A critical use-after-free vulnerability has been discovered in the Linux kernel's net/sched: sch_hfsc (Hierarchical Fair Service Curve) Qdisc traffic control component. This vulnerability can be potentially exploited by an attacker to carry out local privilege escalation, allowing them to gain unauthorized access and control over the affected system.

Vulnerability Details

In a hierarchical packet scheduler implementation, the HFSC scheduler is responsible for managing and allocating bandwidth at each level of the hierarchy. This vulnerability arises when a class with a link-sharing curve (i.e., having the HFSC_FSC flag set) has a parent without a link-sharing curve. In such a scenario, the init_vf() function will call the vttree_insert() function on the parent, but the vttree_remove() function is not called in the update_vf() function. This leads to a dangling pointer that triggers the use-after-free vulnerability.

To better understand the vulnerability, let's take a look at an example code snippet

/* In file hfsc_vtree.c, within the function init_vf() */
if (cl->cl_flags & HFSC_FSC) {
  ...
  vttree_insert(cl->cl_vt, cl->cl_parent);
} else if (cl->cl_parent->cl_flags & HFSC_FSC) {
  // The problematic line; the allocation without freeing the memory
  vttree_insert(cl->cl_vt, cl->cl_parent);
}

Note that the vttree_insert() is called on the parent, but the corresponding vttree_remove() function is not executed in the update_vf() function, leading to a dangling pointer and, subsequently, the use-after-free vulnerability.

1. Linux Kernel's Git Repository - Vulnerability Patch
2. Linux Kernel's Mailing List - Bug Report

Exploit Details

A potential exploit for this vulnerability remains unidentified at this point. However, it may be possible for attackers to exploit the use-after-free vulnerability in affected areas of the Linux kernel to achieve local privilege escalation, which in turn allows them to manipulate and gain control over the affected system.

Mitigation

The vulnerability has been addressed in the Linux kernel's git repository, with the fix available in the following commit: b3d26c5702c7d6c45456326e56d2ccf3f103e60f. We strongly recommend upgrading to a patched version of the kernel as soon as possible to protect against potential exploitation.

Conclusion

The use-after-free vulnerability in the Linux kernel's HFSC Qdisc traffic control component poses significant security risks, particularly related to unauthorized access and privilege escalation. Timely action, including the application of the provided patch, is essential to ensuring system security. Always remain vigilant for potential exploits and maintain regular updates of the Linux kernel for optimal protection.

Timeline

Published on: 09/06/2023 14:15:12 UTC
Last modified on: 10/29/2023 02:40:35 UTC