linux-kernel - Re: [PATCH 1/2] cpu: Re-enable CPU mitigations by default for !X86 architectures

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Date: Fri, 19 Apr 2024 15:37:12 +0100
From: Will Deacon <will@...nel.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Jonathan Corbet <corbet@....net>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Peter Zijlstra <peterz@...radead.org>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Michael Ellerman <mpe@...erman.id.au>,
	Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH 1/2] cpu: Re-enable CPU mitigations by default for !X86
 architectures

On Tue, Apr 16, 2024 at 05:15:06PM -0700, Sean Christopherson wrote:
> Add a generic Kconfig, CPU_MITIGATIONS, to control whether or not CPU
> mitigations are enabled by default, and force it on for all architectures
> except x86.  A recent commit to turn mitigations off by default if
> SPECULATION_MITIGATIONS=n kinda sorta missed that "cpu_mitigations" is
> completely generic, where as SPECULATION_MITIGATIONS is x86 specific.
> 
> Alternatively, SPECULATION_MITIGATIONS could simply be defined in common
> code, but that creates weirdness for x86 because SPECULATION_MITIGATIONS
> ends up being defined twice, and the default behavior would likely depend
> on the arbitrary include order (if the two definitions diverged).
> 
> Ideally, CPU_MITIGATIONS would be unconditionally on by default for all
> architectures, and manually turned off, but there is no way to unselect a
> Kconfig.
> 
> 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
> Fixes: f337a6a21e2f ("x86/cpu: Actually turn off mitigations by default for SPECULATION_MITIGATIONS=n")
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/Kconfig     | 1 +
>  drivers/base/Kconfig | 3 +++
>  kernel/cpu.c         | 4 ++--
>  3 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 4474bf32d0a4..a0eca6313276 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2490,6 +2490,7 @@ config PREFIX_SYMBOLS
>  
>  menuconfig SPECULATION_MITIGATIONS
>  	bool "Mitigations for speculative execution vulnerabilities"
> +	select CPU_MITIGATIONS
>  	default y
>  	help
>  	  Say Y here to enable options which enable mitigations for
> diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
> index 2b8fd6bb7da0..dab19f15fa57 100644
> --- a/drivers/base/Kconfig
> +++ b/drivers/base/Kconfig
> @@ -191,6 +191,9 @@ config GENERIC_CPU_AUTOPROBE
>  config GENERIC_CPU_VULNERABILITIES
>  	bool
>  
> +config CPU_MITIGATIONS
> +	def_bool !X86
> +
>  config SOC_BUS
>  	bool
>  	select GLOB
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 07ad53b7f119..bb0ff275fb46 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -3207,8 +3207,8 @@ enum cpu_mitigations {
>  };
>  
>  static enum cpu_mitigations cpu_mitigations __ro_after_init =
> -	IS_ENABLED(CONFIG_SPECULATION_MITIGATIONS) ? CPU_MITIGATIONS_AUTO :
> -						     CPU_MITIGATIONS_OFF;
> +	IS_ENABLED(CONFIG_CPU_MITIGATIONS) ? CPU_MITIGATIONS_AUTO :
> +					     CPU_MITIGATIONS_OFF;
>  
>  static int __init mitigations_parse_cmdline(char *arg)
>  {
> -- 
> 2.44.0.683.g7961c838ac-goog

Thanks, Sean!

Acked-by: Will Deacon <will@...nel.org>

Will