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

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Message-ID: <YG9d8aOuZKasgw2j@google.com>
Date:   Thu, 8 Apr 2021 19:48:01 +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 12:37 PM, Sean Christopherson wrote:
> > 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.
> > 
> 
> Ah, yeah, forgot about panic_on_warn. I can go back to the original patch
> or do a pr_warn_once(), any pref?

No strong preference.  If you think the print would be helpful for ongoing
development, then it's probably worth adding.