APC 篇——备用 APC 队列

写在前面

  此系列是本人一个字一个字码出来的,包括示例和实验截图。由于系统内核的复杂性,故可能有错误或者不全面的地方,如有错误,欢迎批评指正,本教程将会长期更新。 如有好的建议,欢迎反馈。码字不易,如果本篇文章有帮助你的,如有闲钱,可以打赏支持我的创作。如想转载,请把我的转载信息附在文章后面,并声明我的个人信息和本人博客地址即可,但必须事先通知我

你如果是从中间插过来看的,请仔细阅读 羽夏看Win系统内核——简述 ,方便学习本教程。

  看此教程之前,问几个问题,基础知识储备好了吗?保护模式篇学会了吗?练习做完了吗?没有的话就不要继续了。


🔒 华丽的分割线 🔒


练习及参考

本次答案均为参考,可以与我的答案不一致,但必须成功通过。

1️⃣ 自己编写WriteProcessMemory函数(不使用任何DLL,直接调用0环函数)并在代码中使用。

🔒 点击查看答案 🔒


  话不多说,给个效果图看看,代码见折叠:


🔒 点击查看代码 🔒
```cpp
#include "stdafx.h"
#include <stdlib.h>

#define  _WIN32_WINNT 0x400 //解决 QueueUserAPC 函数未定义
#include <windows.h>

VOID WINAPI APCProc(ULONG Param)
{
    Sleep(1000);
    printf("APC…… 0x%X \n",Param);
}

DWORD WINAPI ThreadProc(VOID* Param)
{
    for (int i =0 ;i<100;i++)
    {
        SleepEx(1000,TRUE); //思考为什么?
        //Sleep(1000);
        printf("Running\n");
    }
    return 0;
}

int main(int argc, char* argv[])
{
    HANDLE hTread=CreateThread(NULL,NULL,ThreadProc,NULL,NULL,NULL);
    Sleep(3000);
    QueueUserAPC(APCProc,hTread,10);
    CloseHandle(hTread);
    system("pause");
    return 0;
}

TerminateThread 分析

  该函数在kernel32.dll当中,我们通过IDA轻松定位到了该函数:

; BOOL __stdcall TerminateThread(HANDLE hThread, DWORD dwExitCode)
                public _TerminateThread@8
_TerminateThread@8 proc near            ; DATA XREF: .text:off_7C802654↑o
hThread         = dword ptr  8
dwExitCode      = dword ptr  0Ch

; FUNCTION CHUNK AT .text:7C8449A4 SIZE 00000016 BYTES
                mov     edi, edi
                push    ebp
                mov     ebp, esp
                cmp     [ebp+hThread], 0
                jz      loc_7C8449A4
                push    [ebp+dwExitCode] ; ExitStatus
                push    [ebp+hThread]   ; ThreadHandle
                call    ds:__imp__NtTerminateThread@8 ; NtTerminateThread(x,x)
                test    eax, eax
                jl      loc_7C8449AD
                xor     eax, eax
                inc     eax

loc_7C81CB49:                           ; CODE XREF: TerminateThread(x,x)+27E92↓j
                pop     ebp
                retn    8
_TerminateThread@8 endp

  发现该函数会调用NtTerminateThread,这个函数来自ntdll.dll当中,如下所示:

; Exported entry 349. NtTerminateThread
; Exported entry 1158. ZwTerminateThread

; =============== S U B R O U T I N E =======================================


; __stdcall ZwTerminateThread(x, x)
                public _ZwTerminateThread@8
_ZwTerminateThread@8 proc near          ; CODE XREF: LdrpGenericExceptionFilter(x,x)+90FD↓p
                                        ; RtlQueryProcessDebugInformation(x,x,x)+10B↓p ...
