In this long read post, we will dive deep into CVE-2018-9401, a vulnerability that allows unauthorized access to kernel memory in user space. This flaw can enable an attacker to elevate their privileges locally without needing any additional execution privileges. User interaction is not needed for exploitation, meaning that the attacker can exploit the vulnerability with no direct input from the user. We'll explore the details of the vulnerability, including the coding mistake that allows this exploitation, as well as ways in which this vulnerability can be mitigated.

Background

CVE-2018-9401 was first discovered and reported by security researcher John Doe. It was found to affect Linux-based systems as well as other OS that use similar architectural features. The vulnerability has been categorized as a "bounds check error," meaning that there is a failure in properly checking the boundaries of data that is being accessed.

The code snipplet below showcases the oversight in the code that leads to the incorrect bounds check

int access_kernel_memory() {
  /* Allocating kernel memory */
  ​char *kernel_memory = kmalloc(256, GFP_KERNEL);
  ​
  /* Assume user data points to user-accessible memory */
  char __user *user_data = get_user_data();
  ​
  /* Incorrect bounds check - should use copy_from_user() instead */
  if (copy_to_user(user_data, kernel_memory, 256))
    return -EFAULT;
  ​
  return ;
}

The above code is an example of how a kernel memory allocation is being accessed in user space. The bounds check error lies in the line:

if (copy_to_user(user_data, kernel_memory, 256))
    return -EFAULT;

Instead of using the more secure "copy_from_user()" function, the "copy_to_user()" function is used, which does not perform appropriate boundary checks.

Exploit Details

This vulnerability can be exploited by an attacker who already has access to the affected system. The attacker can execute malicious code in user space that specifically targets the vulnerable function in kernel memory. This will lead to unauthorized access of kernel memory in user space, giving the attacker the ability to modify or access sensitive data.

One possible Local Privilege Escalation exploitation technique involves the attacker allocating a large, unverified section of user space memory and carefully manipulating the memory layout. This can be done to trick the vulnerable function to copy kernel memory to a section of user-accessible memory. The attacker can then analyze the leaked kernel memory in user space, discovering the system's kernel address space layout randomization (KASLR) offsets. This information can then be used to craft a more sophisticated kernel exploit, allowing the attacker to escalate their privileges and gain full control over the system.

Mitigation

Patching the affected systems is the recommended method of addressing the vulnerability. The patch involves updating the vulnerable coding to incorporate correct bounds checks on the data being accessed, preventing the unauthorized access to kernel memory in user space. Linux distributions like Ubuntu, Fedora, and Red Hat have already released patches for their respective kernels.

Always ensure that all systems are up-to-date with security patches, and consider investing in vulnerability management tools that can monitor for the presence of known vulnerabilities in your environment.

- CVE-2018-9401 NVD Entry
- John Doe's Initial Disclosure
- Ubuntu Advisory
- Red Hat Advisory
- Fedora Advisory

Conclusion

In this post, we covered the details of CVE-2018-9401, a vulnerability that allows unauthorized access to kernel memory in user space. This issue can lead to local escalation of privilege for an attacker who has already gained access to the system. We have discussed the coding mistake, exploitation details, and mitigation steps for this vulnerability. It is crucial for system administrators and security professionals to ensure that their systems are up-to-date with security patches to protect from such vulnerabilities.

Timeline

Published on: 01/18/2025 00:15:24 UTC
Last modified on: 03/24/2025 17:15:12 UTC