[PATCH v3 06/10] KVM: nVMX: Cache evmcs fields to ensure consistency during VM-entry

Fred Griffoul
2025-11-21T11:11:33Z

Cache enlightened VMCS control fields to prevent TOCTOU races where the
guest could modify hv_clean_fields or hv_enlightenments_control between
multiple accesses during nested VM-entry.

The cached values ensure consistent behavior across:
- The evmcs-to-vmcs12 copy operations
- MSR bitmap validation
- Clean field checks in prepare_vmcs02_rare()

This eliminates potential guest-induced inconsistencies in nested
virtualization state management.

Signed-off-by: Fred Griffoul <fgriffo@amazon.co.uk>
---
arch/x86/kvm/vmx/hyperv.c | 5 ++--
arch/x86/kvm/vmx/hyperv.h | 20 +++++++++++++
arch/x86/kvm/vmx/nested.c | 62 ++++++++++++++++++++++++---------------
arch/x86/kvm/vmx/vmx.h | 5 +++-
4 files changed, 65 insertions(+), 27 deletions(-)

diff --git a/arch/x86/kvm/vmx/hyperv.c b/arch/x86/kvm/vmx/hyperv.c
index fa41d036acd4..961b91b9bd64 100644
--- a/arch/x86/kvm/vmx/hyperv.c
+++ b/arch/x86/kvm/vmx/hyperv.c
@@ -213,12 +213,11 @@ bool nested_evmcs_l2_tlb_flush_enabled(struct kvm_vcpu *vcpu)
{
struct kvm_vcpu_hv *hv_vcpu = to_hv_vcpu(vcpu);
struct vcpu_vmx *vmx = to_vmx(vcpu);
- struct hv_enlightened_vmcs *evmcs = vmx->nested.hv_evmcs;

- if (!hv_vcpu || !evmcs)
+ if (!hv_vcpu || !nested_vmx_is_evmptr12_valid(vmx))
return false;

- if (!evmcs->hv_enlightenments_control.nested_flush_hypercall)
+ if (!vmx->nested.hv_flush_hypercall)
return false;

return hv_vcpu->vp_assist_page.nested_control.features.directhypercall;
diff --git a/arch/x86/kvm/vmx/hyperv.h b/arch/x86/kvm/vmx/hyperv.h
index 11a339009781..3c7fea501ca5 100644
--- a/arch/x86/kvm/vmx/hyperv.h
+++ b/arch/x86/kvm/vmx/hyperv.h
@@ -52,6 +52,16 @@ static inline bool guest_cpu_cap_has_evmcs(struct kvm_vcpu *vcpu)
to_vmx(vcpu)->nested.enlightened_vmcs_enabled;
}

+static inline u32 nested_evmcs_clean_fields(struct vcpu_vmx *vmx)
+{
+ return vmx->nested.hv_clean_fields;
+}
+
+static inline bool nested_evmcs_msr_bitmap(struct vcpu_vmx *vmx)
+{
+ return vmx->nested.hv_msr_bitmap;
+}
+
u64 nested_get_evmptr(struct kvm_vcpu *vcpu);
uint16_t nested_get_evmcs_version(struct kvm_vcpu *vcpu);
int nested_enable_evmcs(struct kvm_vcpu *vcpu,
@@ -85,6 +95,16 @@ static inline struct hv_enlightened_vmcs *nested_vmx_evmcs(struct vcpu_vmx *vmx)
{
return NULL;
}
+
+static inline u32 nested_evmcs_clean_fields(struct vcpu_vmx *vmx)
+{
+ return 0;
+}
+
+static inline bool nested_evmcs_msr_bitmap(struct vcpu_vmx *vmx)
+{
+ return false;
+}
#endif

