Re: [RFC][PATCH v2 1/1] KVM: nVMX: Don't leak L1 MMIO regions to L2

[Liran Alon <liran.alon@oracle.com>]

> On 10 Sep 2019, at 1:28, Jim Mattson <jmattson@google.com> wrote:
> 
> If the "virtualize APIC accesses" VM-execution control is set in the
> VMCS, the APIC virtualization hardware is triggered when a page walk
> in VMX non-root mode terminates at a PTE wherein the address of the 4k
> page frame matches the APIC-access address specified in the VMCS. On
> hardware, the APIC-access address may be any valid 4k-aligned physical
> address.
> 
> KVM's nVMX implementation enforces the additional constraint that the
> APIC-access address specified in the vmcs12 must be backed by
> cacheable memory in L1. If not, L0 will simply clear the "virtualize
> APIC accesses" VM-execution control in the vmcs02.
> 
> The problem with this approach is that the L1 guest has arranged the
> vmcs12 EPT tables--or shadow page tables, if the "enable EPT"
> VM-execution control is clear in the vmcs12--so that the L2 guest
> physical address(es)--or L2 guest linear address(es)--that reference
> the L2 APIC map to the APIC-access address specified in the
> vmcs12. Without the "virtualize APIC accesses" VM-execution control in
> the vmcs02, the APIC accesses in the L2 guest will directly access the
> APIC-access page in L1.
> 
> When L0 has no mapping whatsoever for the APIC-access address in L1,
> the L2 VM just loses the intended APIC virtualization. However, when
> the L2 APIC-access address is mapped to an MMIO region in L1, the L2
> guest gets direct access to the L1 MMIO device. For example, if the
> APIC-access address specified in the vmcs12 is 0xfee00000, then L2
> gets direct access to L1's APIC.
> 
> Fixing this correctly is complicated. Since this vmcs12 configuration
> is something that KVM cannot faithfully emulate, the appropriate
> response is to exit to userspace with
> KVM_INTERNAL_ERROR_EMULATION. Sadly, the kvm-unit-tests fail, so I'm
> posting this as an RFC.
> 
> Note that the 'Code' line emitted by qemu in response to this error
> shows the guest %rip two instructions after the
> vmlaunch/vmresume. Hmmm.
> 
> Fixes: fe3ef05c7572 ("KVM: nVMX: Prepare vmcs02 from vmcs01 and vmcs12")
> Reported-by: Dan Cross <dcross@google.com>
> Signed-off-by: Jim Mattson <jmattson@google.com>
> Reviewed-by: Marc Orr <marcorr@google.com>
> Reviewed-by: Peter Shier <pshier@google.com>
> Reviewed-by: Dan Cross <dcross@google.com>