                mov     eax, 102h       ; NtTerminateThread
                mov     edx, 7FFE0300h
                call    dword ptr [edx]
                retn    8
_ZwTerminateThread@8 endp

  可以看出,这个开始进入系统内核了,我们根据PCHunter很容易定义到内核函数为NtTerminateThread,如下所示:

; NTSTATUS __stdcall NtTerminateThread(HANDLE ThreadHandle, NTSTATUS ExitStatus)
_NtTerminateThread@8 proc near          ; DATA XREF: .text:0042AF94↑o

AccessMode      = byte ptr -4
ThreadHandle    = dword ptr  8
ExitStatus      = dword ptr  0Ch

                mov     edi, edi
                push    ebp
                mov     ebp, esp
                push    ecx
                push    ebx
                push    esi
                push    edi
                xor     edi, edi
                mov     eax, large fs:_KPCR.PrcbData.CurrentThread
                cmp     [ebp+ThreadHandle], edi
                mov     esi, eax
                jnz     short loc_4F1E4F
                mov     eax, [esi+_ETHREAD.Tcb.ApcState.Process]
                cmp     [eax+_EPROCESS.ActiveThreads], 1
                jnz     short loc_4F1E8B
                mov     eax, 0C00000DBh
                jmp     short loc_4F1EAA
; ---------------------------------------------------------------------------

loc_4F1E4F:                             ; CODE XREF: NtTerminateThread(x,x)+16↑j
                cmp     [ebp+ThreadHandle], 0FFFFFFFEh
                jz      short loc_4F1E8B
                mov     al, [esi+_ETHREAD.Tcb.PreviousMode]
                push    0               ; HandleInformation
                mov     [ebp+AccessMode], al
                lea     eax, [ebp+ThreadHandle]
                push    eax             ; Object
                push    dword ptr [ebp+AccessMode] ; AccessMode
                push    _PsThreadType   ; ObjectType
                push    1               ; DesiredAccess
                push    [ebp+ThreadHandle] ; Handle
                call    _ObReferenceObjectByHandle@24 ; ObReferenceObjectByHandle(x,x,x,x,x,x)
                mov     edi, eax
                test    edi, edi
                jl      short loc_4F1EA8
                mov     ebx, [ebp+ThreadHandle]
                cmp     ebx, esi
                jnz     short loc_4F1E96
                mov     ecx, ebx        ; Object
                call    @ObfDereferenceObject@4 ; ObfDereferenceObject(x)

loc_4F1E8B:                             ; CODE XREF: NtTerminateThread(x,x)+22↑j
                                        ; NtTerminateThread(x,x)+2F↑j
                push    [ebp+ExitStatus] ; int
                push    esi             ; Response
                call    _PspTerminateThreadByPointer@8 ; PspTerminateThreadByPointer(x,x)
                jmp     short loc_4F1EA8
; ---------------------------------------------------------------------------

loc_4F1E96:                             ; CODE XREF: NtTerminateThread(x,x)+5E↑j
                push    [ebp+ExitStatus] ; int
                push    ebx             ; Response
                call    _PspTerminateThreadByPointer@8 ; PspTerminateThreadByPointer(x,x)
                mov     ecx, ebx        ; Object
                mov     edi, eax
                call    @ObfDereferenceObject@4 ; ObfDereferenceObject(x)

loc_4F1EA8:                             ; CODE XREF: NtTerminateThread(x,x)+57↑j
                                        ; NtTerminateThread(x,x)+70↑j
                mov     eax, edi

loc_4F1EAA:                             ; CODE XREF: NtTerminateThread(x,x)+29↑j
                pop     edi
                pop     esi
                pop     ebx
                leave
                retn    8
_NtTerminateThread@8 endp

  我们要求是分析该函数是怎样终止别的线程的,故里面的细节不要仔细分析,我们看到该函数调用了关键函数PspTerminateThreadByPointer

🔒 点击查看反汇编 🔒
; int __stdcall PspTerminateThreadByPointer(PETHREAD Thread, int a2)
_PspTerminateThreadByPointer@8 proc near
                                        ; CODE XREF: PspUserThreadStartup(x,x)+A9↑p
                                        ; PspSystemThreadStartup(x,x)+3B↑p ...

