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SubjectRe: [PATCH 1/2] KVM: nVMX: Implement EPTP switching for the L1 hypervisor

> > What happens if the root_hpa is dropped by the L0 MMU? I'm not sure
> > what removes it from the shadow EPT list.
>
> That would result in a vmfunc vmexit, which will jump to handle_vmfunc
> and then a call to mmu_alloc_shadow_roots() that will overwrite the shadow
> eptp entry with the new one.
>
> I believe part of your question is also that root_hpa is valid, just not
> tracking the current l1 eptp and in that case, the processor can jump to
> some other guest's page tables which is a problem. In that case, I think it
> should be possible to invalidate that entry in the shadow eptp list.

Exactly. However, I'm not sure it's worth fixing it. There's already
enough complication in the MMU, and there are way more important things
to optimize in nested VMX!

Paolo

> > For a first version of the patch, I would prefer a less optimized
> > version that always goes through L0 when L2 executes VMFUNC.
> > To achieve this effect, you can copy "enable VM functions" secondary
> > execution control from vmcs12 to vmcs02, but leave the VM-function
> > controls to 0 in vmcs02.
>
> Is the current approach prone to other undesired corner cases like the one
> you pointed above ? :) I would be uncomfortable having this in if you feel
> having the cpu jump to a new eptp feels like an interesting exploitable
> interface; however, I think it would be nice to have l2 execute vmfunc
> without exiting to L0.
>
> Thanks for the quick review!
>
> > Paolo
> >
> >> Signed-off-by: Bandan Das <bsd@redhat.com>
> >> ---
> >> arch/x86/include/asm/vmx.h | 5 +++
> >> arch/x86/kvm/vmx.c | 104
> >> +++++++++++++++++++++++++++++++++++++++++++++
> >> 2 files changed, 109 insertions(+)
> >>
> >> diff --git a/arch/x86/include/asm/vmx.h b/arch/x86/include/asm/vmx.h
> >> index 35cd06f..e06783e 100644
> >> --- a/arch/x86/include/asm/vmx.h
> >> +++ b/arch/x86/include/asm/vmx.h
> >> @@ -72,6 +72,7 @@
> >> #define SECONDARY_EXEC_PAUSE_LOOP_EXITING 0x00000400
> >> #define SECONDARY_EXEC_RDRAND 0x00000800
> >> #define SECONDARY_EXEC_ENABLE_INVPCID 0x00001000
> >> +#define SECONDARY_EXEC_ENABLE_VMFUNC 0x00002000
> >> #define SECONDARY_EXEC_SHADOW_VMCS 0x00004000
> >> #define SECONDARY_EXEC_RDSEED 0x00010000
> >> #define SECONDARY_EXEC_ENABLE_PML 0x00020000
> >> @@ -114,6 +115,10 @@
> >> #define VMX_MISC_SAVE_EFER_LMA 0x00000020
> >> #define VMX_MISC_ACTIVITY_HLT 0x00000040
> >>
> >> +/* VMFUNC functions */
> >> +#define VMX_VMFUNC_EPTP_SWITCHING 0x00000001
> >> +#define VMFUNC_EPTP_ENTRIES 512
> >> +
> >> static inline u32 vmx_basic_vmcs_revision_id(u64 vmx_basic)
> >> {
> >> return vmx_basic & GENMASK_ULL(30, 0);
> >> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> >> index ca5d2b9..75049c0 100644
> >> --- a/arch/x86/kvm/vmx.c
> >> +++ b/arch/x86/kvm/vmx.c
