linux-kernel - Re: [PATCH v4 2/5] cpu/speculation: Fix CPU mitigation defaults for !x86

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Date: Fri, 19 Apr 2024 17:09:31 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, 
	Linus Torvalds <torvalds@...ux-foundation.org>, 
	Daniel Sneddon <daniel.sneddon@...ux.intel.com>, 
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Alexandre Chartre <alexandre.chartre@...cle.com>, 
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>, Peter Zijlstra <peterz@...radead.org>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Andrew Cooper <andrew.cooper3@...rix.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Nikolay Borisov <nik.borisov@...e.com>, 
	KP Singh <kpsingh@...nel.org>, Waiman Long <longman@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Ingo Molnar <mingo@...nel.org>, Stephen Rothwell <sfr@...b.auug.org.au>, 
	Michael Ellerman <mpe@...erman.id.au>, Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH v4 2/5] cpu/speculation: Fix CPU mitigation defaults for !x86

On Fri, Apr 19, 2024, Josh Poimboeuf wrote:
> CPU speculative execution mitigations were inadvertently disabled on
> non-x86 arches by the following commit:
> 
>  f337a6a21e2f ("x86/cpu: Actually turn off mitigations by default for SPECULATION_MITIGATIONS=n")
> 
> Fix it by replacing CONFIG_SPECULATION_MITIGATIONS with a new generic
> CONFIG_CPU_MITIGATIONS option and moving the x86-specific mitigations to
> a separate menu which depends on CONFIG_CPU_MITIGATIONS.

Ah drat, I didn't check my mailbox until after Cc'ing Linus my own version[*].

I don't have a strong preference between the two, though I do think it's worth
nothing that this will (obvioulsy) allow disabling mitigations at compile time
on all architectures, which may or may not be desirable.

[*] https://lore.kernel.org/all/20240420000556.2645001-2-seanjc@google.com

> Fixes: f337a6a21e2f ("x86/cpu: Actually turn off mitigations by default for SPECULATION_MITIGATIONS=n")
> Reported-by: Stephen Rothwell <sfr@...b.auug.org.au>
> Reported-by: Michael Ellerman <mpe@...erman.id.au>
> Reported-by: Geert Uytterhoeven <geert@...ux-m68k.org>
> Closes: https://lkml.kernel.org/r/20240413115324.53303a68%40canb.auug.org.au
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> ---
>  arch/Kconfig     | 10 ++++++++++
>  arch/x86/Kconfig | 15 +++------------
>  kernel/cpu.c     |  4 ++--
>  3 files changed, 15 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 9f066785bb71..5c96849eb957 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -11,6 +11,16 @@ source "arch/$(SRCARCH)/Kconfig"
>  
>  menu "General architecture-dependent options"
>  
> +config CPU_MITIGATIONS
> +	bool "Mitigations for CPU speculative execution vulnerabilities"
> +	default y
> +	help
> +	  Say Y here to enable mitigations for CPU speculative execution
> +	  vulnerabilities.
> +
> +	  If you say N, all mitigations will be disabled. You really
> +	  should know what you are doing to say so.