Interval        = _LARGE_INTEGER ptr -0Ch
res             = dword ptr -4
Thread          = dword ptr  8
arg_4           = dword ptr  0Ch

                mov     edi, edi
                push    ebp
                mov     ebp, esp
                sub     esp, 0Ch
                or      dword ptr [ebp+Interval+4], 0FFFFFFFFh
                push    esi
                push    edi
                mov     edi, [ebp+Thread]
                lea     esi, [edi+248h]
                test    byte ptr [esi], 40h
                mov     dword ptr [ebp+Interval], 0FFF0BDC0h
                jz      short loc_4F1B3C
                mov     eax, [edi+220h]
                add     eax, 174h
                push    eax             ; BugCheckParameter3
                push    edi             ; Response
                push    offset aTerminatingCri ; "Terminating critical thread 0x%p (in %s"...
                call    _PspCatchCriticalBreak@12 ; PspCatchCriticalBreak(x,x,x)

loc_4F1B3C:                             ; CODE XREF: PspTerminateThreadByPointer(x,x)+21↑j
                mov     eax, large fs:_KPCR.PrcbData.CurrentThread
                cmp     edi, eax
                jnz     short loc_4F1B54
                xor     eax, eax
                inc     eax
                lock or [esi], eax
                push    [ebp+arg_4]
                call    _PspExitThread@4 ; PspExitThread(x)

loc_4F1B54:                             ; CODE XREF: PspTerminateThreadByPointer(x,x)+42↑j
                test    byte ptr [esi], 10h
                jz      short loc_4F1B63
                mov     eax, 0C0000022h
                jmp     loc_4F1BF6
; ---------------------------------------------------------------------------

loc_4F1B63:                             ; CODE XREF: PspTerminateThreadByPointer(x,x)+55↑j
                push    ebx
                xor     ebx, ebx
                mov     [ebp+res], ebx
                mov     esi, 'xEsP'
                jmp     short loc_4F1B7B
; ---------------------------------------------------------------------------

loc_4F1B70:                             ; CODE XREF: PspTerminateThreadByPointer(x,x)+86↓j
                lea     eax, [ebp+Interval]
                push    eax             ; Interval
                push    ebx             ; Alertable
                push    ebx             ; WaitMode
                call    _KeDelayExecutionThread@12 ; KeDelayExecutionThread(x,x,x)

loc_4F1B7B:                             ; CODE XREF: PspTerminateThreadByPointer(x,x)+6C↑j
                push    esi             ; Tag
                push    30h ; '0'       ; NumberOfBytes
                push    ebx             ; PoolType
                call    _ExAllocatePoolWithTag@12 ; ExAllocatePoolWithTag(x,x,x)
                mov     edi, eax
                cmp     edi, ebx
                jz      short loc_4F1B70
                mov     ecx, [ebp+Thread]
                xor     edx, edx
                inc     edx
                add     ecx, 248h
                mov     eax, [ecx]

loc_4F1B98:                             ; CODE XREF: PspTerminateThreadByPointer(x,x)+9E↓j
                mov     esi, eax
                or      esi, edx
                lock cmpxchg [ecx], esi
                jnz     short loc_4F1B98
                test    dl, al
                jnz     short loc_4F1BEB
                push    [ebp+arg_4]
                push    ebx
                push    offset _PspExitNormalApc@12 ; PspExitNormalApc(x,x,x)
