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

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Date:   Fri, 9 Apr 2021 09:56:27 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Sean Christopherson <seanjc@...gle.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 4/8/21 2:48 PM, Sean Christopherson wrote:
> 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.

For development, I'd want to see it all the time. But since it is guest
triggerable, the _once() method is really needed in production. So in the
latest version I just dropped the message/notification.

Thanks,
Tom

>