linux-kernel - Re: [PATCH 7/7] x86/bugs: Replace CONFIG_SPECTRE_BHI_{ON,OFF} with CONFIG_MITIGATION_SPECTRE

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Date: Thu, 11 Apr 2024 09:48:42 +0200
From: Ingo Molnar <mingo@...nel.org>
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>,
	Sean Christopherson <seanjc@...gle.com>,
	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>
Subject: Re: [PATCH 7/7] x86/bugs: Replace CONFIG_SPECTRE_BHI_{ON,OFF} with
 CONFIG_MITIGATION_SPECTRE_BHI


* Josh Poimboeuf <jpoimboe@...nel.org> wrote:

> For consistency with the other CONFIG_MITIGATION_* options, replace the
> CONFIG_SPECTRE_BHI_{ON,OFF} options with a single
> CONFIG_MITIGATION_SPECTRE_BHI option.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> ---
>  arch/x86/Kconfig           | 17 +++--------------
>  arch/x86/kernel/cpu/bugs.c |  2 +-
>  2 files changed, 4 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index b63b6767a63d..4474bf32d0a4 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2633,27 +2633,16 @@ config MITIGATION_RFDS
>  	  stored in floating point, vector and integer registers.
>  	  See also <file:Documentation/admin-guide/hw-vuln/reg-file-data-sampling.rst>
>  
> -choice
> -	prompt "Clear branch history"
> +config MITIGATION_SPECTRE_BHI
> +	bool "Mitigate Spectre-BHB (Branch History Injection)"
>  	depends on CPU_SUP_INTEL
> -	default SPECTRE_BHI_ON
> +	default y
>  	help
>  	  Enable BHI mitigations. BHI attacks are a form of Spectre V2 attacks
>  	  where the branch history buffer is poisoned to speculatively steer
>  	  indirect branches.
>  	  See <file:Documentation/admin-guide/hw-vuln/spectre.rst>
>  
> -config SPECTRE_BHI_ON
> -	bool "on"
> -	help
> -	  Equivalent to setting spectre_bhi=on command line parameter.
> -config SPECTRE_BHI_OFF
> -	bool "off"
> -	help
> -	  Equivalent to setting spectre_bhi=off command line parameter.
> -
> -endchoice
> -
>  endif
>  
>  config ARCH_HAS_ADD_PAGES
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 74ade6d7caa3..4c46fa2d08c2 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1628,7 +1628,7 @@ enum bhi_mitigations {
>  };
>  
>  static enum bhi_mitigations bhi_mitigation __ro_after_init =
> -	IS_ENABLED(CONFIG_SPECTRE_BHI_ON) ? BHI_MITIGATION_ON : BHI_MITIGATION_OFF;
> +	IS_ENABLED(CONFIG_MITIGATION_SPECTRE_BHI_ON) ? BHI_MITIGATION_ON : BHI_MITIGATION_OFF;

Uhm, after this patch there's no CONFIG_MITIGATION_SPECTRE_BHI_ON anymore, 
which is kindof nasty, as IS_ENABLED() doesn't generate a build failure for 
non-existent Kconfig variables IIRC ...

So AFAICT this patch turns on BHI unconditionally.

I've fixed as per the patch below, but please double check the end result 
in tip:x86/urgent once I've pushed it out..

Thanks,

	Ingo

=====================>
 arch/x86/kernel/cpu/bugs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 80d9018da3d2..25111ad388d9 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1628,7 +1628,7 @@ enum bhi_mitigations {
 };
 
 static enum bhi_mitigations bhi_mitigation __ro_after_init =
-	IS_ENABLED(CONFIG_MITIGATION_SPECTRE_BHI_ON) ? BHI_MITIGATION_ON : BHI_MITIGATION_OFF;
+	IS_ENABLED(CONFIG_MITIGATION_SPECTRE_BHI) ? BHI_MITIGATION_ON : BHI_MITIGATION_OFF;
 
 static int __init spectre_bhi_parse_cmdline(char *str)
 {