                push    offset _ExFreeCallBack@4 ; ExFreeCallBack(x)
                push    offset _PsExitSpecialApc@20 ; PsExitSpecialApc(x,x,x,x,x)
                push    ebx
                push    [ebp+Thread]
                push    edi
                call    _KeInitializeApc@32 ; KeInitializeApc(x,x,x,x,x,x,x,x)
                push    2
                push    ebx
                push    edi
                push    edi
                call    _KeInsertQueueApc@16 ; KeInsertQueueApc(x,x,x,x)
                test    al, al
                jnz     short loc_4F1BE1
                push    ebx             ; Tag
                push    edi             ; P
                call    _ExFreePoolWithTag@8 ; ExFreePoolWithTag(x,x)
                mov     [ebp+res], 0C0000001h
                jmp     short loc_4F1BF2
; ---------------------------------------------------------------------------

loc_4F1BE1:                             ; CODE XREF: PspTerminateThreadByPointer(x,x)+CD↑j
                push    [ebp+Thread]
                call    _KeForceResumeThread@4 ; KeForceResumeThread(x)
                jmp     short loc_4F1BF2
; ---------------------------------------------------------------------------

loc_4F1BEB:                             ; CODE XREF: PspTerminateThreadByPointer(x,x)+A2↑j
                push    ebx             ; Tag
                push    edi             ; P
                call    _ExFreePoolWithTag@8 ; ExFreePoolWithTag(x,x)

loc_4F1BF2:                             ; CODE XREF: PspTerminateThreadByPointer(x,x)+DD↑j
                                        ; PspTerminateThreadByPointer(x,x)+E7↑j
                mov     eax, [ebp+res]
                pop     ebx

loc_4F1BF6:                             ; CODE XREF: PspTerminateThreadByPointer(x,x)+5C↑j
                pop     edi
                pop     esi
                leave
                retn    8
_PspTerminateThreadByPointer@8 endp

  为了方便阅读,用F5翻译为伪代码,如下所示:

int __stdcall PspTerminateThreadByPointer(PETHREAD Thread, int a2)
{
  volatile signed __int32 *v2; // esi
  bool v3; // zf
  PVOID buffer; // edi
  union _LARGE_INTEGER Interval; // [esp+8h] [ebp-Ch] BYREF
  int res; // [esp+10h] [ebp-4h]

  v2 = &Thread[1].WaitBlock[1];
  v3 = (Thread[1].WaitBlock[1].WaitListEntry.Flink & 0x40) == 0;
  Interval.QuadPart = -1000000i64;
  if ( !v3 )
    PspCatchCriticalBreak(
      "Terminating critical thread 0x%p (in %s)\n",
      Thread,
      &Thread[1].WaitListEntry.Flink[46].Blink);
  if ( Thread == KeGetCurrentThread() )
  {
    _InterlockedOr(v2, 1u);
    PspExitThread(a2);
  }
  if ( (*v2 & 0x10) != 0 )
    return 0xC0000022;
  res = 0;
  while ( 1 )
  {
    buffer = ExAllocatePoolWithTag(NonPagedPool, 0x30u, 'xEsP');
    if ( buffer )
      break;
    KeDelayExecutionThread(0, 0, &Interval);
  }
  if ( (_InterlockedOr(&Thread[1].WaitBlock[1], 1u) & 1) != 0 )
  {
    ExFreePoolWithTag(buffer, 0);
  }
  else
  {
    KeInitializeApc(buffer, Thread, 0, PsExitSpecialApc, ExFreeCallBack, PspExitNormalApc, 0, a2);
    if ( KeInsertQueueApc(buffer, buffer, 0, 2) )
    {
      KeForceResumeThread(Thread);
    }
    else
    {
      ExFreePoolWithTag(buffer, 0);
      res = 0xC0000001;
    }
  }
  return res;
}

  根据伪代码,我们可以看出如果发现结束的线程是自身线程,就会调用PspExitThread结束自己,如果不是自己,就会使用KeInitializeApc初始化APC,然后调用KeInsertQueueApc插入APC。如下是泄露的NT3.5内核代码,与之对比:

🔒 点击查看代码 🔒
NTSTATUS
PspTerminateThreadByPointer(
    IN PETHREAD Thread,
    IN NTSTATUS ExitStatus
    )

/*++

Routine Description:

    This function causes the specified thread to terminate.

