[RFC PATCH 4/9] x86/kvm/vmx: Virtualize Intel IA32_SPEC_CTRL

Zhang Chen
2022-12-10T16:19:16Z

Currently KVM disables interception of IA32_SPEC_CTRL after a non-0 is
written to IA32_SPEC_CTRL by guest. Then, guest is allowed to write any
value to hardware.

"virtualize IA32_SPEC_CTRL" is a new tertiary vm-exec control.
It provides VMM the capability to restrict the value of IA32_SPEC_CTRL in
hardware even if the MSR isn't intercepted (e.g., prevent VMs from changing
some bits in IA32_SPEC_CTRL) in an efficient way.

Two new fields are added to VMCS,
IA32_SPEC_CTRL_MASK: Setting a bit in this field prevents guest software
from modifying the corresponding bit in the IA32_SPEC_CTRL MSR
IA32_SPEC_CTRL_SHADOW: This field contains the value that guest software
expects to be in the IA32_SPEC_CTRL MSR

In VMX non-root mode, when IA32_SPEC_CTRL isn't intercepted by VMM,
guest's accesses to IA32_SPEC_CTRL is virtualized by the processor
according to the two new fields:
RDMSR IA32_SPEC_CTRL returns the shadow value.
WRMSR IA32_SPEC_CTRL writes EDX:EAX to the shadow field and calculate
a new value according to the guest value (EDX:EAX), current value of
IA32_SPEC_CTRL MSR and the IA32_SPEC_CTRL_MASK field (specifically,
(cur_val & mask) | (guest_val & ~mask)) and write it to IA32_SPEC_CTRL MSR.

Enable "virtual IA32_SPEC_CTRL" if it is supported. With
"virtual IA32_SPEC_CTRL" enabled, IA32_SPEC_CTRL MSR value seen from
guest p.o.v is different from the value in hardware when guest is running.
We refer to the two values as below:

1. effective value of IA32_SPEC_CTRL. This value is the one programmed in
hardware when the vCPU is running.
2. shadow value of IA32_SPEC_CTRL. This value is returned when rdmsr is
used inside a guest to read IA32_SPEC_CTRL. This value doesn't affect
CPU's enablement of indirect predictor controls.

In KVM, vmx->spec_ctrl always stores the effective value of IA32_SPEC_CTRL
when guest is running (even when "virtual IA32_SPEC_CTRL" is disabled.
In this case, the shadow value is equal to the effective one).
When "virtual IA32_SPEC_CTRL" is enabled, the shadow value of
IA32_SPEC_CTRL is stored in IA32_SPEC_CTRL_SHADOW field in VMCS.

IA32_SPEC_CTRL_MASK is always 0 for now. It means all supported bits
in hardware is allowed to be toggled by guest's wrmsr. The mask will
be changed by following patches.

Note "virtual IA32_SPEC_CTRL" is now used by VMM to enforce some bits
of IA32_SPEC_CTRL to 1 (i.e., enabled some HW mitigations transparently
for guests). In theory, VMM can disable some HW mitigations behind guests.
But to keep this series simple, we leave that for future work.

Co-developed-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Zhang Chen <chen.zhang@intel.com>
---
arch/x86/include/asm/vmx.h | 5 +++++
arch/x86/include/asm/vmxfeatures.h | 2 ++
arch/x86/kvm/vmx/capabilities.h | 5 +++++
arch/x86/kvm/vmx/vmx.c | 30 ++++++++++++++++++++++++++++--
arch/x86/kvm/vmx/vmx.h | 10 +++++++++-
5 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/vmx.h b/arch/x86/include/asm/vmx.h
index 498dc600bd5c..c2efdad491c1 100644
--- a/arch/x86/include/asm/vmx.h
+++ b/arch/x86/include/asm/vmx.h
@@ -81,6 +81,7 @@
* Definitions of Tertiary Processor-Based VM-Execution Controls.
*/
#define TERTIARY_EXEC_IPI_VIRT VMCS_CONTROL_BIT(IPI_VIRT)
+#define TERTIARY_EXEC_VIRT_SPEC_CTRL VMCS_CONTROL_BIT(VIRT_SPEC_CTRL)

#define PIN_BASED_EXT_INTR_MASK VMCS_CONTROL_BIT(INTR_EXITING)
#define PIN_BASED_NMI_EXITING VMCS_CONTROL_BIT(NMI_EXITING)
@@ -233,6 +234,10 @@ enum vmcs_field {
TERTIARY_VM_EXEC_CONTROL_HIGH = 0x00002035,
PID_POINTER_TABLE = 0x00002042,
PID_POINTER_TABLE_HIGH = 0x00002043,
+ IA32_SPEC_CTRL_MASK = 0x0000204A,
+ IA32_SPEC_CTRL_MASK_HIGH = 0x0000204B,
+ IA32_SPEC_CTRL_SHADOW = 0x0000204C,
+ IA32_SPEC_CTRL_SHADOW_HIGH = 0x0000204D,
GUEST_PHYSICAL_ADDRESS = 0x00002400,
GUEST_PHYSICAL_ADDRESS_HIGH = 0x00002401,
VMCS_LINK_POINTER = 0x00002800,
diff --git a/arch/x86/include/asm/vmxfeatures.h b/arch/x86/include/asm/vmxfeatures.h
index c6a7eed03914..d3b7237d9c42 100644
--- a/arch/x86/include/asm/vmxfeatures.h
+++ b/arch/x86/include/asm/vmxfeatures.h
@@ -89,4 +89,6 @@

