CVE-2023-4623: Linux Kernel Use-After-Free Vulnerability

CVE-2023-4623 Overview

A use-after-free vulnerability exists in the Linux kernel's net/sched: sch_hfsc (HFSC qdisc traffic control) component that can be exploited to achieve local privilege escalation. The vulnerability arises from improper memory management in the Hierarchical Fair Service Curve (HFSC) packet scheduler, which is used for traffic shaping and QoS management in Linux networking.

If a class with a link-sharing curve (i.e., with the HFSC_FSC flag set) has a parent without a link-sharing curve, then init_vf() will call vttree_insert() on the parent, but vttree_remove() will be skipped in update_vf(). This leaves a dangling pointer that can cause a use-after-free condition when the memory is subsequently accessed.

Critical Impact

Local attackers with low privileges can exploit this use-after-free vulnerability to escalate privileges on affected Linux systems, potentially gaining root access and full system compromise.

Affected Products

• Linux Kernel (multiple versions)
• Debian Linux 10.0

Discovery Timeline

• 2023-09-06 - CVE-2023-4623 published to NVD
• 2025-03-20 - Last updated in NVD database

Technical Details for CVE-2023-4623

Vulnerability Analysis

This use-after-free vulnerability (CWE-416) resides in the HFSC (Hierarchical Fair Service Curve) qdisc component of the Linux kernel's network scheduling subsystem. The HFSC scheduler is designed to provide guaranteed bandwidth and delay bounds for network traffic, but a flaw in its virtual time tree management creates a memory safety issue.

The vulnerability occurs in the interaction between the init_vf() and update_vf() functions within the HFSC implementation. When configuring traffic classes, the kernel maintains virtual time trees to track scheduling state. The critical flaw manifests when a child class is configured with a link-sharing curve (HFSC_FSC flag) while its parent class lacks such a curve.

In this misconfigured state, init_vf() properly inserts the parent into the virtual time tree by calling vttree_insert(). However, when update_vf() is subsequently executed, it fails to call the corresponding vttree_remove() function to clean up the parent's entry. This asymmetry in insertion and removal operations leaves a dangling pointer in the tree structure.

Root Cause

The root cause is an inconsistent code path in the HFSC qdisc virtual time tree management. The init_vf() function inserts nodes into the vttree structure under certain conditions, but the corresponding cleanup logic in update_vf() does not account for all scenarios where insertion occurred. This creates a dangling pointer when memory is freed but tree references remain, allowing subsequent use of freed memory through the stale pointer.

Attack Vector

This vulnerability requires local access to exploit. An attacker with low-level user privileges can trigger the vulnerable code path by manipulating traffic control configurations through the netlink interface. By crafting specific HFSC class configurations that create the parent-child relationship described above, an attacker can reliably trigger the use-after-free condition. With careful heap manipulation, this can be leveraged to gain arbitrary code execution in kernel context, resulting in complete privilege escalation to root.

The attack flow involves creating HFSC qdisc configurations that cause the dangling pointer condition, then triggering memory operations that reallocate the freed memory with attacker-controlled data before the stale pointer is dereferenced.

Detection Methods for CVE-2023-4623

Indicators of Compromise

• Unexpected kernel crashes or panics related to the network scheduling subsystem
• Suspicious tc (traffic control) command invocations creating HFSC qdiscs with unusual class hierarchies
• Evidence of privilege escalation attempts following network configuration changes
• Kernel log entries indicating memory corruption in net/sched components

Detection Strategies

• Monitor system calls related to netlink and traffic control configuration for anomalous patterns
• Deploy kernel memory corruption detection tools such as KASAN (Kernel Address Sanitizer) to identify use-after-free attempts
• Implement audit logging for tc command usage and HFSC qdisc modifications
• Use SentinelOne Singularity Platform to detect privilege escalation attempts and kernel exploitation behaviors

Monitoring Recommendations

• Enable kernel audit logging for netlink socket operations and traffic control changes
• Monitor for unusual processes spawning with elevated privileges following network configuration activities
• Implement real-time alerting on kernel oops/panic events related to the networking stack
• Review traffic control configurations periodically for unexpected HFSC class hierarchies

How to Mitigate CVE-2023-4623

Immediate Actions Required

• Update the Linux kernel to a patched version containing commit b3d26c5702c7d6c45456326e56d2ccf3f103e60f
• Apply vendor-provided security updates from your Linux distribution
• Restrict access to traffic control utilities and netlink interfaces to trusted administrators only
• Consider applying kernel live patches if available (see Packet Storm Security Notice)

Patch Information

The vulnerability has been addressed in the Linux kernel via commit b3d26c5702c7d6c45456326e56d2ccf3f103e60f. This patch corrects the asymmetric tree management by ensuring that vttree_remove() is properly called in all scenarios where vttree_insert() was invoked, eliminating the dangling pointer condition.

For detailed patch information, refer to the Linux Kernel Commit Details or the Kernel Dance Commit Overview.

Debian users should apply updates per the Debian LTS Security Announcement and Debian LTS Security Update.

Workarounds

• Disable or unload the HFSC qdisc module (sch_hfsc) if not required for network operations
• Restrict access to the tc command and netlink interfaces using Linux capabilities or mandatory access controls
• Implement network namespace isolation to limit the attack surface for unprivileged users
• Apply SELinux or AppArmor policies to restrict traffic control operations to authorized system services

# Disable HFSC qdisc module if not needed
modprobe -r sch_hfsc
echo "blacklist sch_hfsc" >> /etc/modprobe.d/blacklist-hfsc.conf

# Restrict tc command access
chmod 750 /sbin/tc
chown root:network /sbin/tc