覆盖SEH的溢出利用检测思路
BY kruglinski
看到安焦上的一篇《基于栈指纹检测缓冲区溢出的一点思路》，这是在ShellCode已经运行时在它的调用堆栈(被Hook的下级调用函数LoadLibrary)里进行检测，有些利用溢出覆盖SEH Handler，然后任程序运行，因为溢出破坏了堆或栈，肯定会出现异常，这时指向ShellCode的Handler被运行，我在想这一类的溢出利用，既然它想运行，那首先要过操作系统的异常派遣这一关，如果在分派异常时我们就对SEH Handler进行一下检测，或许能在ShellCode运行前就发现它。
我简单看了一下SEH处理流程，一直跟到这两个函数，因为wrk代码不全，所以我选取ReactOS的代码，但并不影响理解。
以下代码来自ReactOS，版权归原作者
VOID
NTAPI
KiUserExceptionDispatcher(PEXCEPTION_RECORD ExceptionRecord,
PCONTEXT Context)
{
EXCEPTION_RECORD NestedExceptionRecord;
NTSTATUS Status;
/* call the vectored exception handlers */
if(RtlpExecuteVectoredExceptionHandlers(ExceptionRecord,
Context) != ExceptionContinueExecution)
{/*VEH??? ReactOS也太强了吧，实现了XP的VEH，兼容度很高啊！*/
goto ContinueExecution;
}
else
{
/* Dispatch the exception and check the result */
if(RtlDispatchException(ExceptionRecord, Context))
{
ContinueExecution:
/* Continue executing */
Status = NtContinue(Context, FALSE);
}
else
{
/* Raise an exception */
Status = NtRaiseException(ExceptionRecord, Context, FALSE);
}
}
/* Setup the Exception record */
NestedExceptionRecord.ExceptionCode = Status;
NestedExceptionRecord.ExceptionFlags = EXCEPTION_NONCONTINUABLE;
NestedExceptionRecord.ExceptionRecord = ExceptionRecord;
NestedExceptionRecord.NumberParameters = Status;
/* Raise the exception */
RtlRaiseException(&NestedExceptionRecord);
}
BOOLEAN
NTAPI
RtlDispatchException(IN PEXCEPTION_RECORD ExceptionRecord,
IN PCONTEXT Context)
{
PEXCEPTION_REGISTRATION_RECORD RegistrationFrame, NestedFrame = NULL;
DISPATCHER_CONTEXT DispatcherContext;
EXCEPTION_RECORD ExceptionRecord2;
EXCEPTION_DISPOSITION Disposition;
ULONG_PTR StackLow, StackHigh;
ULONG_PTR RegistrationFrameEnd;
/* Get the current stack limits and registration frame */
RtlpGetStackLimits(&StackLow, &StackHigh);
RegistrationFrame = RtlpGetExceptionList();
/* Now loop every frame */
while (RegistrationFrame != EXCEPTION_CHAIN_END)
{
/* Find out where it ends */
RegistrationFrameEnd = (ULONG_PTR)RegistrationFrame +
sizeof(EXCEPTION_REGISTRATION_RECORD);
/* Make sure the registration frame is located within the stack */
if ((RegistrationFrameEnd > StackHigh) ||
((ULONG_PTR)RegistrationFrame < StackLow) ||
((ULONG_PTR)RegistrationFrame & 0x3))
{
/* Check if this happened in the DPC Stack */
if (RtlpHandleDpcStackException(RegistrationFrame,
RegistrationFrameEnd,
&StackLow,
&StackHigh))
{
/* Use DPC Stack Limits and restart */
continue;
}
/* Set invalid stack and return false */
ExceptionRecord->ExceptionFlags |= EXCEPTION_STACK_INVALID;
return FALSE;
}
/* Check if logging is enabled */
RtlpCheckLogException(ExceptionRecord,
Context,
RegistrationFrame,
sizeof(*RegistrationFrame));
/* Call the handler */
Disposition = RtlpExecuteHandlerForException(ExceptionRecord,
RegistrationFrame,
Context,
&DispatcherContext,
RegistrationFrame->
Handler);
