概述
调用门是Intel提供的一个机制,用于控制不同权限级(ring0-ring3)的程序函数调用。简单点就是提供了一个ring3 调用ring0 函数的机制。
在intel手册volume3-Chapter 5.83描述如下
Call gates facilitate controlled transfers of program control between different privilege levels.
They are typically used only in operating systems or executives that use the privilege-level protection mechanism
详细可参阅intel Volume3-Chapter5
实现调用门需要构造一个调用门描述符Call-Gate Descriptor放入GDT或者LDT中。

 Segment Selector 指向代码段的段选择子,P表示门是否有效,如果栈转化那么Param Count 指示要从调用方栈拷贝到目标栈的word(16位)数。type固定为1100.
typedef struct _GateDescriptor {
	unsigned int offset_low16 : 16;
	unsigned int selector : 16;
	unsigned int Param_count : 5;
	unsigned int res : 3;
	unsigned int type : 4;
	unsigned int s : 1;
	unsigned int dp1 : 2;
	unsigned int p : 1;
	unsigned int offset_hei16 : 16;
}GateDescriptor;
栈转化
如果调用的代码段是ring3权限(CPL),而目标调用门是ring0(RPL)权限,栈区是不共享的因此需要将栈区的参数拷贝到目标栈中。
因此调用调用门intel会自动按照如下图进行栈拷贝。
 
 但是你需要注意FS寄存器intel并没有保存,但是window在ring3程序fs存储的TIB,在ring0存储KPCR,也就是说在window下你需要手动处理。
调用调用门
call 调用门选择子
jmp 调用门选择子
比如下面的汇编调用0x4bh的选择子
call    0x004B:00000000
但是VC编译器无法编写上面的指令,你只能利用下面的汇编指令
__asm {
		_emit 0x9a;
		_emit 0x00;
		_emit 0x00;
		_emit 0x00;
		_emit 0x00;
		_emit 0x4b;
		_emit 0x00;
	
		//call    0x004B:00000000;
	}
调用门函数的编写
首先函数一般使用裸函数编写,结尾使用retf返回,如下图所示
void Syscall() {
	DbgPrint("[My learning] %s \r\n", __FUNCTION__);
}
//对外提供的调用门函数
__declspec(naked) void SyscallProxy() {
	__asm {
		push ebp;
		mov ebp, esp;
		//windwow ring 0 fs应该指向30h
		//注意!! windbg调试内核的话会自动修改fs为30h
		mov ax, 30h;
		mov fs, ax;
		call Syscall;
		mov esp, ebp;
		pop ebp;
		//这里要返回到ring 3所以应该还原fs
		//ring 3程序固定指向3bh
		mov ax, 3bh;
		mov fs,ax;
		retf 0;
	}
}
为什么要使用裸函数?假设我们代码如下:
void SyscallProxy(){
}
对应的汇编指令
 
 可以发现返回的时候使用ret而不是retf,两个指令最大的差别在于是否会修正cs等。如果直接使用原始的函数那么调用门将不会正确的返回。(cs没有被正确的修正)
实现调用门
我们通过一个驱动程序来编写一个调用门函数。
首先我们需要查看系统哪个GDT表项是空的,让我们插入自己实现的调用门描述符。
//查看gdt表 0到100的表项
dg 0 100

 我们注意到0x48是空白的,所以我们可以利用这个进行插入我们自己的调用门描述符。0x48对应的ring3 的段选择子是0x4bh,
 计算过程如下:
 首先段选择子格式
 
