linux-kernel - Re: [PATCH 04/28] KVM: SVM: Kill the VM instead of the host if MSR interception is buggy

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Message-ID: <aD74GyZmU4Z0dMn1@google.com>
Date: Tue, 3 Jun 2025 06:26:51 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Chao Gao <chao.gao@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Borislav Petkov <bp@...en8.de>, Xin Li <xin@...or.com>, Dapeng Mi <dapeng1.mi@...ux.intel.com>
Subject: Re: [PATCH 04/28] KVM: SVM: Kill the VM instead of the host if MSR
 interception is buggy

On Tue, Jun 03, 2025, Chao Gao wrote:
> On Thu, May 29, 2025 at 04:39:49PM -0700, Sean Christopherson wrote:
> >WARN and kill the VM instead of panicking the host if KVM attempts to set
> >or query MSR interception for an unsupported MSR.  Accessing the MSR
> >interception bitmaps only meaningfully affects post-VMRUN behavior, and
> >KVM_BUG_ON() is guaranteed to prevent the current vCPU from doing VMRUN,
> >i.e. there is no need to panic the entire host.
> >
> >Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> >---
> > arch/x86/kvm/svm/svm.c | 6 ++++--
> > 1 file changed, 4 insertions(+), 2 deletions(-)
> >
> >diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> >index 36a99b87a47f..d5d11cb0c987 100644
> >--- a/arch/x86/kvm/svm/svm.c
> >+++ b/arch/x86/kvm/svm/svm.c
> >@@ -827,7 +827,8 @@ static bool msr_write_intercepted(struct kvm_vcpu *vcpu, u32 msr)
> > 	bit_write = 2 * (msr & 0x0f) + 1;
> > 	tmp       = msrpm[offset];
> 
> not an issue with this patch. but shouldn't the offset be checked against
> MSR_INVALID before being used to index msrpm[]?

Oof, yes.  To some extent, it _is_ a problem with this patch, because using
KVM_BUG_ON() makes the OOB access less fatal.  Though it's just a load, and code
that should be unreachable, but still worth cleaning up.

Anyways, I'll place the KVM_BUG_ON()s in the right location as part of this patch.

Thanks!