linux-kernel - Re: [PATCH v2 3/4] x86/bugs: KVM: Add support for SRSO_MSR

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Message-ID: <20250108181434.GGZ37AiioQkcYbqugO@fat_crate.local>
Date: Wed, 8 Jan 2025 19:14:34 +0100
From: Borislav Petkov <bp@...en8.de>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Borislav Petkov <bp@...nel.org>, X86 ML <x86@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	KVM <kvm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/4] x86/bugs: KVM: Add support for SRSO_MSR_FIX

On Wed, Jan 08, 2025 at 09:18:17AM -0800, Sean Christopherson wrote:
> then my vote is to go with the user_return approach.  It's unfortunate that
> restoring full speculation may be delayed until a CPU exits to userspace or KVM
> is unloaded, but given that enable_virt_at_load is enabled by default, in practice
> it's likely still far better than effectively always running the host with reduced
> speculation.

I guess. Kaplan just said something to that effect so I guess we can start
with that and then see who complains and address it if she cries loud enough.
:-P

> No?  svm_vcpu_load() emits IBPB when switching VMCBs, i.e. when switching between

Bah, nevermind. I got confused by our own whitepaper. /facepalm.

So here's the deal:

The machine has SRSO_USER_KERNEL_NO=1. Which means, you don't need safe-RET.
So we fallback to ibpb-on-vmexit.

Now, if the machine sports BpSpecReduce, we do that and that covers all the
vectors. Otherwise, IBPB-on-VMEXIT it is.

The VM/VM attack vector the paper is talking about and having to IBPB is for
the Spectre v2 side of things. Not SRSO.

Yeah, lemme document that while it is fresh in my head. This is exactly why
I wanted Josh to start mitigation documentation - exactly for such nasties.

Thx.

-- 
Regards/Gruss,
    Boris.

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