Re: [PATCH v9 20/22] KVM: nVMX: Validate FRED-related VMCS fields

[Chao Gao <chao.gao@intel.com>]

On Tue, Jan 20, 2026 at 01:19:55AM -0800, Xin Li wrote:
>>> @@ -3047,22 +3049,11 @@ static int nested_check_vm_entry_controls(struct kvm_vcpu *vcpu,
>>> u8 vector = intr_info & INTR_INFO_VECTOR_MASK;
>>> u32 intr_type = intr_info & INTR_INFO_INTR_TYPE_MASK;
>>> bool has_error_code = intr_info & INTR_INFO_DELIVER_CODE_MASK;
>>> + bool has_nested_exception = vmx->nested.msrs.basic & VMX_BASIC_NESTED_EXCEPTION;
>> 
>> has_error_code reflects whether the to-be-injected event has an error code.
>> Using has_nested_exception for CPU capabilities here is a bit confusing.
>
>Looks better to just remove has_error_code.
>
>> 
>>> bool urg = nested_cpu_has2(vmcs12,
>>>    SECONDARY_EXEC_UNRESTRICTED_GUEST);
>>> bool prot_mode = !urg || vmcs12->guest_cr0 & X86_CR0_PE;
>>> 
>>> - /* VM-entry interruption-info field: interruption type */
>>> - if (CC(intr_type == INTR_TYPE_RESERVED) ||
>>> -     CC(intr_type == INTR_TYPE_OTHER_EVENT &&
>>> -        !nested_cpu_supports_monitor_trap_flag(vcpu)))
>>> - return -EINVAL;
>>> -
>>> - /* VM-entry interruption-info field: vector */
>>> - if (CC(intr_type == INTR_TYPE_NMI_INTR && vector != NMI_VECTOR) ||
>>> -     CC(intr_type == INTR_TYPE_HARD_EXCEPTION && vector > 31) ||
>>> -     CC(intr_type == INTR_TYPE_OTHER_EVENT && vector != 0))
>>> - return -EINVAL;
>>> -
>>> /*
>>>  * Cannot deliver error code in real mode or if the interrupt
>>>  * type is not hardware exception. For other cases, do the
>>> @@ -3086,8 +3077,28 @@ static int nested_check_vm_entry_controls(struct kvm_vcpu *vcpu,
>>> if (CC(intr_info & INTR_INFO_RESVD_BITS_MASK))
>>> return -EINVAL;
>>> 
>>> - /* VM-entry instruction length */
>>> + /*
>>> +  * When the CPU enumerates VMX nested-exception support, bit 13
>>> +  * (set to indicate a nested exception) of the intr info field
>>> +  * may have value 1.  Otherwise bit 13 is reserved.
>>> +  */
>>> + if (CC(!(has_nested_exception && intr_type == INTR_TYPE_HARD_EXCEPTION) &&
>>> +        intr_info & INTR_INFO_NESTED_EXCEPTION_MASK))
>>> + return -EINVAL;
>>> +
>>> switch (intr_type) {
>>> + case INTR_TYPE_EXT_INTR:
>>> + break;
>> 
>> This can be dropped, as the "default" case will handle it.
>
>We don’t have a default case, as all 8 cases are listed (INTR_INFO_INTR_TYPE_MASK is 0x700).
>
>> 
>>> + case INTR_TYPE_RESERVED:
>>> + return -EINVAL;
>> 
>> I think we need to add a CC() statement to make it easier to correlate a
>> VM-entry failure with a specific consistency check.
>
>What do you want me to put in CC()?
>
>CC(intr_type == INTR_TYPE_RESERVED)?

how about this incremental change?

