A critical vulnerability was recently discovered and resolved in the Linux kernel's vsock module. The issue has been assigned the identifier CVE-2025-21756 and is related to the handling of socket bindings. An attacker exploiting this vulnerability could trigger a use-after-free condition, potentially leading to system crashes or arbitrary code execution. In this post, we'll examine the details of the vulnerability, the code changes that were made to fix it, and how to protect your systems.

Vulnerability Details

The vulnerability in the vsock module occurs due to improper handling of socket bindings. Specifically, the issue lies in the following sequence of events:

The vsock_create() function (refcnt=1) calls vsock_insert_unbound() (refcnt=2).

2. The transport's release() function calls vsock_remove_bound() without checking if the socket was bound and moved to the bound list (refcnt=1).
3. The vsock_bind() function assumes the socket is in the unbound list and calls __vsock_remove_bound() before calling __vsock_insert_bound(vsock_bound_sockets()). This results in:
- list_del_init(&vsk->bound_table); // nop
- sock_put(&vsk->sk); // refcnt=

This sequence of events triggers a use-after-free error and generates the following KASAN (Kernel Address Sanitizer) report:

BUG: KASAN: slab-use-after-free in __vsock_bind+x62e/x730
Read of size 4 at addr ffff88816b46a74c by task a.out/2057
[...]

You can view the details of the original report and patch on the Linux kernel mailing list here.

Patch Details

The patch to address this vulnerability involves preserving the socket bindings, including those resulting from an explicit bind() call and those implicitly bound through autobind during connect(). This prevents the socket from unbinding during a transport reassignment and ultimately resolves the use-after-free error.

To apply the patch on your systems, you should update your Linux kernel to the version containing the fix. You can find the patch information here.

Exploit

While no known public exploits currently exist for this vulnerability, it is recommended to apply the patch as soon as possible to ensure your systems are protected. If you cannot immediately update your kernel, you should consider monitoring for unusual activity that might indicate exploitation attempts and implement system hardening measures to reduce the attack surface.

Conclusion

CVE-2025-21756 is a critical vulnerability in the Linux kernel that has been successfully resolved. By understanding the details of this issue and applying the appropriate patches, system administrators can ensure their systems remain secure against this threat. It is critical to keep your systems up-to-date and apply security patches as they become available to protect against vulnerabilities like this.

Timeline

Published on: 02/27/2025 03:15:16 UTC
Last modified on: 04/30/2025 14:15:28 UTC