> >> @@ -240,11 +240,13 @@ struct __packed vmcs12 {
> >> u64 virtual_apic_page_addr;
> >> u64 apic_access_addr;
> >> u64 posted_intr_desc_addr;
> >> + u64 vm_function_control;
> >> u64 ept_pointer;
> >> u64 eoi_exit_bitmap0;
> >> u64 eoi_exit_bitmap1;
> >> u64 eoi_exit_bitmap2;
> >> u64 eoi_exit_bitmap3;
> >> + u64 eptp_list_address;
> >> u64 xss_exit_bitmap;
> >> u64 guest_physical_address;
> >> u64 vmcs_link_pointer;
> >> @@ -441,6 +443,7 @@ struct nested_vmx {
> >> struct page *apic_access_page;
> >> struct page *virtual_apic_page;
> >> struct page *pi_desc_page;
> >> + struct page *shadow_eptp_list;
> >> struct pi_desc *pi_desc;
> >> bool pi_pending;
> >> u16 posted_intr_nv;
> >> @@ -481,6 +484,7 @@ struct nested_vmx {
> >> u64 nested_vmx_cr4_fixed0;
> >> u64 nested_vmx_cr4_fixed1;
> >> u64 nested_vmx_vmcs_enum;
> >> + u64 nested_vmx_vmfunc_controls;
> >> };
> >>
> >> #define POSTED_INTR_ON 0
> >> @@ -1314,6 +1318,22 @@ static inline bool cpu_has_vmx_tsc_scaling(void)
> >> SECONDARY_EXEC_TSC_SCALING;
> >> }
> >>
> >> +static inline bool cpu_has_vmx_vmfunc(void)
> >> +{
> >> + return vmcs_config.cpu_based_exec_ctrl &
> >> + SECONDARY_EXEC_ENABLE_VMFUNC;
> >> +}
> >> +
> >> +static inline u64 vmx_vmfunc_controls(void)
> >> +{
> >> + u64 controls = 0;
> >> +
> >> + if (cpu_has_vmx_vmfunc())
> >> + rdmsrl(MSR_IA32_VMX_VMFUNC, controls);
> >> +
> >> + return controls;
> >> +}
> >> +
> >> static inline bool report_flexpriority(void)
> >> {
> >> return flexpriority_enabled;
> >> @@ -1388,6 +1408,18 @@ static inline bool
> >> nested_cpu_has_posted_intr(struct
> >> vmcs12 *vmcs12)
> >> return vmcs12->pin_based_vm_exec_control & PIN_BASED_POSTED_INTR;
> >> }
> >>
> >> +static inline bool nested_cpu_has_vmfunc(struct vmcs12 *vmcs12)
> >> +{
> >> + return nested_cpu_has2(vmcs12, SECONDARY_EXEC_ENABLE_VMFUNC);
> >> +}
> >> +
> >> +static inline bool nested_cpu_has_eptp_switching(struct vmcs12 *vmcs12)
> >> +{
> >> + return nested_cpu_has_vmfunc(vmcs12) &&
> >> + (vmcs12->vm_function_control &
> >> + VMX_VMFUNC_EPTP_SWITCHING);
> >> +}
> >> +
> >> static inline bool is_nmi(u32 intr_info)
> >> {
> >> return (intr_info & (INTR_INFO_INTR_TYPE_MASK | INTR_INFO_VALID_MASK))
> >> @@ -3143,6 +3175,9 @@ static int vmx_get_vmx_msr(struct kvm_vcpu *vcpu,
> >> u32
> >> msr_index, u64 *pdata)
> >> *pdata = vmx->nested.nested_vmx_ept_caps |
> >> ((u64)vmx->nested.nested_vmx_vpid_caps << 32);
> >> break;
> >> + case MSR_IA32_VMX_VMFUNC:
> >> + *pdata = vmx->nested.nested_vmx_vmfunc_controls;
> >> + break;
> >> default:
> >> return 1;
> >> }
> >> @@ -6959,6 +6994,14 @@ static int enter_vmx_operation(struct kvm_vcpu
> >> *vcpu)
> >> vmx->vmcs01.shadow_vmcs = shadow_vmcs;
> >> }
> >>
> >> + if (vmx_vmfunc_controls() & 1) {
> >> + struct page *page = alloc_page(GFP_KERNEL | __GFP_ZERO);
> >> +
> >> + if (!page)
> >> + goto out_shadow_vmcs;
> >> + vmx->nested.shadow_eptp_list = page;
> >> + }
> >> +
> >> INIT_LIST_HEAD(&(vmx->nested.vmcs02_pool));
