linux-kernel - Re: [PATCH v2] KVM: SVM: Make sure GHCB is mapped before updating

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Message-ID: <YG8/WHFOPX6H1eJf@google.com>
Date:   Thu, 8 Apr 2021 17:37:28 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v2] KVM: SVM: Make sure GHCB is mapped before updating

On Thu, Apr 08, 2021, Tom Lendacky wrote:
> On 4/8/21 12:10 PM, Sean Christopherson wrote:
> > On Thu, Apr 08, 2021, Tom Lendacky wrote:
> >> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> >> index 83e00e524513..7ac67615c070 100644
> >> --- a/arch/x86/kvm/svm/sev.c
> >> +++ b/arch/x86/kvm/svm/sev.c
> >> @@ -2105,5 +2105,8 @@ void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector)
> >>  	 * the guest will set the CS and RIP. Set SW_EXIT_INFO_2 to a
> >>  	 * non-zero value.
> >>  	 */
> >> +	if (WARN_ON_ONCE(!svm->ghcb))
> > 
> > Isn't this guest triggerable?  I.e. send a SIPI without doing the reset hold?
> > If so, this should not WARN.
> 
> Yes, it is a guest triggerable event. But a guest shouldn't be doing that,
> so I thought adding the WARN_ON_ONCE() just to detect it wasn't bad.
> Definitely wouldn't want a WARN_ON().

WARNs are intended only for host issues, e.g. a malicious guest shouldn't be
able to crash the host when running with panic_on_warn.