linux-kernel - Re: [RFC PATCH 07/56] x86/bugs: Reset spectre_v2

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Message-ID: <DDJRFMTS5HX9.3MT8W88VM4U2I@google.com>
Date: Thu, 16 Oct 2025 12:54:03 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: David Kaplan <david.kaplan@....com>, Thomas Gleixner <tglx@...utronix.de>, 
	Borislav Petkov <bp@...en8.de>, Peter Zijlstra <peterz@...radead.org>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, 
	Ingo Molnar <mingo@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>, <x86@...nel.org>, 
	"H . Peter Anvin" <hpa@...or.com>
Cc: Alexander Graf <graf@...zon.com>, Boris Ostrovsky <boris.ostrovsky@...cle.com>, 
	<linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 07/56] x86/bugs: Reset spectre_v2_user mitigations

On Mon Oct 13, 2025 at 2:33 PM UTC, David Kaplan wrote:
> Add function to reset spectre_v2_user mitigations back to their boot-time
> defaults.
>
> Signed-off-by: David Kaplan <david.kaplan@....com>
> ---
>  arch/x86/kernel/cpu/bugs.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 1f56ccb5f641..4ca46f58e384 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -2056,6 +2056,18 @@ static void __init spectre_v2_user_apply_mitigation(void)
>  	}
>  }
>  
> +#ifdef CONFIG_DYNAMIC_MITIGATIONS
> +static void spectre_v2_user_reset_mitigation(void)
> +{
> +	static_branch_disable(&switch_vcpu_ibpb);
> +	static_branch_disable(&switch_mm_always_ibpb);
> +	static_branch_disable(&switch_mm_cond_ibpb);
> +	spectre_v2_user_stibp = SPECTRE_V2_USER_NONE;
> +	spectre_v2_user_ibpb = SPECTRE_V2_USER_NONE;
> +	spectre_v2_user_cmd = SPECTRE_V2_USER_CMD_AUTO;
> +}
> +#endif
> +
>  static const char * const spectre_v2_strings[] = {
>  	[SPECTRE_V2_NONE]			= "Vulnerable",
>  	[SPECTRE_V2_RETPOLINE]			= "Mitigation: Retpolines",
> @@ -3844,5 +3856,6 @@ void arch_cpu_reset_mitigations(void)
>  	spectre_v1_reset_mitigation();
>  	spectre_v2_reset_mitigation();
>  	retbleed_reset_mitigation();
> +	spectre_v2_user_reset_mitigation();
>  }
>  #endif

I think this might be failing to account for task state? E.g. if a
user boots with spectre_v2=off then we ignore the PR_SPEC_DISABLE calls
that would enable IBPB-on-context-switch for that task. Then if they
enable it via this dynamic interface they probably expect their
PR_SPEC_DISABLE to take effect retroactively. I don't think it will with
the current code, do I have that right?