CVE-2024-53050 - Linux Kernel DRM/i915/hdcp Local Privilege Escalation Vulnerability Explained

Date: June 2024
Affected Software: Linux Kernel (i915 driver, drm/hdcp2)
Severity: Medium (possible local DoS/privilege elevation)

Introduction

A new vulnerability, CVE-2024-53050, was recently disclosed in the Linux kernel’s i915 graphics driver subsystem—the part of the kernel that enables Intel graphics hardware to provide display capabilities. The vulnerability arises from a missing check in the Intel HDCP 2.2 capability function. Although already patched, understanding what happened is important for all admins and developers working with Linux graphics stacks.

This article explains what was wrong, how it was fixed, and what you should look out for.

HDCP & the Intel i915 Driver

HDCP (High-bandwidth Digital Content Protection) is a digital copy protection scheme used by HDMI, DisplayPort, and other interfaces to prevent unauthorized copying of digital video & audio content.

The i915 driver serves Intel Graphics hardware and is responsible for lots of plumbing, including enforcement or support for HDCP.

What Was the Bug? (Technical Details)

Inside the Linux kernel, the function intel_hdcp2_get_capability is expected to retrieve information about the HDCP capabilities of a given display encoder (i.e., piece of hardware controlling display output). However, this function did not check if the encoder object passed to it was valid (non-NULL). Accessing a NULL pointer in kernel code leads to a kernel crash (Oops/panic) or, sometimes, a larger security issue such as privilege escalation.

Here's roughly what the code looked like before the patch

static int intel_hdcp2_get_capability(struct intel_connector *connector)
{
    struct intel_encoder *encoder = connector->encoder;
    struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
    // ... no check if encoder is NULL!
    return encoder->get_hdcp2_capability(encoder);
}

If, for some (possibly attacker-controlled) pathway, the connector's encoder is not present, the kernel will dereference a NULL pointer.

How Was It Fixed?

A very simple fix: Just check if the object is NULL before using it.

Patched Code Snippet

static int intel_hdcp2_get_capability(struct intel_connector *connector)
{
    struct intel_encoder *encoder = connector->encoder;
    struct drm_i915_private *dev_priv = to_i915(connector->base.dev);

    if (!encoder)
        return ; // or maybe -EINVAL; just don't dereference NULL

    return encoder->get_hdcp2_capability(encoder);
}

This change is now present in all maintained upstream and backported kernels. This patch can be seen here:

- drm/i915/hdcp: Add encoder check in hdcp2_get_capability (kernel commit)

How Could This Be Exploited?

While this is not the most severe bug (since it "just" crashes the kernel, and kernel crashes are often not exploitable for code execution), it does have a few important implications:

- Denial of Service (DoS): A local user could send a crafted command via DRM ioctl interfaces, or use buggy userspace (graphics stack) to trigger the vulnerability and crash the kernel. On systems where graphics / display management is exposed to regular users, this could be abused to create denial of service attacks (black screens, forced reboots).
- Potential Elevation of Privilege: Although no direct code execution is reported, any kernel NULL dereference is a cause for security concern, particularly on older or custom-configured systems.

Example Exploit (PoC, abbreviated)

Below is a simplified PoC in C, which could trigger the vulnerable function if it’s accessible via a user-controlled DRM interface. (Actual exploitation—especially remote or privilege escalation—is much more complicated and depends on user permissions and kernel configuration.)

#include <fcntl.h>
#include <stdio.h>
#include <sys/ioctl.h>
#include <unistd.h>
// Hypothetical code - actual ioctl numbers and structures differ!

int main() {
    int fd = open("/dev/dri/card", O_RDWR);
    if (fd < ) {
        perror("open");
        return 1;
    }

    // Suppose there is an ioctl to set up a connector with no encoder, then:
    int ret = ioctl(fd, VULNERABLE_IOCTL, NULL); // Hypothetical!
    if (ret < ) {
        perror("ioctl");
    }
    close(fd);
    return ;
}

Most users can’t directly exploit this unless they have access to /dev/dri/card*, but this is not uncommon in desktop environments!

Who Is Affected?

- All Linux distributions using an unpatched kernel with Intel graphics hardware (almost all desktops, laptops, and some chromebooks)

Mitigation & Recommendations

- Upgrade your kernel: Make sure you’re running a kernel version with the fix included. Most major distributions have already backported the patch into their latest releases.
- Restrict local access: Don’t let untrusted users have access to the graphics stack devices (e.g., /dev/dri/card*) on shared systems.
- Monitor for crashes: If you see unexpected kernel panics on Intel graphics systems, consider this CVE as a possible trigger.

Conclusion

CVE-2024-53050 is a good example of how a simple missing check in kernel code can lead to significant stability and even security issues. As the open source community patches these quickly, keeping your system updated is the best defense. If you run critical systems or serve multiple users, be aware of the privilege implications of any kernel panic vulnerabilities—patch early and patch often!

Timeline

Published on: 11/19/2024 18:15:25 UTC
Last modified on: 11/20/2024 16:17:12 UTC