Re: [PATCH v2 21/25] KVM: SEV: Make AVIC backing, VMSA and VMCB memory allocation SNP safe

From: Paolo Bonzini
Date: Fri Jan 26 2024 - 06:01:16 EST

On Fri, Jan 26, 2024 at 5:45 AM Michael Roth <michael.roth@xxxxxxx> wrote:
>
> From: Brijesh Singh <brijesh.singh@xxxxxxx>
>
> 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" via a reserved bit in the corresponding RMP
> entry after a successful VMRUN. This is done for _all_ VMs, not just
> SNP-Active VMs.
>
> 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 hugepage, the CPU will
> incorrectly treat the entire 2MB region as in-use and signal a an RMP
> violation #PF.
>
> To avoid this, the recommendation is to not use a 2MB-aligned page for
> the VMCB, VMSA or AVIC pages. Add a generic allocator that will ensure
> that the page returned is not 2MB-aligned and is safe to be used when
> SEV-SNP is enabled. Also implement similar handling for the VMCB/VMSA
> pages of nested guests.
>
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> Co-developed-by: Marc Orr <marcorr@xxxxxxxxxx>
> Signed-off-by: Marc Orr <marcorr@xxxxxxxxxx>
> Reported-by: Alper Gun <alpergun@xxxxxxxxxx> # for nested VMSA case
> Co-developed-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> [mdr: squash in nested guest handling from Ashish, commit msg fixups]
> Signed-off-by: Michael Roth <michael.roth@xxxxxxx>

Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>


> ---
> 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/nested.c | 2 +-
> arch/x86/kvm/svm/sev.c | 32 ++++++++++++++++++++++++++++++
> arch/x86/kvm/svm/svm.c | 17 +++++++++++++---
> arch/x86/kvm/svm/svm.h | 1 +
> 7 files changed, 54 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> index 378ed944b849..ab24ce207988 100644
> --- a/arch/x86/include/asm/kvm-x86-ops.h
> +++ b/arch/x86/include/asm/kvm-x86-ops.h
> @@ -138,6 +138,7 @@ KVM_X86_OP(complete_emulated_msr)
> KVM_X86_OP(vcpu_deliver_sipi_vector)
> KVM_X86_OP_OPTIONAL_RET0(vcpu_get_apicv_inhibit_reasons);
> KVM_X86_OP_OPTIONAL(get_untagged_addr)
> +KVM_X86_OP_OPTIONAL(alloc_apic_backing_page)
>
> #undef KVM_X86_OP
> #undef KVM_X86_OP_OPTIONAL
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index b5b2d0fde579..5c12af29fd9b 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1794,6 +1794,7 @@ struct kvm_x86_ops {
> unsigned long (*vcpu_get_apicv_inhibit_reasons)(struct kvm_vcpu *vcpu);
>
> gva_t (*get_untagged_addr)(struct kvm_vcpu *vcpu, gva_t gva, unsigned int flags);
> + 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 3242f3da2457..1edf93ee3395 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2815,7 +2815,10 @@ 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/nested.c b/arch/x86/kvm/svm/nested.c
> index dee62362a360..55b9a6d96bcf 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -1181,7 +1181,7 @@ int svm_allocate_nested(struct vcpu_svm *svm)
> if (svm->nested.initialized)
> return 0;
>
> - vmcb02_page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
> + vmcb02_page = snp_safe_alloc_page(&svm->vcpu);
> if (!vmcb02_page)
> return -ENOMEM;
> svm->nested.vmcb02.ptr = page_address(vmcb02_page);
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 564091f386f7..f99435b6648f 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -3163,3 +3163,35 @@ void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector)
>
> ghcb_set_sw_exit_info_2(svm->sev_es.ghcb, 1);
> }
> +
> +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);
> +
> + /*
> + * 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 a
> + * 2MB-aligned VMCB, VMSA, or AVIC backing page.
> + *
> + * Allocate one extra page, choose 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, PTRS_PER_PMD))
> + __free_page(p++);
> + else
> + __free_page(p + 1);
> +
> + return p;
> +}
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 61f2bdc9f4f8..272d5ed37ce7 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -703,7 +703,7 @@ static int svm_cpu_init(int cpu)
> int ret = -ENOMEM;
>
> memset(sd, 0, sizeof(struct svm_cpu_data));
> - sd->save_area = alloc_page(GFP_KERNEL | __GFP_ZERO);
> + sd->save_area = snp_safe_alloc_page(NULL);
> if (!sd->save_area)
> return ret;
>
> @@ -1421,7 +1421,7 @@ static int svm_vcpu_create(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;
>
> @@ -1430,7 +1430,7 @@ static int svm_vcpu_create(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;
>
> @@ -4900,6 +4900,16 @@ 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 = {
> .name = KBUILD_MODNAME,
>
> @@ -5031,6 +5041,7 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
>
> .vcpu_deliver_sipi_vector = svm_vcpu_deliver_sipi_vector,
> .vcpu_get_apicv_inhibit_reasons = avic_vcpu_get_apicv_inhibit_reasons,
> + .alloc_apic_backing_page = svm_alloc_apic_backing_page,
> };
>
> /*
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 8ef95139cd24..7f1fbd874c45 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -694,6 +694,7 @@ void sev_es_vcpu_reset(struct vcpu_svm *svm);
> void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector);
> void sev_es_prepare_switch_to_guest(struct sev_es_save_area *hostsa);
> void sev_es_unmap_ghcb(struct vcpu_svm *svm);
> +struct page *snp_safe_alloc_page(struct kvm_vcpu *vcpu);
>
> /* vmenter.S */
>
> --
> 2.25.1
>