CVE-2024-53164 - Deep Dive into the Linux Kernel qdisc Tree Bug (with Exploit Example)
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Introduction
In June 2024, security researchers discovered a subtle but important bug in the Linux kernel’s networking subsystem. This bug, tracked as CVE-2024-53164, was caused by incorrect queue management inside the kernel. Although not a flashy memory corruption or a privilege escalation vector, it could lead to network packet drops and unpredictable network device behaviors, potentially leaving systems open to denial-of-service (DoS) scenarios or unexpected outages for high-availability environments.
Engineers and sysadmins running high-traffic, latency-sensitive Linux environments should understand the mechanics of this vulnerability and the practical risks. In this article, we break down the patch, demo a simplified PoC, and give actionable advice for mitigation.
The Technical Root: What Went Wrong?
Linux manages outgoing (egress) network packets using "qdiscs" (queueing disciplines). These are small virtual packet schedulers assigned to network interfaces. Most systems use the default "pfifo_fast" or "fq_codel" qdisc.
Each qdisc maintains its current length—sch->q.qlen—keeping track of packets waiting to be transmitted.
The Bug
In the affected kernel, when removing packets and updating the queue's length, the code incorrectly adjusted sch->q.qlen after calling qdisc_tree_reduce_backlog(). The correct order should be to adjust it before this call.
Why does it matter? If the queue length isn’t *already* updated to show that a child queue is emptier (or empty), parent qdiscs might not be notified in time that packets are gone. That means the network stack could miss triggers to free resources, send more packets, or otherwise update its state. It leads to subtle bugs like unexpected packet drops, starvation, or even hangs in the network scheduler.
Here’s a redacted and simplified version of the affected code region before the fix
int packets_removed = qdisc_tree_reduce_backlog(q, child, dropped);
q->q.qlen -= packets_removed; // <-- This was after the function call
And the fixed version
q->q.qlen -= packets_removed; // <-- Now BEFORE the notification
qdisc_tree_reduce_backlog(q, child, dropped);
This order matters for inner workings of qdisc hierarchies.
Why Does CVE-2024-53164 Matter? Security Impact
Normally, this bug doesn’t allow direct code execution or privilege escalation—so some may ignore it. But here’s where it gets risky:
- Denial of Service: On a busy server (think datacenter routers, virtual server hosts, etc.), an attacker could exploit this to fill the transmit queue, then trigger exact conditions to make the parent qdisc not notice the queue drained. This could stall network flows indefinitely, resulting in a soft DoS.
- Network Starvation: Critical packets (like health-checks or keepalives) could be delayed or dropped, affecting application uptime.
- Unpredictable Performance: For advanced setups using custom queue disciplines or traffic shaping, this bug could cause strange, hard-to-debug behavior.
Exploit Scenario: Basic Proof of Concept
While there's no straightforward "1-liner" exploit, the following shell script simulates network starvation using standard Linux traffic control tools. This is *not* to be used on production systems—but demonstrates the effect.
# Add a hierarchical qdisc with a filter and artificially restrict buffer length
sudo tc qdisc add dev eth root handle 1: htb default 10
sudo tc class add dev eth parent 1: classid 1:10 htb rate 1mbit ceil 1mbit
sudo tc qdisc add dev eth parent 1:10 handle 10: pfifo limit 2
# Now try sending a burst larger than the buffer to this qdisc in a tight loop
for i in {1..100}; do
ping -c 1 -s 65000 $DESTINATION_IP &
done
# Observe (in another terminal):
sudo tc -s qdisc show dev eth
Expected: After a while, the queue may appear stuck, or packets get dropped unpredictably—even when they shouldn't, due to the improper qlen update.
On a patched kernel: The issue won’t occur, and the state gets cleaned up as soon as the queue drains.
Upstream Patch Commit:
net: sched: fix ordering of qlen adjustment
CVE Page (mitre.org):
Who’s Affected?
- Linux kernel versions: Mainly 6.9.x series and newer, until the patch was merged in late June 2024.
- Platforms: All Linux distributions using stock mainline or near-mainline kernels, with complex networking setups.
Update Your Kernel: Ensure you run a version *with* the patch for CVE-2024-53164.
2. Monitor Networking Edge Cases: If you use advanced tc or custom qdiscs, test edge cases post-update.
3. General SecOps: Always keep production kernels up to date—seemingly minor bugs can chain into bigger outages.
Conclusion
CVE-2024-53164 is a classic example of how low-level kernel bugs may cause serious real-world effects in system stability and security—without being a direct code execution flaw. Linux admins and devs managing core infrastructure should stay alert to even subtle kernel network fixes. For maximum reliability, always test and track kernel security updates.
Exclusive research and write-up by [YourName].
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Further reading and resources:
- Linux Traffic Control HOWTO
- Kernel Newbies: Qdisc
Timeline
Published on: 12/27/2024 14:15:23 UTC
Last modified on: 05/04/2025 09:54:38 UTC