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Message-ID: <87a4ywaur0.fsf@redhat.com>
Date: Fri, 02 Jan 2026 16:40:03 +0100
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: Fred Griffoul <griffoul@...il.com>, kvm@...r.kernel.org
Cc: seanjc@...gle.com, pbonzini@...hat.com, shuah@...nel.org,
dwmw@...zon.co.uk, linux-kselftest@...r.kernel.org,
linux-kernel@...r.kernel.org, Fred Griffoul <fgriffo@...zon.co.uk>
Subject: Re: [PATCH v4 06/10] KVM: nVMX: Cache evmcs fields to ensure
consistency during VM-entry
Fred Griffoul <griffoul@...il.com> writes:
> From: Fred Griffoul <fgriffo@...zon.co.uk>
>
> 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.
Could you please split the commit to simplify the review and bughunting
if (when?) issues are discovered? In particular, I would've split this
into at least:
- hv_flush_hypercall caching/handling
- hv_msr_bitmap caching/handling
- nested_vmx_is_evmptr12_valid() instead of vmx->nested.hv_evmcs checks
- adding 'bool copy' argument to nested_vmx_handle_enlightened_vmptrld()
and moving copy_enlightened_to_vmcs12() there from nested_vmx_run()
- hv_clean_fields caching/handling.
I want to be extra careful with clean fields. I may had forgotten the
details since I've implemented it but I still remember some pain over
inconsistent state upon migrations, namely
commit d6bf71a18c74de61548ddad44ff95306fe85f829
Author: Vitaly Kuznetsov <vkuznets@...hat.com>
Date: Wed May 26 15:20:22 2021 +0200
KVM: nVMX: Ignore 'hv_clean_fields' data when eVMCS data is copied in vmx_get_nested_state()
Unfortunatelly, I don't think we have good test coverage for all this in
'hyperv_evmcs' selftest :-( I've seen you added evmcs support to
'vmx_l2_switch_test' in this series, thanks a lot for doing that!
>
> Signed-off-by: Fred Griffoul <fgriffo@...zon.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 cb4b85edcb7a..5790e1a26456 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -236,6 +236,9 @@ 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;
> @@ -737,10 +740,10 @@ 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;
> }
>
> @@ -2214,10 +2217,11 @@ 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;
>
> @@ -2297,6 +2301,22 @@ 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;
> @@ -2655,10 +2675,12 @@ 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);
> @@ -2700,8 +2722,7 @@ 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);
> @@ -2792,7 +2813,7 @@ 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)) {
> @@ -2800,7 +2821,8 @@ 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 &&
> @@ -2929,7 +2951,8 @@ 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;
> @@ -3477,7 +3500,7 @@ 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)
> @@ -3867,7 +3890,7 @@ 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;
> @@ -3893,15 +3916,8 @@ 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 90fdf130fd85..cda96196c56c 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
> };
--
Vitaly
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