linux-kernel - Re: [PATCH final?] x86/bugs: KVM: Add support for SRSO_MSR

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Message-ID: <20250303141046.GHZ8W4ZrPEdWA7Hb-b@fat_crate.local>
Date: Mon, 3 Mar 2025 15:10:46 +0100
From: Borislav Petkov <bp@...en8.de>
To: Patrick Bellasi <derkling@...gle.com>
Cc: Sean Christopherson <seanjc@...gle.com>,
	Yosry Ahmed <yosry.ahmed@...ux.dev>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, x86@...nel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Patrick Bellasi <derkling@...bug.net>,
	Brendan Jackman <jackmanb@...gle.com>,
	David Kaplan <David.Kaplan@....com>
Subject: Re: [PATCH final?] x86/bugs: KVM: Add support for SRSO_MSR_FIX

On Wed, Feb 26, 2025 at 06:45:40PM +0000, Patrick Bellasi wrote:
> +
> +	case SRSO_CMD_BP_SPEC_REDUCE:
> +		if (boot_cpu_has(X86_FEATURE_SRSO_BP_SPEC_REDUCE)) {
> +bp_spec_reduce:
> +			pr_notice("Reducing speculation to address VM/HV SRSO attack vector.\n");

Probably not needed anymore as that will be in srso_strings which is issued
later.

> +			srso_mitigation = SRSO_MITIGATION_BP_SPEC_REDUCE;
> +			break;
> +		} else {
> +			srso_mitigation = SRSO_MITIGATION_BP_SPEC_REDUCE_NA;
> +			pr_warn("BP_SPEC_REDUCE not supported!\n");
> +		}

This is the part I'm worried about: user hears somewhere "bp-spec-reduce" is
faster, sets it but doesn't know whether the hw even supports it. Machine
boots, warns which is a single line and waaay buried in dmesg and continues
unmitigated.

So *maybe* we can make this a lot more subtle and say:

srso=__dont_fall_back_to_ibpb_on_vmexit_if_bp_spec_reduce__

(joking about the name but that should be the gist of what it means)

and then act accordingly when that is specified along with a big fat:

WARN_ON(..."You should not use this as a mitigation option if you don't know
what you're doing")

along with a big fat splat in dmesg.

Hmmm...?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette