Re: [PATCH Part2 v5 21/45] KVM: SVM: Make AVIC backing, VMSA and VMCB memory allocation SNP safe

Marc Orr
2021-09-23T19:17:46Z

On Thu, Sep 23, 2021 at 11:09 AM Brijesh Singh <brijesh.singh@amd.com> wrote:
>
>
> On 9/22/21 1:55 PM, Dr. David Alan Gilbert wrote:
> > * Brijesh Singh (brijesh.singh@amd.com) wrote:
> >> Implement a workaround for an SNP erratum where the CPU will incorrectly
> >> signal an RMP violation #PF if a hugepage (2mb or 1gb) collides with the
> >> RMP entry of a VMCB, VMSA or AVIC backing page.
> >>
> >> When SEV-SNP is globally enabled, the CPU marks the VMCB, VMSA, and AVIC
> >> backing pages as "in-use" in the RMP after a successful VMRUN. This is
> >> done for _all_ VMs, not just SNP-Active VMs.
> > Can you explain what 'globally enabled' means?
>
> This means that SNP is enabled in host SYSCFG_MSR.Snp=1. Once its
> enabled then RMP checks are enforced.
>
>
> > Or more specifically, can we trip this bug on public hardware that has
> > the SNP enabled in the bios, but no SNP init in the host OS?
>
> Enabling the SNP support on host is 3 step process:
>
> step1 (bios): reserve memory for the RMP table.
>
> step2 (host): initialize the RMP table memory, set the SYSCFG msr to
> enable the SNP feature
>
> step3 (host): call the SNP_INIT to initialize the SNP firmware (this is
> needed only if you ever plan to launch SNP guest from this host).
>
> The "SNP globally enabled" means the step 1 to 2. The RMP checks are
> enforced as soon as step 2 is completed.

It might be good to update the code to reflect this reply, such that
the new `snp_safe_alloc_page()` function introduced in this patch only
deviates from "normal" page allocation logic when these two conditions
are met. We could introduce a global variable, `snp_globally_enabled`
that defaults to false and gets set to true after we write the SYSCFG
MSR.

> thanks
>
> >
> > Dave
> >
> >> If the hypervisor accesses an in-use page through a writable translation,
> >> the CPU will throw an RMP violation #PF. On early SNP hardware, if an
> >> in-use page is 2mb aligned and software accesses any part of the associated
> >> 2mb region with a hupage, the CPU will incorrectly treat the entire 2mb
> >> region as in-use and signal a spurious RMP violation #PF.
> >>
> >> The recommended is to not use the hugepage for the VMCB, VMSA or
> >> AVIC backing page. Add a generic allocator that will ensure that the page
> >> returns is not hugepage (2mb or 1gb) and is safe to be used when SEV-SNP
> >> is enabled.
> >>
> >> Co-developed-by: Marc Orr <marcorr@google.com>
> >> Signed-off-by: Marc Orr <marcorr@google.com>
> >> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> >> ---
> >> arch/x86/include/asm/kvm-x86-ops.h | 1 +
> >> arch/x86/include/asm/kvm_host.h | 1 +
> >> arch/x86/kvm/lapic.c | 5 ++++-
> >> arch/x86/kvm/svm/sev.c | 35 ++++++++++++++++++++++++++++++
> >> arch/x86/kvm/svm/svm.c | 16 ++++++++++++--
> >> arch/x86/kvm/svm/svm.h | 1 +
> >> 6 files changed, 56 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> >> index a12a4987154e..36a9c23a4b27 100644
> >> --- a/arch/x86/include/asm/kvm-x86-ops.h
> >> +++ b/arch/x86/include/asm/kvm-x86-ops.h
> >> @@ -122,6 +122,7 @@ KVM_X86_OP_NULL(enable_direct_tlbflush)
> >> KVM_X86_OP_NULL(migrate_timers)
> >> KVM_X86_OP(msr_filter_changed)
> >> KVM_X86_OP_NULL(complete_emulated_msr)
> >> +KVM_X86_OP(alloc_apic_backing_page)
> >>
> >> #undef KVM_X86_OP
> >> #undef KVM_X86_OP_NULL
> >> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> >> index 974cbfb1eefe..5ad6255ff5d5 100644
> >> --- a/arch/x86/include/asm/kvm_host.h
> >> +++ b/arch/x86/include/asm/kvm_host.h
> >> @@ -1453,6 +1453,7 @@ struct kvm_x86_ops {
> >> int (*complete_emulated_msr)(struct kvm_vcpu *vcpu, int err);
> >>
> >> void (*vcpu_deliver_sipi_vector)(struct kvm_vcpu *vcpu, u8 vector);
> >> + void *(*alloc_apic_backing_page)(struct kvm_vcpu *vcpu);
> >> };
> >>
> >> struct kvm_x86_nested_ops {
> >> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> >> index ba5a27879f1d..05b45747b20b 100644
> >> --- a/arch/x86/kvm/lapic.c
> >> +++ b/arch/x86/kvm/lapic.c
> >> @@ -2457,7 +2457,10 @@ int kvm_create_lapic(struct kvm_vcpu *vcpu, int timer_advance_ns)
> >>
> >> vcpu->arch.apic = apic;
> >>
> >> - apic->regs = (void *)get_zeroed_page(GFP_KERNEL_ACCOUNT);
> >> + if (kvm_x86_ops.alloc_apic_backing_page)
> >> + apic->regs = static_call(kvm_x86_alloc_apic_backing_page)(vcpu);
> >> + else
> >> + apic->regs = (void *)get_zeroed_page(GFP_KERNEL_ACCOUNT);
> >> if (!apic->regs) {
> >> printk(KERN_ERR "malloc apic regs error for vcpu %x\n",
> >> vcpu->vcpu_id);
> >> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> >> index 1644da5fc93f..8771b878193f 100644
> >> --- a/arch/x86/kvm/svm/sev.c
> >> +++ b/arch/x86/kvm/svm/sev.c
> >> @@ -2703,3 +2703,38 @@ void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector)
> >> break;
> >> }
> >> }
> >> +
> >> +struct page *snp_safe_alloc_page(struct kvm_vcpu *vcpu)
> >> +{
> >> + unsigned long pfn;
> >> + struct page *p;
> >> +
> >> + if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
> >> + return alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);