Arguments:

    ThreadHandle - Supplies a referenced pointer to the thread to terminate.

    ExitStatus - Supplies the exit status associated with the thread.

Return Value:

    TBD

--*/

{

    PKAPC ExitApc;

    PAGED_CODE();

    if ( Thread == PsGetCurrentThread() ) {
        ObDereferenceObject(Thread);
        PspExitThread(ExitStatus);

        //
        // Never Returns
        //

    } else {
        ExitApc = ExAllocatePool(NonPagedPool,(ULONG)sizeof(KAPC));

        if ( !ExitApc ) {
            return STATUS_INSUFFICIENT_RESOURCES;
        }

        KeInitializeApc(
            ExitApc,
            &Thread->Tcb,
            OriginalApcEnvironment,
            PsExitSpecialApc,
            NULL,
            PspExitNormalApc,
            KernelMode,
            (PVOID) ExitStatus
            );


        if ( !KeInsertQueueApc(ExitApc,ExitApc,NULL, 2) ) {
            ExFreePool(ExitApc);

            return STATUS_UNSUCCESSFUL;
        }
    }

    return STATUS_SUCCESS;
}

  可以看出,TerminateThread这个函数是通过APC实现控制其他线程,实现终止线程。

SuspendThread 分析

  该函数和TerminateThread函数调用流程是一样的,我们直接从内核层函数调用开始,三环的自行分析就行了。如下是内核函数:

; NTSTATUS __stdcall NtSuspendThread(HANDLE ThreadHandle, PULONG PreviousSuspendCount)
_NtSuspendThread@8 proc near            ; DATA XREF: .text:0042AF84↑o

var_30          = dword ptr -30h
var_2C          = dword ptr -2Ch
var_28          = dword ptr -28h
var_24          = dword ptr -24h
AccessMode      = byte ptr -20h
Object          = dword ptr -1Ch
ms_exc          = CPPEH_RECORD ptr -18h
ThreadHandle    = dword ptr  8
PreviousSuspendCount= dword ptr  0Ch

; __unwind { // __SEH_prolog
                push    20h
                push    offset stru_402FE8
                call    __SEH_prolog
                xor     ebx, ebx
                mov     [ebp+ms_exc.registration.TryLevel], ebx
                mov     eax, large fs:124h
                mov     [ebp+var_30], eax
                mov     al, [eax+140h]
                mov     [ebp+AccessMode], al
                mov     esi, [ebp+PreviousSuspendCount]
                cmp     al, bl
                jz      short loc_4F3B99
                cmp     esi, ebx
                jz      short loc_4F3B99
                mov     eax, _MmUserProbeAddress
                cmp     esi, eax
                jb      short loc_4F3B95
                mov     [eax], ebx

loc_4F3B95:                             ; CODE XREF: NtSuspendThread(x,x)+35↑j
                mov     eax, [esi]
                mov     [esi], eax

loc_4F3B99:                             ; CODE XREF: NtSuspendThread(x,x)+28↑j
                                        ; NtSuspendThread(x,x)+2C↑j
                or      [ebp+ms_exc.registration.TryLevel], 0FFFFFFFFh
                push    ebx             ; HandleInformation
                lea     eax, [ebp+Object]
                push    eax             ; Object
                push    dword ptr [ebp+AccessMode] ; AccessMode
                push    _PsThreadType   ; ObjectType
                push    2               ; DesiredAccess
                push    [ebp+ThreadHandle] ; Handle
                call    _ObReferenceObjectByHandle@24 ; ObReferenceObjectByHandle(x,x,x,x,x,x)
