[PATCH v2 1/2] KVM: nVMX: Disallow access to vmcs12 fields that aren't supported by "hardware"

Sean Christopherson
2025-12-30T22:02:24Z

Disallow access (VMREAD/VMWRITE) to fields that the loaded incarnation of
KVM doesn't support, e.g. due to lack of hardware support, as a middle
ground between allowing access to any vmcs12 field defined by KVM (current
behavior) and gating access based on the userspace-defined vCPU model (the
most correct, but costly, implementation).

Disallowing access to unsupported fields helps a tiny bit in terms of
closing the virtualization hole (see below), but the main motivation is to
avoid having to weed out unsupported fields when synchronizing between
vmcs12 and a shadow VMCS. Because shadow VMCS accesses are done via
VMREAD and VMWRITE, KVM _must_ filter out unsupported fields (or eat
VMREAD/VMWRITE failures), and filtering out just shadow VMCS fields is
about the same amount of effort, and arguably much more confusing.

As a bonus, this also fixes a KVM-Unit-Test failure bug when running on
_hardware_ without support for TSC Scaling, which fails with the same
signature as the bug fixed by commit ba1f82456ba8 ("KVM: nVMX: Dynamically
compute max VMCS index for vmcs12"):

FAIL: VMX_VMCS_ENUM.MAX_INDEX expected: 19, actual: 17

Dynamically computing the max VMCS index only resolved the issue where KVM
was hardcoding max index, but for CPUs with TSC Scaling, that was "good
enough".

Cc: Xin Li <xin@zytor.com>
Cc: Chao Gao <chao.gao@intel.com>
Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://lore.kernel.org/all/20251026201911.505204-22-xin@zytor.com
Link: https://lore.kernel.org/all/YR2Tf9WPNEzrE7Xg@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/vmx/nested.c | 6 ----
arch/x86/kvm/vmx/vmcs.h | 8 +++++
arch/x86/kvm/vmx/vmcs12.c | 70 +++++++++++++++++++++++++++++++++++++--
arch/x86/kvm/vmx/vmcs12.h | 6 ++--
arch/x86/kvm/vmx/vmx.c | 2 ++
5 files changed, 82 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 6137e5307d0f..9d8f84e3f2da 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -7074,12 +7074,6 @@ void nested_vmx_set_vmcs_shadowing_bitmap(void)
}
}

-/*
- * Indexing into the vmcs12 uses the VMCS encoding rotated left by 6. Undo
- * that madness to get the encoding for comparison.
- */
-#define VMCS12_IDX_TO_ENC(idx) ((u16)(((u16)(idx) >> 6) | ((u16)(idx) << 10)))
-
static u64 nested_vmx_calc_vmcs_enum_msr(void)
{
/*
diff --git a/arch/x86/kvm/vmx/vmcs.h b/arch/x86/kvm/vmx/vmcs.h
index b25625314658..98281e019e38 100644
--- a/arch/x86/kvm/vmx/vmcs.h
+++ b/arch/x86/kvm/vmx/vmcs.h
@@ -11,7 +11,15 @@

#include "capabilities.h"

+/*
+ * Indexing into the vmcs12 uses the VMCS encoding rotated left by 6 as a very
+ * rudimentary compression of the range of indices. The compression ratio is
+ * good enough to allow KVM to use a (very sparsely populated) array without
+ * wasting too much memory, while the "algorithm" is fast enough to be used to
+ * lookup vmcs12 fields on-demand, e.g. for emulation.
+ */
#define ROL16(val, n) ((u16)(((u16)(val) << (n)) | ((u16)(val) >> (16 - (n)))))
+#define VMCS12_IDX_TO_ENC(idx) ((u16)(((u16)(idx) >> 6) | ((u16)(idx) << 10)))

struct vmcs_hdr {
u32 revision_id:31;
diff --git a/arch/x86/kvm/vmx/vmcs12.c b/arch/x86/kvm/vmx/vmcs12.c
index 4233b5ca9461..b92db4768346 100644
--- a/arch/x86/kvm/vmx/vmcs12.c
+++ b/arch/x86/kvm/vmx/vmcs12.c
@@ -9,7 +9,7 @@
FIELD(number, name), \
[ROL16(number##_HIGH, 6)] = VMCS12_OFFSET(name) + sizeof(u32)

