CVE-2023-1829: Linux Kernel Privilege Escalation Flaw

CVE-2023-1829 Overview

A use-after-free vulnerability has been identified in the Linux Kernel traffic control index filter (tcindex) that can be exploited to achieve local privilege escalation. The vulnerability exists in the tcindex_delete function, which fails to properly deactivate filters when using perfect hashes while deleting the underlying structure. This improper handling can subsequently lead to double freeing of the structure, allowing a local attacker to elevate their privileges to root.

Critical Impact

Local attackers with low-level access can exploit this use-after-free vulnerability in the Linux Kernel's tcindex traffic control filter to gain root privileges, potentially compromising the entire system.

Affected Products

• Linux Kernel (multiple versions)
• Systems utilizing traffic control index filtering (tcindex)
• Enterprise Linux distributions based on affected kernel versions

Discovery Timeline

• April 12, 2023 - CVE-2023-1829 published to NVD
• February 13, 2025 - Last updated in NVD database

Technical Details for CVE-2023-1829

Vulnerability Analysis

This use-after-free vulnerability resides in the Linux Kernel's traffic control subsystem, specifically within the tcindex filter implementation. The flaw occurs in the tcindex_delete function, which is responsible for removing tcindex filters. When the filter uses perfect hashes, the function fails to properly deactivate the filter before deleting the underlying data structure.

The improper sequence of operations creates a race condition where the memory associated with the filter structure may be freed while still being referenced elsewhere in the kernel. This leads to a double-free condition, a classic memory corruption vulnerability that can be leveraged by attackers to manipulate kernel memory.

Local attackers can exploit this vulnerability to achieve privilege escalation from a low-privileged user account to root. The attack requires local access to the system but does not require user interaction, making it particularly dangerous in multi-user environments or containerized deployments where unprivileged users may have access.

Root Cause

The root cause of CVE-2023-1829 is improper memory management in the tcindex_delete function within the Linux Kernel's traffic control code. Specifically, the function does not properly deactivate filters in the case of perfect hashes before deleting the underlying memory structure. This oversight allows the structure to be freed while still being referenced, leading to a use-after-free condition that can result in double-free exploitation.

Attack Vector

The attack vector for this vulnerability is local, requiring an attacker to have some level of access to the target system. The exploitation process involves:

1. A local user triggers the vulnerable code path in the tcindex traffic control filter
2. The attacker manipulates the timing of filter deletion operations to trigger the use-after-free condition
3. By carefully crafting memory allocations, the attacker can control the contents of the freed memory
4. The double-free is leveraged to corrupt kernel memory structures
5. The corrupted memory is used to escalate privileges to root

The vulnerability affects the kernel's network traffic control subsystem. An attacker would need to be able to create and manipulate traffic control filters, which typically requires CAP_NET_ADMIN capability or equivalent permissions. However, in certain configurations or containerized environments, these permissions may be more readily available.

Detection Methods for CVE-2023-1829

Indicators of Compromise

• Unexpected privilege escalation events from low-privileged users to root
• Kernel oops or panic messages related to tcindex or traffic control subsystem
• Anomalous traffic control filter creation and deletion activity
• Memory corruption signatures in kernel logs referencing double-free conditions

Detection Strategies

• Monitor kernel logs for use-after-free or double-free related errors in the traffic control subsystem
• Implement runtime kernel integrity monitoring to detect privilege escalation attempts
• Deploy endpoint detection solutions capable of monitoring kernel-level activity
• Track system calls related to traffic control operations (tc commands) for suspicious patterns

Monitoring Recommendations

• Enable kernel auditing for netlink operations related to traffic control
• Configure SentinelOne agents to monitor for local privilege escalation attempts
• Implement alerting on unexpected root shell spawning from non-root processes
• Monitor for rapid creation and deletion of tcindex filters which may indicate exploitation attempts

How to Mitigate CVE-2023-1829

Immediate Actions Required

• Update the Linux Kernel to a version that includes commit 8c710f75256bb3cf05ac7b1672c82b92c43f3d28
• Apply vendor-specific kernel patches from distribution maintainers (Debian, Red Hat, etc.)
• Restrict access to traffic control capabilities where possible
• Consider disabling tcindex if not required in your environment

Patch Information

The vulnerability has been addressed in the Linux Kernel through commit 8c710f75256bb3cf05ac7b1672c82b92c43f3d28. This patch corrects the filter deactivation logic in the tcindex_delete function to properly handle perfect hashes before freeing the underlying structure.

Administrators should update their kernels past this commit to remediate the vulnerability. Multiple Linux distributions have released security advisories addressing this issue, including Debian LTS Advisory and NetApp Security Advisory.

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

Workarounds

• Restrict CAP_NET_ADMIN capability to trusted users and processes only
• Disable or unload the tcindex kernel module if not required: modprobe -r cls_tcindex
• Implement network namespace isolation to limit traffic control access
• Use container security profiles to prevent traffic control operations in unprivileged containers

# Disable tcindex module loading
echo "install cls_tcindex /bin/true" >> /etc/modprobe.d/disable-tcindex.conf
# Remove the module if currently loaded
modprobe -r cls_tcindex
# Verify module is not loaded
lsmod | grep tcindex