> >> vmx->nested.vmcs02_num = 0;
> >>
> >> @@ -7128,6 +7171,11 @@ static void free_nested(struct vcpu_vmx *vmx)
> >> vmx->vmcs01.shadow_vmcs = NULL;
> >> }
> >> kfree(vmx->nested.cached_vmcs12);
> >> +
> >> + if (vmx->nested.shadow_eptp_list) {
> >> + __free_page(vmx->nested.shadow_eptp_list);
> >> + vmx->nested.shadow_eptp_list = NULL;
> >> + }
> >> /* Unpin physical memory we referred to in current vmcs02 */
> >> if (vmx->nested.apic_access_page) {
> >> nested_release_page(vmx->nested.apic_access_page);
> >> @@ -7740,6 +7788,61 @@ static int handle_preemption_timer(struct kvm_vcpu
> >> *vcpu)
> >> return 1;
> >> }
> >>
> >> +static int handle_vmfunc(struct kvm_vcpu *vcpu)
> >> +{
> >> + struct vcpu_vmx *vmx = to_vmx(vcpu);
> >> + struct vmcs12 *vmcs12;
> >> + struct page *page = NULL,
> >> + *shadow_page = vmx->nested.shadow_eptp_list;
> >> + u64 *l1_eptp_list, *shadow_eptp_list;
> >> + u32 index = vcpu->arch.regs[VCPU_REGS_RCX];
> >> + u32 function = vcpu->arch.regs[VCPU_REGS_RAX];
> >> +
> >> + if (is_guest_mode(vcpu)) {
> >> + vmcs12 = get_vmcs12(vcpu);
> >> + if (!nested_cpu_has_ept(vmcs12) ||
> >> + !nested_cpu_has_eptp_switching(vmcs12))
> >> + goto fail;
> >> +
> >> + /*
> >> + * Only function 0 is valid, everything upto 63 injects VMFUNC
> >> + * exit reason to L1, and #UD thereafter
> >> + */
> >> + if (function || !vmcs12->eptp_list_address ||
> >> + index >= VMFUNC_EPTP_ENTRIES)
> >> + goto fail;
> >> +
> >> + page = nested_get_page(vcpu, vmcs12->eptp_list_address);
> >> + if (!page)
> >> + goto fail;
> >> +
> >> + l1_eptp_list = kmap(page);
> >> + if (!l1_eptp_list[index])
> >> + goto fail;
> >> +
> >> + kvm_mmu_unload(vcpu);
> >> + /*
> >> + * TODO: Verify that guest ept satisfies vmentry prereqs
> >> + */
> >> + vmcs12->ept_pointer = l1_eptp_list[index];
> >> + shadow_eptp_list = phys_to_virt(page_to_phys(shadow_page));
> >> + kvm_mmu_reload(vcpu);
> >> + shadow_eptp_list[index] =
> >> + construct_eptp(vcpu->arch.mmu.root_hpa);
> >> + kunmap(page);
> >> +
> >> + return kvm_skip_emulated_instruction(vcpu);
> >> + }
> >> +
> >> +fail:
> >> + if (page)
> >> + kunmap(page);
> >> + nested_vmx_vmexit(vcpu, vmx->exit_reason,
> >> + vmcs_read32(VM_EXIT_INTR_INFO),
> >> + vmcs_readl(EXIT_QUALIFICATION));
> >> + return 1;
> >> +}
> >> +
> >> /*
> >> * The exit handlers return 1 if the exit was handled fully and guest
> >> execution
> >> * may resume. Otherwise they set the kvm_run parameter to indicate what
> >> needs
> >> @@ -7790,6 +7893,7 @@ static int (*const kvm_vmx_exit_handlers[])(struct
> >> kvm_vcpu *vcpu) = {
> >> [EXIT_REASON_XSAVES] = handle_xsaves,
> >> [EXIT_REASON_XRSTORS] = handle_xrstors,
> >> [EXIT_REASON_PML_FULL] = handle_pml_full,
> >> + [EXIT_REASON_VMFUNC] = handle_vmfunc,
> >> [EXIT_REASON_PREEMPTION_TIMER] = handle_preemption_timer,
> >> };
> >>
> >> --
> >> 2.9.4
> >>
> >>
>

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