CVE-2023-1281 - Privilege Escalation via Use-After-Free in Linux Kernel's Traffic Control (tcindex) Filter

In early 2023, a serious security vulnerability was discovered in the Linux kernel's networking subsystem, specifically within the traffic control index filter (tcindex). Known as CVE-2023-1281, this use-after-free bug can allow a local attacker to escalate their privileges to root. In this article, we’ll break down the vulnerability in simple terms, show you how it can be exploited, and discuss how to protect your systems.

What is the tcindex filter?

The tcindex filter in Linux is part of the traffic control mechanism, which allows network administrators to classify, filter, and control traffic on network devices. It's an important feature, but like any complex piece of code, it is susceptible to bugs.

Affected Versions

- From Linux 4.14 up to (but not including) the fix in git commit ee059170b1f7e94e55fa6cadee544e176a6e59c2

Summary

The use-after-free bug occurs when the imperfect hash area in the tcindex filter is modified (updated) while packets are being processed. This can lead to the execution of code that accesses memory that has already been freed.

More specifically, when a packet is being filtered, the function tcf_exts_exec() is called. However, if the extension (tcf_ext) it operates on is destroyed by another thread or process at the same time, it results in a use-after-free. Since the kernel runs with high privileges, exploiting this bug can allow an attacker to execute arbitrary code with root privileges.

How the Vulnerability Works

The vulnerability is triggered by a time-of-check-to-time-of-use (TOCTOU) race condition. Let’s break it down:

1. Packet flows through tcindex filter: As packets traverse the network stack, the tcindex filter examines them and calls extensions for additional actions.
2. Hash area updated: At the same time, another process or thread updates (deletes or modifies) the imperfect hash table.
3. Freed memory used: If a filter extension or action is destroyed during processing, the already-freed pointer is used when tcf_exts_exec() is called.
4. Potential exploit: A local attacker can race the update and traversal, arranging for controlled memory content to be used after free—leading to privilege escalation.

Below is an excerpt (simplified for clarity) from the affected code

/* Simplified excerpt from net/sched/cls_tcindex.c */

static int tcindex_classify(struct sk_buff *skb, const struct tcf_proto *tp,
                            struct tcf_result *res)
{
    // ... code omitted ...

    struct tcindex_filter_result *r = /* look up from imperfect hash */;
    if (r) {
        int r = tcf_exts_exec(tp->exts, skb);
        // tcf_exts_exec can be called with a freed 'exts' pointer if
        // the hash area is updated concurrently!
    }

    // ... code omitted ...
}

The fix ensures that the extensions are not destroyed while still being used, typically by improving reference counting or atomic access to shared structures.

1. Prepare Two Processes

- Process A: Continuously sends packets to be filtered by tcindex. This triggers the code path where tcf_exts_exec() is called.
- Process B: Rapidly adds or deletes tcindex filters in a loop, racing to update (free) the imperfect hash area while packets are being processed.

2. Win the Race

With enough attempts, Process B can cause the hash area and its extensions to be freed while Process A is still processing a packet—resulting in a use-after-free when tcf_exts_exec() is called with a dangling pointer.

3. Hijack Execution

If the attacker can arrange memory allocations so that after the filter is freed, their controlled data occupies the freed space, tcf_exts_exec() may execute attacker-supplied code.

4. Escalate Privileges

By placing the right data and function pointers, the attacker can get code execution in the kernel, allowing them to elevate privileges to root.

Simple Proof-of-Concept Skeleton (Pseudocode)

// ATTACKER: Rapid add/remove tcindex filters
while (true) {
    system("tc filter add ...");  // Add filter
    system("tc filter del ...");  // Delete filter
}

// ATTACKER: Send crafted packets to hit the filter
while (true) {
    send_packet();
}

The rest of the exploit depends on precise timing and detailed knowledge of kernel heap allocation, but this rough layout demonstrates the basic idea.

References and Further Reading

- Original Patch and Fix Commit
- CVE-2023-1281 Red Hat Security Advisory
- Linux Traffic Control Documentation
- Use-After-Free Vulnerabilities Explained

How to Protect Your System

1. Patch Immediately: Update your Linux kernel to a version including or after the fix commit ee059170b1f7e94e55fa6cadee544e176a6e59c2.
2. Limit Local Users: Only trusted users should have access to environments where tcindex filters can be manipulated.
3. Monitor Unusual Syscalls or Packet Activity: Use tools like auditd or seccomp to identify and restrict suspicious behavior.

Conclusion

CVE-2023-1281 highlights how complex interactions in the Linux kernel can open doors for privilege escalation—especially when race conditions and use-after-free bugs are involved. Always keep your systems up to date and remain vigilant for new threats.

If you manage Linux servers, patch now to keep attackers at bay!


*Written exclusively for your security awareness by a Linux enthusiast.*

Timeline

Published on: 03/22/2023 14:15:00 UTC
Last modified on: 05/03/2023 14:15:00 UTC