 index: 1001(第9个gdt项)
 TI :0
 RPL:11
 RPL 表示当前权限因为是ring3 所以是11
 组合上面的数据后就是 1001011 也就是4bh
#pragma push
#pragma pack(1)
typedef struct _GDTR {
	short limit;
	int base;
}GDTR;
#pragma pop
typedef struct _GateDescriptor {
	unsigned int offset_low16 : 16;
	unsigned int selector : 16;
	unsigned int Param_count : 5;
	unsigned int res : 3;
	unsigned int type : 4;
	unsigned int s : 1;
	unsigned int dp1 : 2;
	unsigned int p : 1;
	unsigned int offset_hei16 : 16;
}GateDescriptor;
void Syscall() {
	//KdBreakPoint();
	DbgPrint("[My learning] %s \r\n", __FUNCTION__);
}
//对外暴露的调用门函数
__declspec(naked) void SyscallProxy() {
	__asm {
		//int 3;
		push ebp;
		mov ebp, esp;
		//push fs;
		mov ax, 30h;
		mov fs, ax;
		call Syscall;
		mov esp, ebp;
		pop ebp;
		mov ax, 3bh;
		mov fs,ax;
		retf 0;
	}
}
//安装调用门到GDT中
void InstallGate() {
	//KdBreakPoint();
	DbgPrint("[My learning] %s \r\n", __FUNCTION__);
	GateDescriptor gate = { 0 };
	//指向代码段的选择子,因为ring0代码段是gdt第1个项目且DPL是0
	gate.selector = 0x8;
	//函数
	gate.offset_low16 = (ULONG)SyscallProxy & 0xffff;
	gate.offset_hei16 = ((ULONG)SyscallProxy >> 16) & 0xffff;
	//参数是0
	gate.Param_count = 0;
	//固定数值
	gate.type = 0xc;
	gate.s = 0;
	gate.p = 1;
	//权限因为是给ring3准备的所以是3 
	gate.dp1 = 3;
	
	KAFFINITY mask = KeQueryActiveProcessors();
	KAFFINITY shift = 1;
	while (mask)
	{
		KeSetSystemAffinityThread(shift);
		GDTR gdt = { 0 };
		__asm sgdt  gdt;
		DbgPrint("[My learning] %s base:%p limit %p \r\n", __FUNCTION__, gdt.base, gdt.limit);
		GateDescriptor*pGate = (GateDescriptor*)gdt.base;
		if (MmIsAddressValid(pGate))
		{
			pGate[9] = gate;
		}
		shift <<= 1;
		mask >>= 1;
	}
}
//卸载函数
void UnInstallGate() {
	//KdBreakPoint();
	DbgPrint("[My learning] %s \r\n", __FUNCTION__);
	KAFFINITY mask = KeQueryActiveProcessors();
	KAFFINITY shift = 1;
	while (mask)
	{
		KeSetSystemAffinityThread(shift);
		GDTR gdt = { 0 };
		__asm sgdt  gdt;
		DbgPrint("[My learning] %s base:%p limit %p \r\n", __FUNCTION__, gdt.base, gdt.limit);
		GateDescriptor*pGate = (GateDescriptor*)gdt.base;
		if (MmIsAddressValid(pGate))
		{
			pGate[9].p = 0;
		}
		shift <<= 1;
		mask >>= 1;
	}
}
结合驱动代码
#include<ntifs.h>
#include <Ntddk.h>
#include<intrin.h>
#pragma push
#pragma pack(1)
typedef struct _GDTR {
	short limit;
	int base;
}GDTR;
#pragma pop
typedef struct _GateDescriptor {
	unsigned int offset_low16 : 16;
	unsigned int selector : 16;
	unsigned int Param_count : 5;
	unsigned int res : 3;
	unsigned int type : 4;
	unsigned int s : 1;
	unsigned int dp1 : 2;
	unsigned int p : 1;
	unsigned int offset_hei16 : 16;
}GateDescriptor;
void Syscall() {
	//KdBreakPoint();
	DbgPrint("[My learning] %s \r\n", __FUNCTION__);
}
__declspec(naked) void SyscallProxy() {
	__asm {
		//int 3;
		push ebp;
		mov ebp, esp;
		//push fs;
		mov ax, 30h;
		mov fs, ax;
		call Syscall;
		mov esp, ebp;
		pop ebp;
		mov ax, 3bh;
		mov fs,ax;
		retf 0;
	}
}
void InstallGate() {
	//KdBreakPoint();
	DbgPrint("[My learning] %s \r\n", __FUNCTION__);
	GateDescriptor gate = { 0 };
	gate.selector = 0x8;
	gate.offset_low16 = (ULONG)SyscallProxy & 0xffff;
	gate.offset_hei16 = ((ULONG)SyscallProxy >> 16) & 0xffff;
	gate.Param_count = 0;
	gate.type = 0xc;
	gate.s = 0;
	gate.dp1 = 3;
	gate.p = 1;
	KAFFINITY mask = KeQueryActiveProcessors();
	KAFFINITY shift = 1;
	while (mask)
	{
		KeSetSystemAffinityThread(shift);
		GDTR gdt = { 0 };
		__asm sgdt  gdt;
		DbgPrint("[My learning] %s base:%p limit %p \r\n", __FUNCTION__, gdt.base, gdt.limit);
		GateDescriptor*pGate = (GateDescriptor*)gdt.base;
		if (MmIsAddressValid(pGate))
		{
			pGate[9] = gate;
		}
		shift <<= 1;
		mask >>= 1;
	}
}
void UnInstallGate() {
	//KdBreakPoint();
	DbgPrint("[My learning] %s \r\n", __FUNCTION__);
	KAFFINITY mask = KeQueryActiveProcessors();
	KAFFINITY shift = 1;
	while (mask)
	{
		KeSetSystemAffinityThread(shift);
		GDTR gdt = { 0 };
		__asm sgdt  gdt;
		DbgPrint("[My learning] %s base:%p limit %p \r\n", __FUNCTION__, gdt.base, gdt.limit);
		GateDescriptor*pGate = (GateDescriptor*)gdt.base;
		if (MmIsAddressValid(pGate))
		{
			pGate[9].p = 0;
		}
		shift <<= 1;
		mask >>= 1;
	}
}
//这个函数被注册用于驱动卸载调用
VOID myUnload(
	struct _DRIVER_OBJECT* DriverObject
) {
	UNREFERENCED_PARAMETER(DriverObject);
	DbgPrint("hello  drive unloaded");
	PDEVICE_OBJECT DeviceObject = DriverObject->DeviceObject;
	UnInstallGate();
	if (DriverObject->DeviceObject != NULL)
	{
		DbgPrint("驱动文件不为空执行删除");
		IoDeleteDevice(DeviceObject);
		UNICODE_STRING symbolDevName;
		RtlInitUnicodeString(&symbolDevName, L"\\DosDevices\\MytestDriver");
		IoDeleteSymbolicLink(&symbolDevName);
	}
}
//驱动被加载的时候会调用此函数
NTSTATUS
DriverEntry(
	_In_ struct _DRIVER_OBJECT* DriverObject,
	_In_ PUNICODE_STRING    RegistryPath
)
{
	//如果你没有用到参数需要告诉系统。
	UNREFERENCED_PARAMETER(RegistryPath);
	InstallGate();
	//打印信息
	DbgPrint("[My learning]  drive loaded");
	
