linux-kernel - RE: [PATCH v5 03/16] x86/bugs: Restructure MMIO mitigation

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Message-ID:
 <LV3PR12MB9265795E7CF35195B7FE02A594852@LV3PR12MB9265.namprd12.prod.outlook.com>
Date: Thu, 24 Apr 2025 20:31:25 +0000
From: "Kaplan, David" <David.Kaplan@....com>
To: Borislav Petkov <bp@...en8.de>
CC: Thomas Gleixner <tglx@...utronix.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" <x86@...nel.org>, "H .
 Peter Anvin" <hpa@...or.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v5 03/16] x86/bugs: Restructure MMIO mitigation

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: Borislav Petkov <bp@...en8.de>
> Sent: Thursday, April 24, 2025 4:19 PM
> To: Kaplan, David <David.Kaplan@....com>
> Cc: Thomas Gleixner <tglx@...utronix.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>; linux-kernel@...r.kernel.org
> Subject: Re: [PATCH v5 03/16] x86/bugs: Restructure MMIO mitigation
>
> Caution: This message originated from an External Source. Use proper caution
> when opening attachments, clicking links, or responding.
>
>
> On Fri, Apr 18, 2025 at 11:17:08AM -0500, David Kaplan wrote:
> > +static void __init mmio_apply_mitigation(void) {
> > +     if (mmio_mitigation == MMIO_MITIGATION_OFF)
> > +             return;
> >
> >       /*
> > -      * X86_FEATURE_CLEAR_CPU_BUF could be enabled by other VERW based
> > -      * mitigations, disable KVM-only mitigation in that case.
> > +      * Only enable the VMM mitigation if the CPU buffer clear mitigation is
> > +      * not being used.
> >        */
> > -     if (boot_cpu_has(X86_FEATURE_CLEAR_CPU_BUF))
> > +     if (verw_mitigation_selected) {
> > +             setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
> >               static_branch_disable(&cpu_buf_vm_clear);
> > -     else
> > +     } else {
> >               static_branch_enable(&cpu_buf_vm_clear);
> > +     }
>
> Sorry, but I'm still not happy about this.
>
> After this patch, we have:
>
>         /*
>          * Enable CPU buffer clear mitigation for host and VMM, if also affected
>          * by MDS or TAA.
>          */
>         if (boot_cpu_has_bug(X86_BUG_MDS) || taa_vulnerable())
>                 verw_mitigation_selected = true;
>
> in the select function.
>
> The comment is wrong. The code does: enable the VERW mitigation for MMIO if
> affected by MDS or TAA. verw_mitigation_selected doesn't have any bearing on
> whether this should be a host or VMM mitigation - as its name says, a VERW
> mitigation has been selected.

verw_mitigation_selected implies that X86_FEATURE_CLEAR_CPU_BUF will be enabled, which does a VERW on kernel/vmm exits.

So I'm not sure the comment is really wrong, but it can be rephrased.

>
> Then in the apply function:
>
>         /*
>          * Only enable the VMM mitigation if the CPU buffer clear mitigation is
>          * not being used.
>          */
>         if (verw_mitigation_selected) {
>                 setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
>                 static_branch_disable(&cpu_buf_vm_clear);
>         } else {
>                 static_branch_enable(&cpu_buf_vm_clear);
>         }
>
> Comment is again wrong. verw_mitigation_selected doesn't mean the CPU buffer
> clear mitigation is not being used.

But it kind of does.  !verw_mitigation_selected means that the X86_FEATURE bit there isn't set.  So the VMM-based mitigation (the static branch) is only used if the broader X86_FEATURE_CLEAR_CPU_BUF is not being used.

>
> Yes yes, it boils down to the same thing in the end but reading it confusing as hell.
> verw_mitigation_selected means what its name is: a VERW mitigation has been
> selected and nothing else.
>
> Renaming the var is probably unnecessary churn but you can fix the comments and
> still rename the key:
>
> ---
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c index
> c97ded4d55e5..4a5bd6214508 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -157,8 +157,8 @@
> DEFINE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
>   * X86_FEATURE_CLEAR_CPU_BUF, and should only be enabled when KVM-only
>   * mitigation is required.
>   */
> -DEFINE_STATIC_KEY_FALSE(cpu_buf_vm_clear);
> -EXPORT_SYMBOL_GPL(cpu_buf_vm_clear);
> +DEFINE_STATIC_KEY_FALSE(clear_cpu_buf_vm);
> +EXPORT_SYMBOL_GPL(clear_cpu_buf_vm);
>
>  void __init cpu_select_mitigations(void)  { @@ -528,10 +528,7 @@ static void
> __init mmio_select_mitigation(void)
>         if (mmio_mitigation == MMIO_MITIGATION_OFF)
>                 return;
>
> -       /*
> -        * Enable CPU buffer clear mitigation for host and VMM, if also affected
> -        * by MDS or TAA.
> -        */
> +       /* Enable full VERW mitigation if also affected by MDS or TAA.
> + */
>         if (boot_cpu_has_bug(X86_BUG_MDS) || taa_vulnerable())
>                 verw_mitigation_selected = true;  } @@ -568,14 +565,14 @@ static void
> __init mmio_apply_mitigation(void)
>                 return;
>
>         /*
> -        * Only enable the VMM mitigation if the CPU buffer clear mitigation is
> -        * not being used.
> +        * Full VERW mitigation selection enables host and VMENTER buffer clearing,
> +        * otherwise buffer clearing only on VMENTER.
>          */
>         if (verw_mitigation_selected) {
>                 setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
> -               static_branch_disable(&cpu_buf_vm_clear);
> +               static_branch_disable(&clear_cpu_buf_vm);
>         } else {
> -               static_branch_enable(&cpu_buf_vm_clear);
> +               static_branch_enable(&clear_cpu_buf_vm);
>         }
>
>         /*
> @@ -681,7 +678,7 @@ static void __init md_clear_update_mitigation(void)
>                 taa_select_mitigation();
>         }
>         /*
> -        * MMIO_MITIGATION_OFF is not checked here so that cpu_buf_vm_clear
> +        * MMIO_MITIGATION_OFF is not checked here so that
> + clear_cpu_buf_vm
>          * gets updated correctly as per X86_FEATURE_CLEAR_CPU_BUF state.
>          */
>         if (boot_cpu_has_bug(X86_BUG_MMIO_STALE_DATA)) {
> ---
>

I'm ok with this patch, as long as 'full VERW mitigation' is considered a clear enough term.  I think the updated comment in the apply function does explain what that means, so if that's good enough I'm ok.

--David Kaplan