I prefer to make has_error_code and has_nested_exception consistent, and add a
CC() statement before all "return -EINVAL" statements for debugging.

t a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 8682709d8759..f13df70405d9 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -3049,7 +3049,7 @@ static int nested_check_vm_entry_controls(struct kvm_vcpu *vcpu,
		u8 vector = intr_info & INTR_INFO_VECTOR_MASK;
		u32 intr_type = intr_info & INTR_INFO_INTR_TYPE_MASK;
		bool has_error_code = intr_info & INTR_INFO_DELIVER_CODE_MASK;
-		bool has_nested_exception = vmx->nested.msrs.basic & VMX_BASIC_NESTED_EXCEPTION;
+		bool has_nested_exception = intr_info & INTR_INFO_NESTED_EXCEPTION_MASK;
		bool urg = nested_cpu_has2(vmcs12,
					   SECONDARY_EXEC_UNRESTRICTED_GUEST);
		bool prot_mode = !urg || vmcs12->guest_cr0 & X86_CR0_PE;
@@ -3077,20 +3077,10 @@ static int nested_check_vm_entry_controls(struct kvm_vcpu *vcpu,
		if (CC(intr_info & INTR_INFO_RESVD_BITS_MASK))
			return -EINVAL;
 
-		/*
-		 * When the CPU enumerates VMX nested-exception support, bit 13
-		 * (set to indicate a nested exception) of the intr info field
-		 * may have value 1.  Otherwise bit 13 is reserved.
-		 */
-		if (CC(!(has_nested_exception && intr_type == INTR_TYPE_HARD_EXCEPTION) &&
-		       intr_info & INTR_INFO_NESTED_EXCEPTION_MASK))
+		if (CC(intr_type == INTR_TYPE_RESERVED))
			return -EINVAL;
 
		switch (intr_type) {
-		case INTR_TYPE_EXT_INTR:
-			break;
-		case INTR_TYPE_RESERVED:
-			return -EINVAL;
		case INTR_TYPE_NMI_INTR:
			if (CC(vector != NMI_VECTOR))
				return -EINVAL;
@@ -3098,6 +3088,13 @@ static int nested_check_vm_entry_controls(struct kvm_vcpu *vcpu,
		case INTR_TYPE_HARD_EXCEPTION:
			if (CC(vector > 31))
				return -EINVAL;
+			/*
+			 * When the CPU enumerates VMX nested-exception support, bit 13
+			 * (set to indicate a nested exception) of the intr info field
+			 * may have value 1.  Otherwise bit 13 is reserved.
+			 */
+			if (CC(has_nested_exception && !(vmx->nested.msrs.basic & VMX_BASIC_NESTED_EXCEPTION)))
+				return -EINVAL;
			break;
		case INTR_TYPE_SOFT_EXCEPTION:
		case INTR_TYPE_SOFT_INTR:
@@ -3108,6 +3105,9 @@ static int nested_check_vm_entry_controls(struct kvm_vcpu *vcpu,
				return -EINVAL;
			break;
		case INTR_TYPE_OTHER_EVENT:
+			if (CC(vector > 3))
+				return -EINVAL;
+
			switch (vector) {
			case 0:
				if (CC(!nested_cpu_supports_monitor_trap_flag(vcpu)))
@@ -3121,7 +3121,7 @@ static int nested_check_vm_entry_controls(struct kvm_vcpu *vcpu,
					return -EINVAL;
				break;
			default:
-				return -EINVAL;
+				break;
			}
			break;
		}
@@ -3454,14 +3454,15 @@ static int nested_vmx_check_guest_state(struct kvm_vcpu *vcpu,
		}
		if (vmcs12->guest_cr4 & X86_CR4_FRED) {
			unsigned int ss_dpl = VMX_AR_DPL(vmcs12->guest_ss_ar_bytes);
+
+			if (CC(ss_dpl == 1 || ss_dpl == 2))
+				return -EINVAL;
+
			switch (ss_dpl) {
			case 0:
				if (CC(!(vmcs12->guest_cs_ar_bytes & VMX_AR_L_MASK)))
					return -EINVAL;
				break;
-			case 1:
-			case 2:
-				return -EINVAL;
			case 3:
				if (CC(vmcs12->guest_rflags & X86_EFLAGS_IOPL))
					return -EINVAL;