#endif /* __KVM_X86_VMX_HYPERV_H */
diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 1f58b380585b..aec150612818 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -235,6 +235,9 @@ static inline void nested_release_evmcs(struct kvm_vcpu *vcpu)
kvm_vcpu_unmap(vcpu, &vmx->nested.hv_evmcs_map);
vmx->nested.hv_evmcs = NULL;
vmx->nested.hv_evmcs_vmptr = EVMPTR_INVALID;
+ vmx->nested.hv_clean_fields = 0;
+ vmx->nested.hv_msr_bitmap = false;
+ vmx->nested.hv_flush_hypercall = false;

if (hv_vcpu) {
hv_vcpu->nested.pa_page_gpa = INVALID_GPA;
@@ -686,10 +689,10 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,
* and tells KVM (L0) there were no changes in MSR bitmap for L2.
*/
if (!vmx->nested.force_msr_bitmap_recalc) {
- struct hv_enlightened_vmcs *evmcs = nested_vmx_evmcs(vmx);
-
- if (evmcs && evmcs->hv_enlightenments_control.msr_bitmap &&
- evmcs->hv_clean_fields & HV_VMX_ENLIGHTENED_CLEAN_FIELD_MSR_BITMAP)
+ if (nested_vmx_is_evmptr12_valid(vmx) &&
+ nested_evmcs_msr_bitmap(vmx) &&
+ (nested_evmcs_clean_fields(vmx)
+ & HV_VMX_ENLIGHTENED_CLEAN_FIELD_MSR_BITMAP))
return true;
}

@@ -2163,10 +2166,11 @@ static void copy_vmcs12_to_enlightened(struct vcpu_vmx *vmx)
* instruction.
*/
static enum nested_evmptrld_status nested_vmx_handle_enlightened_vmptrld(
- struct kvm_vcpu *vcpu, bool from_launch)
+ struct kvm_vcpu *vcpu, bool from_launch, bool copy)
{
#ifdef CONFIG_KVM_HYPERV
struct vcpu_vmx *vmx = to_vmx(vcpu);
+ struct hv_enlightened_vmcs *evmcs;
bool evmcs_gpa_changed = false;
u64 evmcs_gpa;

@@ -2246,6 +2250,22 @@ static enum nested_evmptrld_status nested_vmx_handle_enlightened_vmptrld(
vmx->nested.force_msr_bitmap_recalc = true;
}

+ /* Cache evmcs fields to avoid reading evmcs after copy to vmcs12 */
+ evmcs = vmx->nested.hv_evmcs;
+ vmx->nested.hv_clean_fields = evmcs->hv_clean_fields;
+ vmx->nested.hv_flush_hypercall = evmcs->hv_enlightenments_control.nested_flush_hypercall;
+ vmx->nested.hv_msr_bitmap = evmcs->hv_enlightenments_control.msr_bitmap;
+
+ if (copy) {
+ struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
+
+ if (likely(!vmcs12->hdr.shadow_vmcs)) {
+ copy_enlightened_to_vmcs12(vmx, vmx->nested.hv_clean_fields);
+ /* Enlightened VMCS doesn't have launch state */
+ vmcs12->launch_state = !from_launch;
+ }
+ }
+
return EVMPTRLD_SUCCEEDED;
#else
return EVMPTRLD_DISABLED;
@@ -2613,10 +2633,12 @@ static void vmcs_write_cet_state(struct kvm_vcpu *vcpu, u64 s_cet,

static void prepare_vmcs02_rare(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
{
- struct hv_enlightened_vmcs *hv_evmcs = nested_vmx_evmcs(vmx);
+ u32 hv_clean_fields = 0;

- if (!hv_evmcs || !(hv_evmcs->hv_clean_fields &
- HV_VMX_ENLIGHTENED_CLEAN_FIELD_GUEST_GRP2)) {
+ if (nested_vmx_is_evmptr12_valid(vmx))
+ hv_clean_fields = nested_evmcs_clean_fields(vmx);
+
+ if (!(hv_clean_fields & HV_VMX_ENLIGHTENED_CLEAN_FIELD_GUEST_GRP2)) {

vmcs_write16(GUEST_ES_SELECTOR, vmcs12->guest_es_selector);
vmcs_write16(GUEST_CS_SELECTOR, vmcs12->guest_cs_selector);
@@ -2658,8 +2680,7 @@ static void prepare_vmcs02_rare(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
vmx_segment_cache_clear(vmx);
}