/* Tertiary Processor-Based VM-Execution Controls, word 3 */
#define VMX_FEATURE_IPI_VIRT ( 3*32+ 4) /* Enable IPI virtualization */
+#define VMX_FEATURE_VIRT_SPEC_CTRL ( 3*32+ 7) /* Virtualize IA32_SPEC_CTRL */
+
#endif /* _ASM_X86_VMXFEATURES_H */
diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
index 07254314f3dd..a9a0adcd403b 100644
--- a/arch/x86/kvm/vmx/capabilities.h
+++ b/arch/x86/kvm/vmx/capabilities.h
@@ -138,6 +138,11 @@ static inline bool cpu_has_tertiary_exec_ctrls(void)
CPU_BASED_ACTIVATE_TERTIARY_CONTROLS;
}

+static inline bool cpu_has_virt_spec_ctrl(void)
+{
+ return vmcs_config.cpu_based_3rd_exec_ctrl & TERTIARY_EXEC_VIRT_SPEC_CTRL;
+}
+
static inline bool cpu_has_vmx_virtualize_apic_accesses(void)
{
return vmcs_config.cpu_based_2nd_exec_ctrl &
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 63247c57c72c..407061b369b4 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1898,7 +1898,10 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
!guest_has_spec_ctrl_msr(vcpu))
return 1;

- msr_info->data = to_vmx(vcpu)->spec_ctrl;
+ if (cpu_has_virt_spec_ctrl())
+ msr_info->data = vmcs_read64(IA32_SPEC_CTRL_SHADOW);
+ else
+ msr_info->data = to_vmx(vcpu)->spec_ctrl;
break;
case MSR_IA32_SYSENTER_CS:
msr_info->data = vmcs_read32(GUEST_SYSENTER_CS);
@@ -2160,10 +2163,22 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
!guest_has_spec_ctrl_msr(vcpu))
return 1;

- if (kvm_spec_ctrl_test_value(data))
+ if (kvm_spec_ctrl_test_value(data | vmx->spec_ctrl_mask))
return 1;

vmx->spec_ctrl = data;
+
+ if (cpu_has_virt_spec_ctrl()) {
+ vmcs_write64(IA32_SPEC_CTRL_SHADOW, data);
+ /*
+ * Some bits are allowed to be toggled by guest.
+ * Update the effective value of IA32_SPEC_CTRL
+ * MSR according to the value written by guest
+ * but keep bits in the mask set.
+ */
+ vmx->spec_ctrl = data | vmx->spec_ctrl_mask;
+ }
+
if (!data)
break;

@@ -4673,6 +4688,11 @@ static void init_vmcs(struct vcpu_vmx *vmx)
if (cpu_has_vmx_xsaves())
vmcs_write64(XSS_EXIT_BITMAP, VMX_XSS_EXIT_BITMAP);

+ if (cpu_has_virt_spec_ctrl()) {
+ vmcs_write64(IA32_SPEC_CTRL_SHADOW, 0);
+ vmcs_write64(IA32_SPEC_CTRL_MASK, vmx->spec_ctrl_mask);
+ }
+
if (enable_pml) {
vmcs_write64(PML_ADDRESS, page_to_phys(vmx->pml_pg));
vmcs_write16(GUEST_PML_INDEX, PML_ENTITY_NUM - 1);
@@ -4738,6 +4758,12 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
__vmx_vcpu_reset(vcpu);

vmx->rmode.vm86_active = 0;
+
+ if (cpu_has_virt_spec_ctrl()) {
+ vmx->spec_ctrl_mask = 0;
+ vmcs_write64(IA32_SPEC_CTRL_MASK, vmx->spec_ctrl_mask);
+ vmcs_write64(IA32_SPEC_CTRL_SHADOW, 0);
+ }
vmx->spec_ctrl = 0;

vmx->msr_ia32_umwait_control = 0;
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index a3da84f4ea45..c5a41ae14237 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -290,7 +290,14 @@ struct vcpu_vmx {
u64 msr_guest_kernel_gs_base;
#endif

+ /* The value of hardware IA32_SPEC_CTRL MSR when guest is running */
u64 spec_ctrl;
+ /*
+ * The bits KVM doesn't allow guest to toggle.
+ * A bit set in the mask should always be set in guest
+ * IA32_SPEC_CTRL_MSR.
+ */
+ u64 spec_ctrl_mask;
u32 msr_ia32_umwait_control;

/*
@@ -589,7 +596,8 @@ static inline u8 vmx_get_rvi(void)

#define KVM_REQUIRED_VMX_TERTIARY_VM_EXEC_CONTROL 0
#define KVM_OPTIONAL_VMX_TERTIARY_VM_EXEC_CONTROL \
- (TERTIARY_EXEC_IPI_VIRT)
+ (TERTIARY_EXEC_IPI_VIRT | \
+ TERTIARY_EXEC_VIRT_SPEC_CTRL)

#define BUILD_CONTROLS_SHADOW(lname, uname, bits) \
static inline void lname##_controls_set(struct vcpu_vmx *vmx, u##bits val) \
--
2.25.1