KVM: nVMX: Disallow access to vmcs12 fields that aren't supported by "hardware"

Disallow access (VMREAD/VMWRITE), both emulated and via a shadow VMCS, to
VMCS 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 functionally correct, but
very 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".

Reviewed-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Xin Li <xin@zytor.com>
Cc: Xiaoyao Li <xiaoyao.li@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
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://patch.msgid.link/20260115173427.716021-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 61113ead3d7bd2e759c26a116c401e57034ad381..f7950d8465a975bdb3fbacf688614ddedf923090 100644 (file)
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -86,6 +86,9 @@ static void init_vmcs_shadow_fields(void)
                        pr_err("Missing field from shadow_read_only_field %x\n",
                               field + 1);
 
+               if (get_vmcs12_field_offset(field) < 0)
+                       continue;
+
                clear_bit(field, vmx_vmread_bitmap);
                if (field & 1)
 #ifdef CONFIG_X86_64
@@ -111,6 +114,9 @@ static void init_vmcs_shadow_fields(void)
                          field <= GUEST_TR_AR_BYTES,
                          "Update vmcs12_write_any() to drop reserved bits from AR_BYTES");
 
+               if (get_vmcs12_field_offset(field) < 0)
+                       continue;
+
                /*
                 * PML and the preemption timer can be emulated, but the
                 * processor cannot vmwrite to fields that don't exist
@@ -7074,12 +7080,6 @@ 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)
 {
        /*
@@ -7407,6 +7407,12 @@ __init int nested_vmx_hardware_setup(int (*exit_handlers[])(struct kvm_vcpu *))
 {
        int i;
 
+       /*
+        * Note!  The set of supported vmcs12 fields is consumed by both VMX
+        * MSR and shadow VMCS setup.
+        */
+       nested_vmx_setup_vmcs12_fields();
+
        nested_vmx_setup_ctls_msrs(&vmcs_config, vmx_capability.ept);
 
        if (!cpu_has_vmx_shadow_vmcs())

diff --git a/arch/x86/kvm/vmx/vmcs.h b/arch/x86/kvm/vmx/vmcs.h
index 9aa204c876618cde72bc2e2d83fd5662e6060f1e..66d747e265b1274a27e0a3c73236eca81b214d0d 100644 (file)
--- 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) ROL16(idx, 10)
 #define ENC_TO_VMCS12_IDX(enc) ROL16(enc, 6)
 
 struct vmcs_hdr {

diff --git a/arch/x86/kvm/vmx/vmcs12.c b/arch/x86/kvm/vmx/vmcs12.c
index c2ac9e1a50b3212d87caf16de57e10ffec22220a..1ebe67c384ad4db7ec216e6690984f6c7bd5718b 100644 (file)
--- a/arch/x86/kvm/vmx/vmcs12.c
+++ b/arch/x86/kvm/vmx/vmcs12.c
@@ -9,7 +9,7 @@
        FIELD(number, name),                                            \
        [ENC_TO_VMCS12_IDX(number##_HIGH)] = 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 7a5fdd9b27ba0c7fdcd02b54f6b01feab9b4f346..21cd1b75e4fdfc921c23dc83145f501a36ddbd7e 100644 (file)
--- 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)
 {