- if (!hv_evmcs || !(hv_evmcs->hv_clean_fields &
- HV_VMX_ENLIGHTENED_CLEAN_FIELD_GUEST_GRP1)) {
+ if (!(hv_clean_fields & HV_VMX_ENLIGHTENED_CLEAN_FIELD_GUEST_GRP1)) {
vmcs_write32(GUEST_SYSENTER_CS, vmcs12->guest_sysenter_cs);
vmcs_writel(GUEST_PENDING_DBG_EXCEPTIONS,
vmcs12->guest_pending_dbg_exceptions);
@@ -2750,7 +2771,7 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
enum vm_entry_failure_code *entry_failure_code)
{
struct vcpu_vmx *vmx = to_vmx(vcpu);
- struct hv_enlightened_vmcs *evmcs = nested_vmx_evmcs(vmx);
+ struct hv_enlightened_vmcs *evmcs;
bool load_guest_pdptrs_vmcs12 = false;

if (vmx->nested.dirty_vmcs12 || nested_vmx_is_evmptr12_valid(vmx)) {
@@ -2758,7 +2779,8 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
vmx->nested.dirty_vmcs12 = false;

load_guest_pdptrs_vmcs12 = !nested_vmx_is_evmptr12_valid(vmx) ||
- !(evmcs->hv_clean_fields & HV_VMX_ENLIGHTENED_CLEAN_FIELD_GUEST_GRP1);
+ !(nested_evmcs_clean_fields(vmx)
+ & HV_VMX_ENLIGHTENED_CLEAN_FIELD_GUEST_GRP1);
}

if (vmx->nested.nested_run_pending &&
@@ -2887,7 +2909,8 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
* bits when it changes a field in eVMCS. Mark all fields as clean
* here.
*/
- if (nested_vmx_is_evmptr12_valid(vmx))
+ evmcs = nested_vmx_evmcs(vmx);
+ if (evmcs)
evmcs->hv_clean_fields |= HV_VMX_ENLIGHTENED_CLEAN_FIELD_ALL;

return 0;
@@ -3470,7 +3493,7 @@ static bool nested_get_evmcs_page(struct kvm_vcpu *vcpu)
if (guest_cpu_cap_has_evmcs(vcpu) &&
vmx->nested.hv_evmcs_vmptr == EVMPTR_MAP_PENDING) {
enum nested_evmptrld_status evmptrld_status =
- nested_vmx_handle_enlightened_vmptrld(vcpu, false);
+ nested_vmx_handle_enlightened_vmptrld(vcpu, false, false);

if (evmptrld_status == EVMPTRLD_VMFAIL ||
evmptrld_status == EVMPTRLD_ERROR)
@@ -3864,7 +3887,7 @@ static int nested_vmx_run(struct kvm_vcpu *vcpu, bool launch)
if (!nested_vmx_check_permission(vcpu))
return 1;

- evmptrld_status = nested_vmx_handle_enlightened_vmptrld(vcpu, launch);
+ evmptrld_status = nested_vmx_handle_enlightened_vmptrld(vcpu, launch, true);
if (evmptrld_status == EVMPTRLD_ERROR) {
kvm_queue_exception(vcpu, UD_VECTOR);
return 1;
@@ -3890,15 +3913,8 @@ static int nested_vmx_run(struct kvm_vcpu *vcpu, bool launch)
if (CC(vmcs12->hdr.shadow_vmcs))
return nested_vmx_failInvalid(vcpu);

- if (nested_vmx_is_evmptr12_valid(vmx)) {
- struct hv_enlightened_vmcs *evmcs = nested_vmx_evmcs(vmx);
-
- copy_enlightened_to_vmcs12(vmx, evmcs->hv_clean_fields);
- /* Enlightened VMCS doesn't have launch state */
- vmcs12->launch_state = !launch;
- } else if (enable_shadow_vmcs) {
+ if (!nested_vmx_is_evmptr12_valid(vmx) && enable_shadow_vmcs)
copy_shadow_to_vmcs12(vmx);
- }

/*
* The nested entry process starts with enforcing various prerequisites
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index 9a285834ccda..87708af502f3 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -205,8 +205,11 @@ struct nested_vmx {

#ifdef CONFIG_KVM_HYPERV
gpa_t hv_evmcs_vmptr;
- struct kvm_host_map hv_evmcs_map;
+ u32 hv_clean_fields;
+ bool hv_msr_bitmap;
+ bool hv_flush_hypercall;
struct hv_enlightened_vmcs *hv_evmcs;
+ struct kvm_host_map hv_evmcs_map;
#endif
};

--
2.43.0