linux-kernel - Re: [PATCH Part2 v5 27/45] KVM: SVM: Add KVM_SEV_SNP_LAUNCH

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Message-ID: <CAA03e5HmBHbt4YhE+0Sd0UKf_Nqeip4fRj73pdPfSSmNaKZvBQ@mail.gmail.com>
Date:   Wed, 18 May 2022 13:21:18 -0700
From:   Marc Orr <marcorr@...gle.com>
To:     "Kalra, Ashish" <Ashish.Kalra@....com>
Cc:     x86 <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>, linux-coco@...ts.linux.dev,
        linux-mm@...ck.org,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Alper Gun <alpergun@...gle.com>
Subject: Re: [PATCH Part2 v5 27/45] KVM: SVM: Add KVM_SEV_SNP_LAUNCH_FINISH command

> @@ -2364,16 +2467,29 @@ static void sev_flush_guest_memory(struct vcpu_svm *svm, void *va,
>  void sev_free_vcpu(struct kvm_vcpu *vcpu)
>  {
>         struct vcpu_svm *svm;
> +       u64 pfn;
>
>         if (!sev_es_guest(vcpu->kvm))
>                 return;
>
>         svm = to_svm(vcpu);
> +       pfn = __pa(svm->vmsa) >> PAGE_SHIFT;
>
>         if (vcpu->arch.guest_state_protected)
>                 sev_flush_guest_memory(svm, svm->vmsa, PAGE_SIZE);
> +
> +       /*
> +        * If its an SNP guest, then VMSA was added in the RMP entry as
> +        * a guest owned page. Transition the page to hyperivosr state
> +        * before releasing it back to the system.
> +        */
> +       if (sev_snp_guest(vcpu->kvm) &&
> +           host_rmp_make_shared(pfn, PG_LEVEL_4K, false))
> +               goto skip_vmsa_free;
> +
>         __free_page(virt_to_page(svm->vmsa));
>
> +skip_vmsa_free:
>         if (svm->ghcb_sa_free)
>                 kfree(svm->ghcb_sa);
>  }

Hi Ashish. We're still working with this patch set internally. We
found a bug that I wanted to report in this patch. Above, we need to
flush the VMSA page, `svm->vmsa`, _after_ we call
`host_rmp_make_shared()` to mark the page is shared. Otherwise, the
host gets an RMP violation when it tries to flush the guest-owned VMSA
page.

The bug was silent, at least on our Milan platforms, bef reo
d45829b351ee6 ("KVM: SVM: Flush when freeing encrypted pages even on
SME_COHERENT CPUs"), because the `sev_flush_guest_memory()` helper was
a noop on platforms with the SME_COHERENT feature. However, after
d45829b351ee6, we unconditionally do the flush to keep the IO address
space coherent. And then we hit this bug.

Thanks,
Marc