-const unsigned short vmcs12_field_offsets[] = {
+static const u16 kvm_supported_vmcs12_field_offsets[] __initconst = {
FIELD(VIRTUAL_PROCESSOR_ID, virtual_processor_id),
FIELD(POSTED_INTR_NV, posted_intr_nv),
FIELD(GUEST_ES_SELECTOR, guest_es_selector),
@@ -158,4 +158,70 @@ const unsigned short vmcs12_field_offsets[] = {
FIELD(HOST_SSP, host_ssp),
FIELD(HOST_INTR_SSP_TABLE, host_ssp_tbl),
};
-const unsigned int nr_vmcs12_fields = ARRAY_SIZE(vmcs12_field_offsets);
+
+u16 vmcs12_field_offsets[ARRAY_SIZE(kvm_supported_vmcs12_field_offsets)] __ro_after_init;
+unsigned int nr_vmcs12_fields __ro_after_init;
+
+#define VMCS12_CASE64(enc) case enc##_HIGH: case enc
+
+static __init bool cpu_has_vmcs12_field(unsigned int idx)
+{
+ switch (VMCS12_IDX_TO_ENC(idx)) {
+ case VIRTUAL_PROCESSOR_ID:
+ return cpu_has_vmx_vpid();
+ case POSTED_INTR_NV:
+ return cpu_has_vmx_posted_intr();
+ VMCS12_CASE64(TSC_MULTIPLIER):
+ return cpu_has_vmx_tsc_scaling();
+ case TPR_THRESHOLD:
+ VMCS12_CASE64(VIRTUAL_APIC_PAGE_ADDR):
+ return cpu_has_vmx_tpr_shadow();
+ VMCS12_CASE64(APIC_ACCESS_ADDR):
+ return cpu_has_vmx_virtualize_apic_accesses();
+ VMCS12_CASE64(POSTED_INTR_DESC_ADDR):
+ return cpu_has_vmx_posted_intr();
+ case GUEST_INTR_STATUS:
+ return cpu_has_vmx_virtual_intr_delivery();
+ VMCS12_CASE64(VM_FUNCTION_CONTROL):
+ VMCS12_CASE64(EPTP_LIST_ADDRESS):
+ return cpu_has_vmx_vmfunc();
+ VMCS12_CASE64(EPT_POINTER):
+ return cpu_has_vmx_ept();
+ VMCS12_CASE64(XSS_EXIT_BITMAP):
+ return cpu_has_vmx_xsaves();
+ VMCS12_CASE64(ENCLS_EXITING_BITMAP):
+ return cpu_has_vmx_encls_vmexit();
+ VMCS12_CASE64(GUEST_IA32_PERF_GLOBAL_CTRL):
+ VMCS12_CASE64(HOST_IA32_PERF_GLOBAL_CTRL):
+ return cpu_has_load_perf_global_ctrl();
+ case SECONDARY_VM_EXEC_CONTROL:
+ return cpu_has_secondary_exec_ctrls();
+ case GUEST_S_CET:
+ case GUEST_SSP:
+ case GUEST_INTR_SSP_TABLE:
+ case HOST_S_CET:
+ case HOST_SSP:
+ case HOST_INTR_SSP_TABLE:
+ return cpu_has_load_cet_ctrl();
+
+ /* KVM always emulates PML and the VMX preemption timer in software. */
+ case GUEST_PML_INDEX:
+ case VMX_PREEMPTION_TIMER_VALUE:
+ default:
+ return true;
+ }
+}
+
+void __init nested_vmx_setup_vmcs12_fields(void)
+{
+ unsigned int i;
+
+ for (i = 0; i < ARRAY_SIZE(kvm_supported_vmcs12_field_offsets); i++) {
+ if (!kvm_supported_vmcs12_field_offsets[i] ||
+ !cpu_has_vmcs12_field(i))
+ continue;
+
+ vmcs12_field_offsets[i] = kvm_supported_vmcs12_field_offsets[i];
+ nr_vmcs12_fields = i + 1;
+ }
+}
diff --git a/arch/x86/kvm/vmx/vmcs12.h b/arch/x86/kvm/vmx/vmcs12.h
index 4ad6b16525b9..f2c0721fe3e2 100644
--- a/arch/x86/kvm/vmx/vmcs12.h
+++ b/arch/x86/kvm/vmx/vmcs12.h
@@ -374,8 +374,10 @@ static inline void vmx_check_vmcs12_offsets(void)
CHECK_OFFSET(guest_pml_index, 996);
}

-extern const unsigned short vmcs12_field_offsets[];
-extern const unsigned int nr_vmcs12_fields;
+extern u16 vmcs12_field_offsets[] __ro_after_init;
+extern unsigned int nr_vmcs12_fields __ro_after_init;
+
+void __init nested_vmx_setup_vmcs12_fields(void);

static inline short get_vmcs12_field_offset(unsigned long field)
{
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 6b96f7aea20b..e5ad3853f51d 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -8670,6 +8670,8 @@ __init int vmx_hardware_setup(void)
* can hide/show features based on kvm_cpu_cap_has().
*/
if (nested) {
+ nested_vmx_setup_vmcs12_fields();
+
nested_vmx_setup_ctls_msrs(&vmcs_config, vmx_capability.ept);

r = nested_vmx_hardware_setup(kvm_vmx_exit_handlers);
--
2.52.0.351.gbe84eed79e-goog