                cmp     eax, ebx
                jl      short loc_4F3C15
                lea     eax, [ebp+var_24]
                push    eax
                push    [ebp+Object]
                call    _PsSuspendThread@8 ; PsSuspendThread(x,x)
                mov     edi, eax
                mov     ecx, [ebp+Object] ; Object
                call    @ObfDereferenceObject@4 ; ObfDereferenceObject(x)
                mov     [ebp+ms_exc.registration.TryLevel], 1
                cmp     esi, ebx
                jz      short loc_4F3BF5
                mov     eax, [ebp+var_24]
                mov     [esi], eax
                jmp     short loc_4F3BF5
; ---------------------------------------------------------------------------

loc_4F3BE1:                             ; DATA XREF: .text:stru_402FE8↑o
                mov     eax, [ebp+ms_exc.exc_ptr]
                mov     eax, [eax]
                mov     eax, [eax]
                mov     [ebp+var_28], eax
                xor     eax, eax
                inc     eax
                retn
; ---------------------------------------------------------------------------

loc_4F3BEF:                             ; DATA XREF: .text:stru_402FE8↑o
                mov     esp, [ebp+ms_exc.old_esp]
                mov     edi, [ebp+var_28]

loc_4F3BF5:                             ; CODE XREF: NtSuspendThread(x,x)+7C↑j
                                        ; NtSuspendThread(x,x)+83↑j
                or      [ebp+ms_exc.registration.TryLevel], 0FFFFFFFFh
                mov     eax, edi
                jmp     short loc_4F3C15
; ---------------------------------------------------------------------------

loc_4F3BFD:                             ; DATA XREF: .text:stru_402FE8↑o
                mov     eax, [ebp+ms_exc.exc_ptr]
                mov     eax, [eax]
                mov     eax, [eax]
                mov     [ebp+var_2C], eax
                xor     eax, eax
                inc     eax
                retn
; ---------------------------------------------------------------------------

loc_4F3C0B:                             ; DATA XREF: .text:stru_402FE8↑o
                mov     esp, [ebp+ms_exc.old_esp]
                or      [ebp+ms_exc.registration.TryLevel], 0FFFFFFFFh
                mov     eax, [ebp+var_2C]

loc_4F3C15:                             ; CODE XREF: NtSuspendThread(x,x)+5B↑j
                                        ; NtSuspendThread(x,x)+9F↑j
                call    __SEH_epilog
                retn    8
; } // starts at 4F3B5C
_NtSuspendThread@8 endp

  我们注意到,该函数调用了PsSuspendThread函数来实现功能,点击去看看,为了方便阅读,我直接放上它的伪代码:

unsigned int __stdcall PsSuspendThread(PETHREAD Thread, _DWORD *PreviousSuspendCount)
{
  unsigned int v2; // esi
  int v4; // [esp+18h] [ebp-1Ch]

  v2 = 0;
  v4 = 0;
  if ( Thread == KeGetCurrentThread() )
  {
    v4 = KeSuspendThread(Thread);
    goto LABEL_10;
  }
  if ( ExAcquireRundownProtection(&Thread[1].WaitBlock[0].WaitListEntry.Blink) )
  {
    if ( (Thread[1].WaitBlock[1].WaitListEntry.Flink & 1) == 0 )
    {
      v4 = KeSuspendThread(Thread);