Continuing my other comment, above: if we introduce a
`snp_globally_enabled` var, we could use that here, rather than
`cpu_feature_enabled(X86_FEATURE_SEV_SNP)`.

> >> +
> >> + /*
> >> + * Allocate an SNP safe page to workaround the SNP erratum where
> >> + * the CPU will incorrectly signal an RMP violation #PF if a
> >> + * hugepage (2mb or 1gb) collides with the RMP entry of VMCB, VMSA
> >> + * or AVIC backing page. The recommeded workaround is to not use the
> >> + * hugepage.
> >> + *
> >> + * Allocate one extra page, use a page which is not 2mb aligned
> >> + * and free the other.
> >> + */
> >> + p = alloc_pages(GFP_KERNEL_ACCOUNT | __GFP_ZERO, 1);
> >> + if (!p)
> >> + return NULL;
> >> +
> >> + split_page(p, 1);
> >> +
> >> + pfn = page_to_pfn(p);
> >> + if (IS_ALIGNED(__pfn_to_phys(pfn), PMD_SIZE)) {
> >> + pfn++;
> >> + __free_page(p);
> >> + } else {
> >> + __free_page(pfn_to_page(pfn + 1));
> >> + }
> >> +
> >> + return pfn_to_page(pfn);
> >> +}
> >> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> >> index 25773bf72158..058eea8353c9 100644
> >> --- a/arch/x86/kvm/svm/svm.c
> >> +++ b/arch/x86/kvm/svm/svm.c
> >> @@ -1368,7 +1368,7 @@ static int svm_create_vcpu(struct kvm_vcpu *vcpu)
> >> svm = to_svm(vcpu);
> >>
> >> err = -ENOMEM;
> >> - vmcb01_page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
> >> + vmcb01_page = snp_safe_alloc_page(vcpu);
> >> if (!vmcb01_page)
> >> goto out;
> >>
> >> @@ -1377,7 +1377,7 @@ static int svm_create_vcpu(struct kvm_vcpu *vcpu)
> >> * SEV-ES guests require a separate VMSA page used to contain
> >> * the encrypted register state of the guest.
> >> */
> >> - vmsa_page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
> >> + vmsa_page = snp_safe_alloc_page(vcpu);
> >> if (!vmsa_page)
> >> goto error_free_vmcb_page;
> >>
> >> @@ -4539,6 +4539,16 @@ static int svm_vm_init(struct kvm *kvm)
> >> return 0;
> >> }
> >>
> >> +static void *svm_alloc_apic_backing_page(struct kvm_vcpu *vcpu)
> >> +{
> >> + struct page *page = snp_safe_alloc_page(vcpu);
> >> +
> >> + if (!page)
> >> + return NULL;
> >> +
> >> + return page_address(page);
> >> +}
> >> +
> >> static struct kvm_x86_ops svm_x86_ops __initdata = {
> >> .hardware_unsetup = svm_hardware_teardown,
> >> .hardware_enable = svm_hardware_enable,
> >> @@ -4667,6 +4677,8 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
> >> .complete_emulated_msr = svm_complete_emulated_msr,
> >>
> >> .vcpu_deliver_sipi_vector = svm_vcpu_deliver_sipi_vector,
> >> +
> >> + .alloc_apic_backing_page = svm_alloc_apic_backing_page,
> >> };
> >>
> >> static struct kvm_x86_init_ops svm_init_ops __initdata = {
> >> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> >> index d1f1512a4b47..e40800e9c998 100644
> >> --- a/arch/x86/kvm/svm/svm.h
> >> +++ b/arch/x86/kvm/svm/svm.h
> >> @@ -575,6 +575,7 @@ void sev_es_create_vcpu(struct vcpu_svm *svm);
> >> void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector);
> >> void sev_es_prepare_guest_switch(struct vcpu_svm *svm, unsigned int cpu);
> >> void sev_es_unmap_ghcb(struct vcpu_svm *svm);
> >> +struct page *snp_safe_alloc_page(struct kvm_vcpu *vcpu);
> >>
> >> /* vmenter.S */
> >>
> >> --
> >> 2.17.1
> >>
> >>