kvm: vmx: Nested VM-entry prereqs for event inj.

This patch extends the checks done prior to a nested VM entry.
Specifically, it extends the check_vmentry_prereqs function with checks
for fields relevant to the VM-entry event injection information, as
described in the Intel SDM, volume 3.

This patch is motivated by a syzkaller bug, where a bad VM-entry
interruption information field is generated in the VMCS02, which causes
the nested VM launch to fail. Then, KVM fails to resume L1.

While KVM should be improved to correctly resume L1 execution after a
failed nested launch, this change is justified because the existing code
to resume L1 is flaky/ad-hoc and the test coverage for resuming L1 is
sparse.
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Marc Orr <marcorr@google.com>
[Removed comment whose parts were describing previous revisions and the
 rest was obvious from function/variable naming. - Radim]
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/include/asm/vmx.h

@@ -114,6 +114,7 @@
 #define VMX_MISC_PREEMPTION_TIMER_RATE_MASK	0x0000001f
 #define VMX_MISC_SAVE_EFER_LMA			0x00000020
 #define VMX_MISC_ACTIVITY_HLT			0x00000040
+#define VMX_MISC_ZERO_LEN_INS			0x40000000
 
 /* VMFUNC functions */
 #define VMX_VMFUNC_EPTP_SWITCHING               0x00000001
@@ -351,11 +352,13 @@ enum vmcs_field {
 #define VECTORING_INFO_VALID_MASK       	INTR_INFO_VALID_MASK
 
 #define INTR_TYPE_EXT_INTR              (0 << 8) /* external interrupt */
+#define INTR_TYPE_RESERVED              (1 << 8) /* reserved */
 #define INTR_TYPE_NMI_INTR		(2 << 8) /* NMI */
 #define INTR_TYPE_HARD_EXCEPTION	(3 << 8) /* processor exception */
 #define INTR_TYPE_SOFT_INTR             (4 << 8) /* software interrupt */
 #define INTR_TYPE_PRIV_SW_EXCEPTION	(5 << 8) /* ICE breakpoint - undocumented */
 #define INTR_TYPE_SOFT_EXCEPTION	(6 << 8) /* software exception */
+#define INTR_TYPE_OTHER_EVENT           (7 << 8) /* other event */
 
 /* GUEST_INTERRUPTIBILITY_INFO flags. */
 #define GUEST_INTR_STATE_STI		0x00000001

arch/x86/kvm/vmx.c

@@ -1705,6 +1705,17 @@ static inline bool nested_cpu_has_vmwrite_any_field(struct kvm_vcpu *vcpu)
 		MSR_IA32_VMX_MISC_VMWRITE_SHADOW_RO_FIELDS;
 }
 
+static inline bool nested_cpu_has_zero_length_injection(struct kvm_vcpu *vcpu)
+{
+	return to_vmx(vcpu)->nested.msrs.misc_low & VMX_MISC_ZERO_LEN_INS;
+}
+
+static inline bool nested_cpu_supports_monitor_trap_flag(struct kvm_vcpu *vcpu)
+{
+	return to_vmx(vcpu)->nested.msrs.procbased_ctls_high &
+			CPU_BASED_MONITOR_TRAP_FLAG;
+}
+
 static inline bool nested_cpu_has(struct vmcs12 *vmcs12, u32 bit)
 {
 	return vmcs12->cpu_based_vm_exec_control & bit;
@@ -11620,6 +11631,62 @@ static int check_vmentry_prereqs(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12)
 	    !nested_cr3_valid(vcpu, vmcs12->host_cr3))
 		return VMXERR_ENTRY_INVALID_HOST_STATE_FIELD;
 
+	/*
+	 * From the Intel SDM, volume 3:
+	 * Fields relevant to VM-entry event injection must be set properly.
+	 * These fields are the VM-entry interruption-information field, the
+	 * VM-entry exception error code, and the VM-entry instruction length.
+	 */
+	if (vmcs12->vm_entry_intr_info_field & INTR_INFO_VALID_MASK) {
+		u32 intr_info = vmcs12->vm_entry_intr_info_field;
+		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 should_have_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 (intr_type == INTR_TYPE_RESERVED ||
+		    (intr_type == INTR_TYPE_OTHER_EVENT &&
+		     !nested_cpu_supports_monitor_trap_flag(vcpu)))
+			return VMXERR_ENTRY_INVALID_CONTROL_FIELD;
+
+		/* VM-entry interruption-info field: vector */
+		if ((intr_type == INTR_TYPE_NMI_INTR && vector != NMI_VECTOR) ||
+		    (intr_type == INTR_TYPE_HARD_EXCEPTION && vector > 31) ||
+		    (intr_type == INTR_TYPE_OTHER_EVENT && vector != 0))
+			return VMXERR_ENTRY_INVALID_CONTROL_FIELD;
+
+		/* VM-entry interruption-info field: deliver error code */
+		should_have_error_code =
+			intr_type == INTR_TYPE_HARD_EXCEPTION && prot_mode &&
+			x86_exception_has_error_code(vector);
+		if (has_error_code != should_have_error_code)
+			return VMXERR_ENTRY_INVALID_CONTROL_FIELD;
+
+		/* VM-entry exception error code */
+		if (has_error_code &&
+		    vmcs12->vm_entry_exception_error_code & GENMASK(31, 15))
+			return VMXERR_ENTRY_INVALID_CONTROL_FIELD;
+
+		/* VM-entry interruption-info field: reserved bits */
+		if (intr_info & INTR_INFO_RESVD_BITS_MASK)
+			return VMXERR_ENTRY_INVALID_CONTROL_FIELD;
+
+		/* VM-entry instruction length */
+		switch (intr_type) {
+		case INTR_TYPE_SOFT_EXCEPTION:
+		case INTR_TYPE_SOFT_INTR:
+		case INTR_TYPE_PRIV_SW_EXCEPTION:
+			if ((vmcs12->vm_entry_instruction_len > 15) ||
+			    (vmcs12->vm_entry_instruction_len == 0 &&
+			     !nested_cpu_has_zero_length_injection(vcpu)))
+				return VMXERR_ENTRY_INVALID_CONTROL_FIELD;
+		}
+	}
+
 	return 0;
 }
 

arch/x86/kvm/x86.h

@@ -110,6 +110,15 @@ static inline bool is_la57_mode(struct kvm_vcpu *vcpu)
 #endif
 }
 
+static inline bool x86_exception_has_error_code(unsigned int vector)
+{
+	static u32 exception_has_error_code = BIT(DF_VECTOR) | BIT(TS_VECTOR) |
+			BIT(NP_VECTOR) | BIT(SS_VECTOR) | BIT(GP_VECTOR) |
+			BIT(PF_VECTOR) | BIT(AC_VECTOR);
+
+	return (1U << vector) & exception_has_error_code;
+}
+
 static inline bool mmu_is_nested(struct kvm_vcpu *vcpu)
 {
 	return vcpu->arch.walk_mmu == &vcpu->arch.nested_mmu;