Reviewed-by: Liran Alon <liran.alon@oracle.com>

> ---
> arch/x86/include/asm/kvm_host.h |  2 +-
> arch/x86/kvm/vmx/nested.c       | 65 +++++++++++++++++++++------------
> arch/x86/kvm/x86.c              |  9 ++++-
> 3 files changed, 49 insertions(+), 27 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 74e88e5edd9cf..e95acf8c82b47 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1191,7 +1191,7 @@ struct kvm_x86_ops {
> 	int (*set_nested_state)(struct kvm_vcpu *vcpu,
> 				struct kvm_nested_state __user *user_kvm_nested_state,
> 				struct kvm_nested_state *kvm_state);
> -	void (*get_vmcs12_pages)(struct kvm_vcpu *vcpu);
> +	int (*get_vmcs12_pages)(struct kvm_vcpu *vcpu);
> 
> 	int (*smi_allowed)(struct kvm_vcpu *vcpu);
> 	int (*pre_enter_smm)(struct kvm_vcpu *vcpu, char *smstate);
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index ced9fba32598d..04b5069d4a9b3 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -2871,7 +2871,7 @@ static int nested_vmx_check_vmentry_hw(struct kvm_vcpu *vcpu)
> static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,
> 						 struct vmcs12 *vmcs12);
> 
> -static void nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
> +static int nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
> {
> 	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
> 	struct vcpu_vmx *vmx = to_vmx(vcpu);
> @@ -2891,19 +2891,33 @@ static void nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
> 			vmx->nested.apic_access_page = NULL;
> 		}
> 		page = kvm_vcpu_gpa_to_page(vcpu, vmcs12->apic_access_addr);
> -		/*
> -		 * If translation failed, no matter: This feature asks
> -		 * to exit when accessing the given address, and if it
> -		 * can never be accessed, this feature won't do
> -		 * anything anyway.
> -		 */
> -		if (!is_error_page(page)) {
> +		if (likely(!is_error_page(page))) {
> 			vmx->nested.apic_access_page = page;
> 			hpa = page_to_phys(vmx->nested.apic_access_page);
> 			vmcs_write64(APIC_ACCESS_ADDR, hpa);
> 		} else {
> -			secondary_exec_controls_clearbit(vmx,
> -				SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES);
> +			/*
> +			 * Since there is no backing page, we can't
> +			 * just rely on the usual L1 GPA -> HPA
> +			 * translation mechanism to do the right
> +			 * thing. We'd have to assign an appropriate
> +			 * HPA for the L1 APIC-access address, and
> +			 * then we'd have to modify the MMU to ensure
> +			 * that the L1 APIC-access address is mapped
> +			 * to the assigned HPA if and only if an L2 VM
> +			 * with that APIC-access address and the
> +			 * "virtualize APIC accesses" VM-execution
> +			 * control set in the vmcs12 is running. For
> +			 * now, just admit defeat.
> +			 */
> +			pr_warn_ratelimited("Unsupported vmcs12 APIC-access address 0x%llx\n",
> +				vmcs12->apic_access_addr);
> +			vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
> +			vcpu->run->internal.suberror =
> +				KVM_INTERNAL_ERROR_EMULATION;
> +			vcpu->run->internal.ndata = 1;
> +			vcpu->run->internal.data[0] = vmcs12->apic_access_addr;
> +			return -EINTR;
> 		}
> 	}
> 
> @@ -2948,6 +2962,7 @@ static void nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
> 		exec_controls_setbit(vmx, CPU_BASED_USE_MSR_BITMAPS);
> 	else
> 		exec_controls_clearbit(vmx, CPU_BASED_USE_MSR_BITMAPS);
> +	return 0;
> }
> 
> /*
> @@ -2986,11 +3001,11 @@ static void load_vmcs12_host_state(struct kvm_vcpu *vcpu,
> /*
>  * If from_vmentry is false, this is being called from state restore (either RSM
>  * or KVM_SET_NESTED_STATE).  Otherwise it's called from vmlaunch/vmresume.
> -+ *
> -+ * Returns:
> -+ *   0 - success, i.e. proceed with actual VMEnter
> -+ *   1 - consistency check VMExit
> -+ *  -1 - consistency check VMFail
> + *
> + * Returns:
> + * -EINTR  - exit to userspace
> + * -EINVAL - VMentry failure; continue
> + *  0      - success, i.e. proceed with actual VMEnter
>  */
> int nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu, bool from_vmentry)
> {
> @@ -2999,6 +3014,7 @@ int nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu, bool from_vmentry)
> 	bool evaluate_pending_interrupts;
> 	u32 exit_reason = EXIT_REASON_INVALID_STATE;
> 	u32 exit_qual;
> +	int r;
> 
> 	evaluate_pending_interrupts = exec_controls_get(vmx) &
> 		(CPU_BASED_VIRTUAL_INTR_PENDING | CPU_BASED_VIRTUAL_NMI_PENDING);
> @@ -3035,11 +3051,15 @@ int nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu, bool from_vmentry)
> 	prepare_vmcs02_early(vmx, vmcs12);
> 
> 	if (from_vmentry) {
> -		nested_get_vmcs12_pages(vcpu);
> +		r = nested_get_vmcs12_pages(vcpu);
> +		if (unlikely(r))
> +			return r;
> 
> 		if (nested_vmx_check_vmentry_hw(vcpu)) {
> 			vmx_switch_vmcs(vcpu, &vmx->vmcs01);
> -			return -1;
> +			r = nested_vmx_failValid(vcpu,
> +						 VMXERR_ENTRY_INVALID_CONTROL_FIELD);
> +			return r ? -EINVAL : -EINTR;
> 		}
> 
> 		if (nested_vmx_check_guest_state(vcpu, vmcs12, &exit_qual))
> @@ -3119,14 +3139,14 @@ int nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu, bool from_vmentry)
> 	vmx_switch_vmcs(vcpu, &vmx->vmcs01);
> 
> 	if (!from_vmentry)
> -		return 1;
> +		return -EINVAL;
> 
> 	load_vmcs12_host_state(vcpu, vmcs12);
> 	vmcs12->vm_exit_reason = exit_reason | VMX_EXIT_REASONS_FAILED_VMENTRY;
> 	vmcs12->exit_qualification = exit_qual;
> 	if (enable_shadow_vmcs || vmx->nested.hv_evmcs)
> 		vmx->nested.need_vmcs12_to_shadow_sync = true;
> -	return 1;
> +	return -EINVAL;
> }
> 
> /*
> @@ -3200,11 +3220,8 @@ static int nested_vmx_run(struct kvm_vcpu *vcpu, bool launch)
> 	vmx->nested.nested_run_pending = 1;
> 	ret = nested_vmx_enter_non_root_mode(vcpu, true);
> 	vmx->nested.nested_run_pending = !ret;
> -	if (ret > 0)
> -		return 1;
> -	else if (ret)
> -		return nested_vmx_failValid(vcpu,
> -			VMXERR_ENTRY_INVALID_CONTROL_FIELD);
> +	if (ret)
> +		return ret != -EINTR;
> 
> 	/* Hide L1D cache contents from the nested guest.  */
> 	vmx->vcpu.arch.l1tf_flush_l1d = true;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 290c3c3efb877..5ddbf16c8b108 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7803,8 +7803,13 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> 	bool req_immediate_exit = false;
> 
> 	if (kvm_request_pending(vcpu)) {
> -		if (kvm_check_request(KVM_REQ_GET_VMCS12_PAGES, vcpu))
> -			kvm_x86_ops->get_vmcs12_pages(vcpu);
> +		if (kvm_check_request(KVM_REQ_GET_VMCS12_PAGES, vcpu)) {
> +			r = kvm_x86_ops->get_vmcs12_pages(vcpu);
> +			if (unlikely(r)) {
> +				r = 0;
> +				goto out;
> +			}
> +		}
> 		if (kvm_check_request(KVM_REQ_MMU_RELOAD, vcpu))
> 			kvm_mmu_unload(vcpu);
> 		if (kvm_check_request(KVM_REQ_MIGRATE_TIMER, vcpu))
> -- 
> 2.23.0.162.g0b9fbb3734-goog
>