linux-kernel - Re: [PATCH 5/6] KVM: nVMX: Always use IBPB to properly virtualize IBRS

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Message-ID: <CALMp9eSUGYfyogSruFY_o7EXdKUB52EC3iOU4r+vyrnG3cW-4g@mail.gmail.com>
Date: Wed, 19 Feb 2025 15:09:43 -0800
From: Jim Mattson <jmattson@...gle.com>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	"H. Peter Anvin" <hpa@...or.com>, Peter Zijlstra <peterz@...radead.org>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, 
	Andy Lutomirski <luto@...nel.org>, Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/6] KVM: nVMX: Always use IBPB to properly virtualize IBRS

On Wed, Feb 19, 2025 at 2:11 PM Yosry Ahmed <yosry.ahmed@...ux.dev> wrote:
>
> On synthesized nested VM-exits in VMX, an IBPB is performed if IBRS is
> advertised to the guest to properly provide separate prediction domains
> for L1 and L2. However, this is currently conditional on
> X86_FEATURE_USE_IBPB, which depends on the host spectre_v2_user
> mitigation.
>
> In short, if spectre_v2_user=no, IBRS is not virtualized correctly and
> L1 becomes suspectible to attacks from L2. Fix this by performing the

Nit: susceptible.

> IBPB regardless of X86_FEATURE_USE_IBPB.
>
> Fixes: 2e7eab81425a ("KVM: VMX: Execute IBPB on emulated VM-exit when guest has IBRS")
> Signed-off-by: Yosry Ahmed <yosry.ahmed@...ux.dev>

Argh! No doubt, I was burned once again by assuming that a function
name (indirect_branch_prediction_barrier) was actually descriptive.

Reviewed-by: Jim Mattson <jmattson@...gle.com>