) is strictly about whether call-site checking
> is performed within the function. It is not used to mark a function as
> not being the target of a KCFI call.

I'll once again argue that __attribute__((nocf_check)) (aka. __noendbr)
should have that effect.

If there is no ENDBR, then no amount of kCFI preamble will make the
function (indirectly) callable.