CVE-2024-27027 - Linux Kernel dpll_xa_ref_*_del() Multiple Registration Vulnerability Explained

A recent vulnerability, CVE-2024-27027, was identified and fixed in the Linux Kernel, specifically relating to the DPLL (Digital Phase-Locked Loop) subsystems. This bug could trigger kernel warnings and potentially destabilize systems dealing with multiple pin registrations on the same DPLL device. In this exclusive article, we’ll break down what went wrong, how it was fixed, show affected code, and demonstrate a PoC (Proof of Concept) for understanding the exploit.

What’s the DPLL Subsystem?

The DPLL subsystem in Linux manages timing and synchronization hardware, helping ensure clock signals across devices stay in phase. In environments like telecoms or network hardware, DPLLs are critical for robust operation.

The Vulnerability

Whenever the same pin is registered multiple times with a DPLL device, removal functions (dpll_xa_ref_dpll_del() and dpll_xa_ref_pin_del()) mishandle their reference lists. Instead of always removing entries, they only took action when the reference count dropped to zero.

This results in warnings like

WARNING: CPU: 5 PID: 2212 at drivers/dpll/dpll_core.c:143 dpll_xa_ref_pin_del.isra.+x21e/x230
WARNING: CPU: 5 PID: 2212 at drivers/dpll/dpll_core.c:223 __dpll_pin_unregister+x2b3/x2c

Original (buggy) deletion code

// Simplified for clarity
void dpll_xa_ref_pin_del() {
    // ... locate registration entry ...
    if (--ref->count == ) {
        list_del(&ref->node);
        kfree(ref);
    }
    // Else does nothing, entry still remains!
}

Patched version

void dpll_xa_ref_pin_del() {
    // ... locate registration entry ...
    list_del(&ref->node);       // Remove always!
    kfree(ref);                 // Free always!
}

The fix:
The registration is *always* removed and memory freed regardless of the reference count.

Exploit & Proof of Concept

This bug primarily manifests as kernel warnings (and potential instability), not as a clear RCE or privilege escalation. Still, attackers might abuse this to trigger local DoS (Denial of Service) conditions by creating repeated pin registrations & unregistrations, filling up kernel message logs and possibly destabilizing the DPLL subsystem.

PoC Outline (Requires local access & ability to interact with DPLL devices)

// Pseudo C code: Not an actual kernel module
for (int i = ; i < 10; i++) {
    dpll_register_pin(device, pin_id);
}
for (int i = ; i < 10; i++) {
    dpll_unregister_pin(device, pin_id);
}
// Triggers warning if kernel isn't patched

How Was This Fixed?

The patch changes the logic so that registration entries are always removed and memory is always freed, no matter the reference count. This prevents list corruption and those nasty warnings.

Patch commit:

dpll: fix dpll_xa_ref_*_del() for multiple registrations

References

- Linux Kernel Commit: cd7f2148bc5fa944f2e9e30c3fbf65ff5b56fd8e
- CVE Record: CVE-2024-27027
- Bug Discussion: LKML mail

Conclusion

If you work with specialized Linux hardware or kernel modules, make sure to update to a patched kernel to avoid hitting issues with DPLL pin registration. Even if there’s no current public kernel exploit, having a stable and warning-free system is reason enough.

Stay safe, update early!

*Written exclusive for our tech reading community. Feel free to share, but always credit.*

Timeline

Published on: 05/01/2024 13:15:48 UTC
Last modified on: 05/04/2025 09:02:35 UTC