From b65699be2c606d2687593516d93e66d16a713b61 Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Tue, 30 Jun 2026 13:28:26 -0700 Subject: [PATCH] KVM: x86: Reject nested CAP enablement if nested virtualization is disabled Add a flag to explicitly track if nested virtualization is enabled, and use it enumerate that various nested CAPs are unsupported, and to reject enablement of said CAPs. When the nested ops hooks were moved to their own structure, KVM's NULL-by-default behavior was deliberately dropped, with the changelog asserting that all was well. That wasn't quite true; there is no danger to KVM, but now KVM is over-reporting support for KVM_CAP_NESTED_STATE and KVM_CAP_HYPERV_ENLIGHTENED_VMCS. Fixes: 33b22172452f ("KVM: x86: move nested-related kvm_x86_ops to a separate struct") Reviewed-by: Vitaly Kuznetsov Link: https://patch.msgid.link/20260630202828.440724-2-seanjc@google.com Signed-off-by: Sean Christopherson --- arch/x86/include/asm/kvm_host.h | 2 ++ arch/x86/kvm/hyperv.c | 3 ++- arch/x86/kvm/svm/svm.c | 1 + arch/x86/kvm/vmx/vmx.c | 1 + arch/x86/kvm/x86.c | 12 +++++++----- 5 files changed, 13 insertions(+), 6 deletions(-) diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index fdb8953aeeb1..a46504077b3e 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -1729,6 +1729,8 @@ struct kvm_x86_ops { }; struct kvm_x86_nested_ops { + bool enabled; + void (*leave_nested)(struct kvm_vcpu *vcpu); bool (*is_exception_vmexit)(struct kvm_vcpu *vcpu, u8 vector, u32 error_code); diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c index 9d38cb644668..72036670c1ac 100644 --- a/arch/x86/kvm/hyperv.c +++ b/arch/x86/kvm/hyperv.c @@ -2791,7 +2791,8 @@ int kvm_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid, }; int i, nent = ARRAY_SIZE(cpuid_entries); - if (kvm_x86_ops.nested_ops->get_evmcs_version) + if (kvm_x86_ops.nested_ops->enabled && + kvm_x86_ops.nested_ops->get_evmcs_version) evmcs_ver = kvm_x86_ops.nested_ops->get_evmcs_version(vcpu); if (cpuid->nent < nent) diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index ef69a51ab27f..163440fca59f 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -5646,6 +5646,7 @@ static __init int svm_hardware_setup(void) if (r) return r; } + svm_nested_ops.enabled = nested; /* * KVM's MMU doesn't support using 2-level paging for itself, and thus diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index b25b978d7a9d..5ef6bdb334c7 100644 --- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -8786,6 +8786,7 @@ __init int vmx_hardware_setup(void) if (r) return r; } + vmx_nested_ops.enabled = nested; kvm_set_posted_intr_wakeup_handler(pi_wakeup_handler); diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 5ee6d0c33009..566a017eab1d 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -2355,7 +2355,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext) r &= ~KVM_X2APIC_ENABLE_SUPPRESS_EOI_BROADCAST; break; case KVM_CAP_NESTED_STATE: - r = kvm_x86_ops.nested_ops->get_state ? + r = kvm_x86_ops.nested_ops->enabled ? kvm_x86_ops.nested_ops->get_state(NULL, NULL, 0) : 0; break; #ifdef CONFIG_KVM_HYPERV @@ -2363,7 +2363,8 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext) r = kvm_x86_ops.enable_l2_tlb_flush != NULL; break; case KVM_CAP_HYPERV_ENLIGHTENED_VMCS: - r = kvm_x86_ops.nested_ops->enable_evmcs != NULL; + r = kvm_x86_ops.nested_ops->enabled && + kvm_x86_ops.nested_ops->enable_evmcs != NULL; break; #endif case KVM_CAP_SMALLER_MAXPHYADDR: @@ -3375,7 +3376,8 @@ static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu, uint16_t vmcs_version; void __user *user_ptr; - if (!kvm_x86_ops.nested_ops->enable_evmcs) + if (!kvm_x86_ops.nested_ops->enabled || + !kvm_x86_ops.nested_ops->enable_evmcs) return -ENOTTY; r = kvm_x86_ops.nested_ops->enable_evmcs(vcpu, &vmcs_version); if (!r) { @@ -3741,7 +3743,7 @@ long kvm_arch_vcpu_ioctl(struct file *filp, u32 user_data_size; r = -EINVAL; - if (!kvm_x86_ops.nested_ops->get_state) + if (!kvm_x86_ops.nested_ops->enabled) break; BUILD_BUG_ON(sizeof(user_data_size) != sizeof(user_kvm_nested_state->size)); @@ -3771,7 +3773,7 @@ long kvm_arch_vcpu_ioctl(struct file *filp, int idx; r = -EINVAL; - if (!kvm_x86_ops.nested_ops->set_state) + if (!kvm_x86_ops.nested_ops->enabled) break; r = -EFAULT;