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

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Message-ID: <20250905085103.GT3245006@noisy.programming.kicks-ass.net>
Date: Fri, 5 Sep 2025 10:51:03 +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,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2 2/7] kcfi: Add core Kernel Control Flow Integrity
 infrastructure

On Thu, Sep 04, 2025 at 05:24:10PM -0700, Kees Cook wrote:
> +- The check-call instruction sequence must be treated a single unit: it
> +  cannot be rearranged or split or optimized. The pattern is that
> +  indirect calls, "call *$target", get converted into:
> +
> +    mov $target_expression, %target ; only present if the expression was
> +                                    ; not already %target register
> +    load -$offset(%target), %tmp    ; load the typeid hash at target
> +    cmp $hash, %tmp                 ; compare expected typeid with loaded
> +    je .Lcheck_passed               ; jump to the indirect call
> +  .Lkcfi_trap$N:                    ; label of trap insn
> +    trap                            ; trap on failure, but arranged so
> +                                    ; "permissive mode" falls through
> +  .Lkcfi_call$N:                    ; label of call insn
> +    call *%target                   ; actual indirect call
> +
> +  This pattern of call immediately after trap provides for the
> +  "permissive" checking mode automatically: the trap gets handled,
> +  a warning emitted, and then execution continues after the trap to
> +  the call.

I know it is far too late to do anything here. But I've recently dug
through a bunch of optimization manual and the like and that Jcc is
about as bad as it gets :/

The old optimization manual states that forward jumps are assumed
not-taken; while backward jumps are assumed taken.

The new wisdom is that any Jcc must be assumed not-taken; that is, the
fallthrough case has the best throughput.

Here we have a forward branch which is assumed taken :-(