/* Check if this is a nested frame */
if (RegistrationFrame == NestedFrame)
{
/* Mask out the flag and the nested frame */
ExceptionRecord->ExceptionFlags &= ~EXCEPTION_NESTED_CALL;
NestedFrame = NULL;
}
/* Handle the dispositions */
switch (Disposition)
{
/* Continue searching */
case ExceptionContinueExecution:
/* Check if it was non-continuable */
if (ExceptionRecord->ExceptionFlags & EXCEPTION_NONCONTINUABLE)
{
/* Set up the exception record */
ExceptionRecord2.ExceptionRecord = ExceptionRecord;
ExceptionRecord2.ExceptionCode =
STATUS_NONCONTINUABLE_EXCEPTION;
ExceptionRecord2.ExceptionFlags = EXCEPTION_NONCONTINUABLE;
ExceptionRecord2.NumberParameters = 0;
/* Raise the exception */
RtlRaiseException(&ExceptionRecord2);
}
else
{
/* Return to caller */
return TRUE;
}
/* Continue searching */
case ExceptionContinueSearch:
break;
/* Nested exception */
case ExceptionNestedException:
/* Turn the nested flag on */
ExceptionRecord->ExceptionFlags |= EXCEPTION_NESTED_CALL;
/* Update the current nested frame */
if (DispatcherContext.RegistrationPointer > NestedFrame)
{
/* Get the frame from the dispatcher context */
NestedFrame = DispatcherContext.RegistrationPointer;
}
break;
/* Anything else */
default:
/* Set up the exception record */
ExceptionRecord2.ExceptionRecord = ExceptionRecord;
ExceptionRecord2.ExceptionCode = STATUS_INVALID_DISPOSITION;
ExceptionRecord2.ExceptionFlags = EXCEPTION_NONCONTINUABLE;
ExceptionRecord2.NumberParameters = 0;
/* Raise the exception */
RtlRaiseException(&ExceptionRecord2);
break;
}
/* Go to the next frame */
RegistrationFrame = RegistrationFrame->Next;
}
/* Unhandled, return false */
return FALSE;
}
然后我们可以为需要保护的进程Hook KiUserExceptionDispatcher，在这里面检测Handler是否安全，我能想到的可能不太安全的Handler有四种情况，也许有更多，我只简单的实现了第一个策略(就是遍历一下SEH链)，下面是相关的代码片段。
//SEHChecker.cpp
inline DWORD __fastcall GetFsDword(DWORD dwOffset)
{
__asm mov eax,DWORD PTR fs:[ecx]
}
策略:
1. Handler在栈区域
2. Handler在堆区域
3. Handler在全局数据区
4. Handler在正常的代码页中，但第一条指令是jmp xxx，或者Handler前一段是不影响ShellCode的指令，后面带有一个jmp xxx，跳到Handler中，怎么检测？
BOOL AnyUnsafeHandler(void)
{
struct SEHChain{
SEHChain *pNext;
void *pHandler;
};
SEHChain* pChain=(SEHChain*)GetFsDword(0);
DWORD dwStackBase=GetFsDword(4);
DWORD dwStackLimit=GetFsDword(8);
BOOL bRet=FALSE;
do
{
bRet=((DWORD)(pChain->pHandler)>=dwStackLimit)&&((DWORD)(pChain->pHandler)<=dwStackBase);
pChain=pChain->pNext;
}while(!bRet&&(pChain!=(SEHChain*)-1));
return bRet;
}
VOID
WINAPI
HookedUserExceptionDispatcher(PEXCEPTION_RECORD ExceptionRecord,
PCONTEXT Context)
{
if(AnyUnsafeHandler())
{
ExitThread(0);
}
TrampolineUserExceptionDispatcher(ExceptionRecord,Context);
}
在检测到非安全的Handler时我为什么要用ExitThread呢，因为基于TIB的Seh Chain是线程相关的，它不是Final型的SEH Handler(不懂的参考一下Hume大侠的经典文章<<SEH in ASM>>)，所以直接用ExitThread把可能出现危险的线程给退掉，如果是主线程则进程会退出，需要的话同时记录一下日志，以供管理员分析受攻击情况。