调用门学习笔记

学习自大佬  https://bbs.pediy.com/thread-62263.htm

10年前的帖子，现在拿来学习还看了一下午。。。。菜鸡努力学习中

原理帖子中讲的很清楚了，只是自己实现了一遍代码。

//如果以ULONG对齐粒度就gg
#pragma pack(1) 
typedef struct
{
    USHORT TableLimit;
    ULONG TableBase;
}GDT,*PGDT;
//根据上图写出结构体
typedef struct
{
    unsigned short  offset_0_15;
    unsigned short  selector;

    unsigned char    param_count : 5;
    unsigned char    some_bits : 3;

    unsigned char    type : 4;
    unsigned char    app_system : 1;
    unsigned char    dpl : 2;
    unsigned char    present : 1;

    unsigned short  offset_16_31;
} CALLGATE_DESCRIPTOR;


void AddGate(ULONG Fun)
{
    CALLGATE_DESCRIPTOR* CallGate;
    GDT* gdt = ExAllocatePool(NonPagedPool,sizeof(GDT));
    ULONG pos = 0,count = 0;
    USHORT Limit = 0;
    USHORT CallGateSel = 0;
    _asm
    {
        mov eax,gdt
        sgdt [eax]
    }
    Limit = gdt->TableLimit;
    CallGate = gdt->TableBase;
    CallGate++;//调试发现第一项为空(第一项为系统保留)
    count = (Limit + 1) / 8;//Limit里面是字节
    while (pos < count)
    {    
        if (CallGate->present == 0)
        {
            //找到空闲位置
            CallGate->offset_0_15 = Fun & 0xFFFF;//低16位偏移
            CallGate->selector = 0x8;
            CallGate->param_count = 0;
            CallGate->some_bits = 0;
            CallGate->type = 0xC;
            CallGate->app_system = 0;
            CallGate->dpl = 0x3;
            CallGate->present = 0x1;
            CallGate->offset_16_31 = Fun >> 0x10;//右移16位，保存高16位
            CallGateSel = (USHORT)((pos * sizeof(CALLGATE_DESCRIPTOR)) | 0x3);
            break;
        }
        pos ++;
        CallGate++;
    }
}

__declspec(naked) void MyFun()
{
　　__asm
　　{
　　　　pushad
　　　　pushfd
　　　　call Ring0Run
　　　　popfd
　　　　popad
　　　　retf
　　}
}

void Ring0Run()
{    
    DbgPrint("My CallGate Run....");
}