      if ( (Thread[1].WaitBlock[1].WaitListEntry.Flink & 1) == 0 )
      {
LABEL_8:
        ExReleaseRundownProtection(&Thread[1].WaitBlock[0].WaitListEntry.Blink);
        goto LABEL_10;
      }
      KeForceResumeThread(Thread);
      v4 = 0;
    }
    v2 = 0xC000004B;
    goto LABEL_8;
  }
  v2 = 0xC000004B;
LABEL_10:
  if ( PreviousSuspendCount )
    *PreviousSuspendCount = v4;
  return v2;
}

  而这个函数又调用KeSuspendThread函数进行实现,我们直接看看其伪代码:

int __stdcall KeSuspendThread(PETHREAD Thread)
{
  int res; // edi
  struct _KLOCK_QUEUE_HANDLE LockHandle; // [esp+Ch] [ebp-Ch] BYREF

  KeAcquireInStackQueuedSpinLockRaiseToSynch(&Thread->ApcQueueLock, &LockHandle);
  res = Thread->SuspendCount;
  if ( res == 0x7F )
  {
    KeReleaseInStackQueuedSpinLock(&LockHandle);
    ExRaiseStatus(0xC000004A);
  }
  if ( Thread->ApcQueueable == 1 )
  {
    ++Thread->SuspendCount;
    if ( !res && !Thread->FreezeCount && !KiInsertQueueApc(&Thread->SuspendApc, 0) )
      --Thread->SuspendSemaphore.Header.SignalState;
  }
  KeReleaseInStackQueuedSpinLock(&LockHandle);
  return res;
}

  我们可以看出,SuspendThread这个函数也是通过APC实现控制其他线程,实现挂起线程,如下同是NT3.5泄露代码与之对比:

🔒 点击查看代码 🔒
ULONG
KeSuspendThread (
    IN PKTHREAD Thread
    )

/*++

Routine Description:

    This function suspends the execution of a thread. If the suspend count
    overflows the maximum suspend count, then a condition is raised.

Arguments:

    Thread  - Supplies a pointer to a dispatcher object of type thread.

Return Value:

    The previous suspend count.

--*/

{

    ULONG OldCount;
    KIRQL OldIrql;

    ASSERT_THREAD(Thread);

    //
    // Raise IRQL to dispatcher level and lock dispatcher database.
    //

    KiLockDispatcherDatabase(&OldIrql);

    //
    // Capture the current suspend count.
    //

    OldCount = Thread->SuspendCount;

    //
    // If the suspend count is at its maximum value, then unlock dispatcher
    // database, lower IRQL to its previous value, and raise an error
    // condition.
    //

    if (OldCount == MAXIMUM_SUSPEND_COUNT) {

        //
        // Unlock the dispatcher database and raise an exception.
        //

        KiUnlockDispatcherDatabase(OldIrql);
        ExRaiseStatus(STATUS_SUSPEND_COUNT_EXCEEDED);
    }

    //
    // Increment the suspend count. If the thread was not previously suspended,
    // then queue the thread's suspend APC.
    //

    Thread->SuspendCount += 1;
    if ((OldCount == 0) && (Thread->FreezeCount == 0)) {
        if (KiInsertQueueApc(&Thread->SuspendApc, RESUME_INCREMENT) == FALSE) {
            Thread->SuspendSemaphore.Header.SignalState -= 1;
        }
    }

    //
    // Unlock dispatcher database and lower IRQL to its previous
    // value.
    //

    KiUnlockDispatcherDatabase(OldIrql);

    //
    // Return the previous suspend count.
    //

    return OldCount;
}

备用 APC 队列

  在上一篇中我们讲过,如果想让线程做什么事情,就给它的APC队列里面挂一个APC。在介绍备用APC队列之前,我们先看看下面的结构体:

kd> dt _KTHREAD
nt!_KTHREAD
    ...
    +0x034 ApcState         : _KAPC_STATE
    ...
    +0x138 ApcStatePointer  : [2] Ptr32 _KAPC_STATE
    ...
    +0x14c SavedApcState    : _KAPC_STATE
    ...
    +0x165 ApcStateIndex    : UChar
    +0x166 ApcQueueable     : UChar
    ...

