CVE-2024-53069 - Fixing a NULL Pointer Dereference in Linux Kernel Qualcomm SCM Driver

A critical vulnerability, CVE-2024-53069, was identified and resolved in the Linux kernel’s Qualcomm SCM (Secure Channel Manager) driver. This bug could crash affected systems by causing a NULL pointer dereference. While this issue is mostly relevant to systems using Qualcomm hardware on Linux, understanding the problem—what caused it, how it was fixed, and its security impact—is important for kernel developers, device integrators, and anyone running Linux on Qualcomm platforms.

This long read will break down the vulnerability in simple terms, show the problematic code snippet, and explain how the bug was fixed. You’ll get insights into how device trees and hardware drivers interact in Linux, along with links to official references and the upstream fix.

Background: What Is the Qualcomm SCM Driver?

The Qualcomm SCM API is a secure interface for communicating with the chipset’s secure world, often used for manipulating protected hardware resources and boot processes. In Linux, this is implemented with the qcom_scm driver, which exposes these functions to other kernel code.

When Linux boots on Qualcomm devices, it tries to match the driver with relevant hardware described in the device tree. Sometimes, a system may not include a Secure Channel Manager, or may not enable it in the device tree. In such cases, when other software tries to use SCM services, the corresponding pointer (__scm) ends up as NULL.

The Problem: NULL Pointer Dereference

A NULL pointer dereference is one of the most common, yet dangerous, bugs in C programming. It happens when code tries to access or modify data through a pointer that hasn’t been initialized to a valid memory address (i.e., it points to NULL). In the kernel, such bugs usually cause a system crash or panic, opening up risks for possible privilege escalation or denial-of-service attacks.

Real Scenario

In this particular case, some calls to SCM functions were possible even when the underlying SCM driver hadn't been loaded—often because the hardware wasn't present or enabled. The code didn’t always check if the __scm pointer was valid before trying to use it.

Here's a simplified version of what the problematic code looked like

static struct qcom_scm *__scm;

int qcom_scm_call(...arguments...)
{
    // ... code ...
    if (__scm->version == X) {
        // Dereference without checking if __scm is NULL!
    }
    // ... code ...
}

If __scm was NULL (for example, because there was no SCM entry in device-tree), dereferencing it (__scm->version) led to a NULL pointer dereference, causing a kernel oops or panic.

The Fix: Check for NULL Pointers

The fix is a classic programming safeguard: Always check your pointers before using them. The developer added a check at the start of each function that might use __scm. If __scm is NULL, the function safely returns an error instead of trying to access memory at address .

Here’s how the new, safe approach looks

int qcom_scm_call(...arguments...)
{
    if (!__scm)
        return -ENODEV; // Return error if SCM driver isn't available

    // Safe to use __scm now
    if (__scm->version == X) {
        // process
    }
    // ... rest of function ...
}

This fix prevents the kernel from crashing in unexpected hardware combinations, where the SCM is not present.

Security Impact

Any kernel NULL pointer dereference can be critical. Attackers may exploit such bugs (depending on circumstances) for denial of service (DoS) or, under rare conditions, even escalate privileges. In this specific bug, an unprivileged process may be able to trigger the crash if other kernel components call the vulnerable function without properly verifying hardware state.

Who’s affected?

Devices running Linux with Qualcomm hardware.

- Systems with device trees not listing an SCM entry, but using kernel features or drivers that eventually call SCM APIs.

Official Patch:

Firmware: qcom: scm: fix a NULL-pointer dereference (lkml.org)
- CVE-2024-53069 at NVD (as available)
- Linux Kernel Source on Git
- What is Device Tree? (elinux wiki)
- Common Kernel Security Vulnerabilities

Conclusion

CVE-2024-53069 is a reminder that even small oversights—like forgetting to check if a pointer is NULL—can crash an entire system. For kernel contributors and device maintainers, it’s crucial to validate all hardware dependencies before accessing them, especially in a modular world like Linux.

If you maintain devices or kernels, double-check your use of platform-specific hardware, and, when in doubt, always check your pointers!


*If you found this write-up helpful, please share it with fellow Linux developers and keep your security patches up to date!*


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Published on: 11/19/2024 18:15:26 UTC
Last modified on: 11/22/2024 22:26:20 UTC