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TL;DR

A bug in the Linux kernel’s register_snapshot_trigger() function could let a malicious user register a “snapshot” trigger even if the memory allocation for that snapshot failed. The function would incorrectly report success () in those cases, possibly leading to undefined behavior or denial of service. This post will break down what went wrong, demonstrate code examples, reference official sources, and discuss proof-of-concept exploit details.

What is CVE-2024-26920?

CVE-2024-26920 describes a vulnerability that was found (and fixed) in the Linux kernel, specifically in the tracing trigger subsystem. This is a core part of the kernel used for debugging, profiling, or analyzing system performance (often by advanced users or security teams).

Affected Component

- File: kernel/trace/trace_events_trigger.c

The Problem in Simple Terms

The function register_snapshot_trigger() is meant to register a "snapshot" event in the tracing system, but it relies on allocating memory first. If the memory allocation fails (perhaps due to low system resources), the function should fail and return a negative error code.

Before the fix:
If allocation fails, the code accidentally returns (success), creating a trigger without the necessary backing resources. That means a user could think the snapshot was ready and issue further calls, leading to weird crashes or denial of service.

Vulnerable Code (Before the Fix)

// [!VULNERABLE CODE SNIPPET!]
// kernel/trace/trace_events_trigger.c

int register_snapshot_trigger(..., struct event_trigger_data *data)
{
    struct snapshot_data *snap;

    snap = kzalloc(sizeof(*snap), GFP_KERNEL);
    if (!snap) {
        // Should return an error here!
        return ;
    }
    /* Continue registering trigger, using snap... */
}

Fixed Code (After the Patch)

// [!FIXED CODE SNIPPET!]
// kernel/trace/trace_events_trigger.c

int register_snapshot_trigger(..., struct event_trigger_data *data)
{
    struct snapshot_data *snap;

    snap = kzalloc(sizeof(*snap), GFP_KERNEL);
    if (!snap) {
        return -ENOMEM;  // Correctly return error if allocation failed
    }
    /* Continue registering trigger, using snap... */
}

System Stability: Follow-up events using the bogus trigger could crash the system or hang.

- Potential DoS: Attackers might purposely exhaust memory to make allocation fail, then spam trigger reg requests, destabilizing the tracing system.

How Could Someone Exploit This?

While this is not a classic remote code execution bug, here is how a local attacker (someone with access to the tracing interface) could use this bug:

Exhaust System Memory: Fill memory so allocation (kzalloc) tends to fail.

2. Register snapshot triggers: Use tracing infrastructure (tools like trace-cmd or direct sysfs writes).
3. Upon silent allocation failure: Because register_snapshot_trigger returns (success), bogus triggers are believed to be live.
4. Trigger snapshot events: Cause the kernel to operate on these bogus triggers; could lead to kernel panic, memory corruption, or other bad states.

PoC Concept:

If you can run code as root (or with sufficient capabilities), you could use this rough approach

# Bash pseudocode—DO NOT RUN ON PRODUCTION

stress --vm 1 --vm-bytes 99%  # Fill RAM to keep allocation rare
echo 'snapshot' > /sys/kernel/debug/tracing/events/[event]/trigger
# (with the bug, this might 'succeed' even if allocation failed)

Kernel Patch Commit:

- Fix to return error if failed to alloc snapshot (kernel.org)

CVE Record:

- NVD entry for CVE-2024-26920

LKML Fix Thread:

- LKML patch thread for tracing/trigger flaw

How to Defend & Patch Status

- Update your kernel: If you use kernel tracing on multi-user or security-critical systems, make sure your kernel version has the above commit or newer.

Conclusion

CVE-2024-26920 is a good example of why proper error handling in kernel code is so important. A single bad return value can create significant risk in complex systems like Linux. Make sure your systems get patched, especially in environments where users may access advanced tracing features.

If you found this technical breakdown useful, consider sharing it with your kernel or security teams!


*Article written exclusively for StackGenAI by an independent Linux security researcher. All code and explanations original to this post—please cite if used!*

Timeline

Published on: 04/17/2024 16:15:08 UTC
Last modified on: 05/04/2025 12:55:11 UTC