  上面展示的就是KTHREAD结构体中与APC相关的内容。下面我们来详细介绍它们的用途。

ApcState 与 SavedApcState

  它们存储的是一个结构体,我们上篇简单介绍过,再拿来看看:

kd> dt _KAPC_STATE
ntdll!_KAPC_STATE
   +0x000 ApcListHead      : [2] _LIST_ENTRY
   +0x010 Process          : Ptr32 _KPROCESS
   +0x014 KernelApcInProgress : UChar
   +0x015 KernelApcPending : UChar
   +0x016 UserApcPending   : UChar

  这个结构体存储了它的“养父”是谁以及APC相关的信息。有了ApcState了,为啥还要有SavedApcState?我们可以考虑一个事情,线程APC队列中的APC函数都是与进程相关联的,具体点说:A进程的T线程中的所有APC函数,要访问的内存地址都是A进程的。但线程是可以挂靠到其他的进程:比如A进程的线程T,通过修改Cr3(改为B进程的页目录基址),就可以访问B进程地址空间,即所谓“进程挂靠”。
  当T线程挂靠B进程后,APC队列中存储的却仍然是原来的APC。具体点说,比如某个APC函数要读取一个地址为0x12345678的数据,如果此时进行读取,读到的将是B进程的地址空间,这样逻辑就错误了。为了避免混乱,在T线程挂靠B进程时,会将ApcState中的值暂时存储到SavedApcState中,等回到原进程A时,再将APC队列恢复,这就是所谓的备用APC队列。
  当线程是否处于挂靠环境下,ApcState的意义是不一样的。我们设个条件:A进程的T线程挂靠B进程,A是T的所属进程,B是T的挂靠进程。那么如果处于挂靠环境,ApcState存储是就是B进程相关的APC函数,而SavedApcState存储的是A进程相关的APC函数。在正常情况下,当前进程就是所属进程A,如果是挂靠情况下,当前进程就是挂靠进程B。

ApcStatePointer

  为了操作方便,_KTHREAD结构体中定义了一个指针数组ApcStatePointer ,一共两个成员。不同情况下的存储的值我们用下面的表格进行展示:

情况 ApcStatePointer[0] ApcStatePointer[1]
正常情况 ApcState SavedApcState
挂靠情况 SavedApcState ApcState

ApcStateIndex

  用来标识当前线程处于什么状态。如果值为0则为正常状态;如果值为1则为挂靠状态。

ApcQueueable

  用于表示是否可以向线程的APC队列中插入APC。一个线程不可能每时每刻都能被插入APC的。比如当线程正在执行退出的代码时,会将这个值设置为0 ,如果此时执行插入APC的代码(KiInsertQueueApc函数),在插入函数中会判断这个值的状态,如果为0,则插入失败。当线程为系统内核线程,没有3环的部分,如果插入3环的APC,这也是不行的。

ApcStatePointer 与 ApcStateIndex 组合寻址

  正常情况下,向ApcState队列中插入APC时,ApcStatePointer[0]指向ApcState,此时ApcStateIndex的值为0,ApcStatePointer[ApcStateIndex]指向ApcState
  挂靠情况下,向ApcState队列中插入APC时,ApcStatePointer[1]指向ApcState,此时ApcStateIndex的值为1,ApcStatePointer[ApcStateIndex]指向ApcState
  于是我们可以下一个结论:无论什么环境下,ApcStatePointer[ApcStateIndex]指向的都是ApcState,也就是该值总是表示线程当前使用的APC状态。

本节练习

本节的答案将会在下一节进行讲解,务必把本节练习做完后看下一个讲解内容。不要偷懒,实验是学习本教程的捷径。

  俗话说得好,光说不练假把式,如下是本节相关的练习。如果练习没做好,就不要看下一节教程了,越到后面,不做练习的话容易夹生了,开始还明白,后来就真的一点都不明白了。本节练习不多,请保质保量的完成,本篇参考将会在正文给出。

1️⃣ 分析NtReadVirtualMemory在挂靠时如何备份和恢复APC队列的。

下一篇

  APC 篇—— APC 挂入

posted @ 2022-01-26 17:32  寂静的羽夏  阅读(987)  评论(0编辑  收藏  举报