	DriverObject->DriverUnload = myUnload;
	
	UNICODE_STRING ustrDevName;
	RtlInitUnicodeString(&ustrDevName, L"\\Device\\MytestDriver");
	PDEVICE_OBJECT  pDevObj = NULL;
	auto ret = IoCreateDevice(DriverObject, 0, &ustrDevName, FILE_DEVICE_UNKNOWN, FILE_DEVICE_SECURE_OPEN, FALSE, &pDevObj);
	if (NT_SUCCESS(ret))
	{
		//指定IO模式
		pDevObj->Flags |= DO_DIRECT_IO;
		DbgPrint("IoCreateDevice 成功 \r\n");
	}
	else {
		DbgPrint("IoCreateDevice 失败 %d\r\n", ret);
		return STATUS_FAIL_CHECK;
	}
	UNICODE_STRING symbolDevName;
	RtlInitUnicodeString(&symbolDevName, L"\\DosDevices\\MytestDriver");
	ret = IoCreateSymbolicLink(&symbolDevName, &ustrDevName);
	if (NT_SUCCESS(ret))
	{
		DbgPrint("IoCreateSymbolicLink 成功 \r\n");
	}
	else {
		DbgPrint("IoCreateSymbolicLink 失败%d\r\n", ret);
		IoDeleteDevice(pDevObj);
		return STATUS_FAIL_CHECK;
	}
	return STATUS_SUCCESS;
}
最后ring3 层的调用代码
// ring3Demo.cpp : 此文件包含 "main" 函数。程序执行将在此处开始并结束。
//
#include <iostream>
int main()
{
	__asm {
		_emit 0x9a;
		_emit 0x00;
		_emit 0x00;
		_emit 0x00;
		_emit 0x00;
		_emit 0x4b;
		_emit 0x00;
		
	}
	printf("调用完毕syscall");
	system("pause");
	return 0;
}



















