KVM: nVMX: Synthesize nested VM-Exit for supported emulation intercepts

When emulating an instruction on behalf of L2 that L1 wants to intercept,
generate a nested VM-Exit instead of injecting a #UD into L2.  Now that
(most of) the necessary information is available, synthesizing a VM-Exit
isn't terribly difficult.

Punt on decoding the ModR/M for descriptor table exits for now.  There is
no evidence that any hypervisor intercepts descriptor table accesses *and*
uses the EXIT_QUALIFICATION to expedite emulation, i.e. it's not worth
delaying basic support for.

To avoid doing more harm than good, e.g. by putting L2 into an infinite
or effectively corrupting its code stream, inject #UD if the instruction
length is nonsensical.

Link: https://lore.kernel.org/r/20250201015518.689704-11-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/vmx/vmx.c

@@ -8008,20 +8008,13 @@ static __init void vmx_set_cpu_caps(void)
 }
 
 static bool vmx_is_io_intercepted(struct kvm_vcpu *vcpu,
-				  struct x86_instruction_info *info)
+				  struct x86_instruction_info *info,
+				  unsigned long *exit_qualification)
 {
 	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
 	unsigned short port;
 	int size;
-
-	if (info->intercept == x86_intercept_in ||
-	    info->intercept == x86_intercept_ins) {
-		port = info->src_val;
-		size = info->dst_bytes;
-	} else {
-		port = info->dst_val;
-		size = info->src_bytes;
-	}
+	bool imm;
 
 	/*
 	 * If the 'use IO bitmaps' VM-execution control is 0, IO instruction
@@ -8033,6 +8026,30 @@ static bool vmx_is_io_intercepted(struct kvm_vcpu *vcpu,
 	if (!nested_cpu_has(vmcs12, CPU_BASED_USE_IO_BITMAPS))
 		return nested_cpu_has(vmcs12, CPU_BASED_UNCOND_IO_EXITING);
 
+	if (info->intercept == x86_intercept_in ||
+	    info->intercept == x86_intercept_ins) {
+		port = info->src_val;
+		size = info->dst_bytes;
+		imm  = info->src_type == OP_IMM;
+	} else {
+		port = info->dst_val;
+		size = info->src_bytes;
+		imm  = info->dst_type == OP_IMM;
+	}
+
+
+	*exit_qualification = ((unsigned long)port << 16) | (size - 1);
+
+	if (info->intercept == x86_intercept_ins ||
+	    info->intercept == x86_intercept_outs)
+		*exit_qualification |= BIT(4);
+
+	if (info->rep_prefix)
+		*exit_qualification |= BIT(5);
+
+	if (imm)
+		*exit_qualification |= BIT(6);
+
 	return nested_vmx_check_io_bitmaps(vcpu, port, size);
 }
 
@@ -8042,6 +8059,9 @@ int vmx_check_intercept(struct kvm_vcpu *vcpu,
 			struct x86_exception *exception)
 {
 	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
+	unsigned long exit_qualification = 0;
+	u32 vm_exit_reason;
+	u64 exit_insn_len;
 
 	switch (info->intercept) {
 	case x86_intercept_rdpid:
@@ -8062,8 +8082,10 @@ int vmx_check_intercept(struct kvm_vcpu *vcpu,
 	case x86_intercept_ins:
 	case x86_intercept_out:
 	case x86_intercept_outs:
-		if (!vmx_is_io_intercepted(vcpu, info))
+		if (!vmx_is_io_intercepted(vcpu, info, &exit_qualification))
 			return X86EMUL_CONTINUE;
+
+		vm_exit_reason = EXIT_REASON_IO_INSTRUCTION;
 		break;
 
 	case x86_intercept_lgdt:
@@ -8076,11 +8098,25 @@ int vmx_check_intercept(struct kvm_vcpu *vcpu,
 	case x86_intercept_str:
 		if (!nested_cpu_has2(vmcs12, SECONDARY_EXEC_DESC))
 			return X86EMUL_CONTINUE;
+
+		if (info->intercept == x86_intercept_lldt ||
+		    info->intercept == x86_intercept_ltr ||
+		    info->intercept == x86_intercept_sldt ||
+		    info->intercept == x86_intercept_str)
+			vm_exit_reason = EXIT_REASON_LDTR_TR;
+		else
+			vm_exit_reason = EXIT_REASON_GDTR_IDTR;
+		/*
+		 * FIXME: Decode the ModR/M to generate the correct exit
+		 *        qualification for memory operands.
+		 */
 		break;
 
 	case x86_intercept_hlt:
 		if (!nested_cpu_has(vmcs12, CPU_BASED_HLT_EXITING))
 			return X86EMUL_CONTINUE;
+
+		vm_exit_reason = EXIT_REASON_HLT;
 		break;
 
 	case x86_intercept_pause:
@@ -8096,15 +8132,21 @@ int vmx_check_intercept(struct kvm_vcpu *vcpu,
 		    !nested_cpu_has(vmcs12, CPU_BASED_PAUSE_EXITING))
 			return X86EMUL_CONTINUE;
 
+		vm_exit_reason = EXIT_REASON_PAUSE_INSTRUCTION;
 		break;
 
 	/* TODO: check more intercepts... */
 	default:
-		break;
+		return X86EMUL_UNHANDLEABLE;
 	}
 
-	/* FIXME: produce nested vmexit and return X86EMUL_INTERCEPTED.  */
-	return X86EMUL_UNHANDLEABLE;
+	exit_insn_len = abs_diff((s64)info->next_rip, (s64)info->rip);
+	if (!exit_insn_len || exit_insn_len > X86_MAX_INSTRUCTION_LENGTH)
+		return X86EMUL_UNHANDLEABLE;
+
+	__nested_vmx_vmexit(vcpu, vm_exit_reason, 0, exit_qualification,
+			    exit_insn_len);
+	return X86EMUL_INTERCEPTED;
 }
 
 #ifdef CONFIG_X86_64