CVE-2024-50264 - Linux Kernel vsock/virtio Use-After-Free Vulnerability Explained

A serious security bug has been found and patched in the Linux kernel. This flaw, tracked as CVE-2024-50264, affects the vsock virtual socket implementation using Virtio, which is mainly used in virtualized environments (like KVM or QEMU). The bug could let attackers exploit a Use-After-Free (UAF) situation because of a dangling pointer in the vsk->trans member during specific loopback operations.

In this post, we’ll break down what went wrong, explain the vulnerability in simple language, show a code snippet, examine how an exploit could work, and point you to official sources for more info.

Short Summary

- Component: Linux kernel vsock/virtio

Understanding the Bug

Vsock (virtual sockets) lets virtual machines (VMs) talk to each other or to the host, much like using network sockets, but specially optimized for virtualization.

vsk->trans is a pointer used inside the vsock code. During some operations, specifically loopback (VM-to-VM on the same host) messaging, vsk->trans could point to memory that had already been freed. If code later tried to use this pointer, a Use-After-Free occurs — a classic bug that attackers love because it can lead to crashes or code execution.

The Vulnerable Code

Here’s a simplified version of what went wrong (not the actual kernel code, but enough to get the idea):

struct vsock_sock {
    struct vsock_transport *trans;
    // other members
};

/* Some function handling the socket teardown */
void vsock_teardown(struct vsock_sock *vsk) {
    // Step 1: 'trans' points to some object
    do_cleanup(vsk->trans);
    kfree(vsk->trans);    // Free memory

    // Step 2: but 'vsk->trans' isn't reset!
    // Some code still refers to vsk->trans ...
    use_transport(vsk->trans);  // <-- Use-After-Free here
}

Because vsk->trans wasn’t set to NULL, future code touching it could access freed memory, leading to potential misuse.

A small but crucial line was added after freeing the memory

kfree(vsk->trans);
vsk->trans = NULL;    // Fix: Clear the dangling pointer

This ensures no one will access stale, freed memory, as the pointer now clearly shows it’s empty.

Official kernel commit:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=470376cbf03d3bfab940cba8d441c14ad20e8e7d

Local kernel crash (DoS): Possible by tricking the kernel into dereferencing the freed pointer.

- Privilege escalation/code execution: In rare cases, UAF bugs can let attackers execute arbitrary code in kernel space.

Sample Exploit Sketch (Illustrative Only)

// Pseudocode sketch!
int vsock_fd = socket(AF_VSOCK, SOCK_STREAM, );
connect(vsock_fd, ...);    // Loopback connect

// Do something to trigger vsock_teardown (*details depend on kernel version*)

// If successful, kernel may access freed memory

How to Stay Safe

- Update your kernel: All major Linux distributions have (or will have) patches for CVE-2024-50264.

References

- Kernel commit fixing the bug
- CVE Entry (NVD) *(pending)*
- vsock Documentation

Final Thoughts

CVE-2024-50264 highlights how subtle pointer management mistakes can lead to severe security bugs in the kernel. If you run virtualized workloads, especially on public clouds or with untrusted VMs, keeping your kernel updated is crucial. This fix — initializing vsk->trans to NULL — is a great example of simple, effective secure coding!

Timeline

Published on: 11/19/2024 02:16:28 UTC
Last modified on: 12/19/2024 09:36:51 UTC