CVE-2024-53129 - Practical Guide to the Linux Kernel drm/rockchip VOP Vulnerability

CVE-2024-53129 refers to a recently addressed vulnerability in the Linux Kernel, specifically tied to the Rockchip Direct Rendering Manager (DRM) Video Output Processor (VOP) driver. The bug involves a subtle programming mistake: dereferencing a pointer before confirming it's safe (non-NULL). In this post, we'll break down what happened, show you the vulnerable code, discuss the fix, and explain how this could have been exploited in real-world scenarios.

> This guide is built on direct Linux kernel source analysis. References and proof-of-concept (POC) code are included for researchers and defenders.

The bug sits in the function vop_plane_atomic_async_check() within the file

drivers/gpu/drm/rockchip/rockchip_drm_vop.c

The code used state without verifying its validity, only checking it *afterwards*. In systems programming, this is a classic recipe for dereferencing invalid memory, leading to kernel panics or, in rare cases, potential escalation if attackers can manipulate the pointer.

Warning generated by static analysis

drivers/gpu/drm/rockchip/rockchip_drm_vop.c:1096
vop_plane_atomic_async_check() warn: variable dereferenced before check 'state' (see line 1077)

Vulnerable Code Snippet

int vop_plane_atomic_async_check(struct drm_plane *plane, struct drm_plane_state *state)
{
    struct drm_crtc *crtc = state->crtc;
    struct drm_crtc_state *crtc_state;

    // ... (some code)

    // 'state' is dereferenced here before a NULL check!
    crtc_state = drm_atomic_get_existing_crtc_state(state->state, crtc);

    if (!crtc_state)
        return -EINVAL;  // Error: crtc_state is NULL

    // ... (rest of function)
}

Here, state->crtc and state->state are used before the code checks if crtc_state is NULL. If state itself is NULL, any dereference will cause a crash.

## The Patch / Fix

Developers resolved this by ensuring the function immediately returns if state is NULL, without trying to use it.

Fixed Code Example

int vop_plane_atomic_async_check(struct drm_plane *plane, struct drm_plane_state *state)
{
    if (!state)
        return -EINVAL;

    struct drm_crtc *crtc = state->crtc;
    struct drm_crtc_state *crtc_state;

    crtc_state = drm_atomic_get_existing_crtc_state(state->state, crtc);

    if (!crtc_state)
        return -EINVAL;

    // Rest of the fixed function...
}

Source reference:
Mainline patch commit

Related discussion:
Kernel mailing list reference

Exploitability Assessment

This flaw is a local Denial of Service (DoS) at minimum. If an attacker has permission to use the DRM subsystem (which is unlikely for unprivileged users, but common in containerized desktop or embedded scenarios), they might craft input triggering a NULL pointer dereference, causing a kernel panic or hang.

Conceptual Exploit (Local DoS)

A malicious userspace process could use IOCTL calls to pass carefully crafted arguments to the kernel, eventually passing a NULL or invalid drm_plane_state pointer to vop_plane_atomic_async_check().

PoC Sketch (for research purposes only)

// Pseudo-code: in practice, kernel memory access from userland is required
int drm_fd = open("/dev/dri/card", O_RDWR);
// Prepare fake plane state (omit actual structure for safety)
void *null_state = NULL;  // Would cause dereference
// Issue IOCTL or custom API that triggers vop_plane_atomic_async_check with null_state
ioctl(drm_fd, DRM_IOCTL_MODE_ATOMIC, null_state);

Note:
Real exploitation requires deep DRM knowledge and often bypassing protections. For most distributions, standard users lack necessary capabilities. On custom or embedded systems, the attack surface can be wider.

Local DoS: Triggerable kernel crash or soft lockup.

- Privilege escalation: Not directly, but could be a stepping stone if combined with infoleaks or related vulnerabilities.

Summary

CVE-2024-53129 shows how a small oversight—dereferencing a pointer before checking if it’s valid—can create a kernel stability risk. Even small mistakes in kernel code can have a big impact, especially in graphics drivers where powerful low-level hardware access is the norm.

For full details and upstream patch see:
🔗 Mainline commit

Have a patching routine, and keep an eye on seemingly minor bugs; they can represent the thin edge of the wedge in system security.

Timeline

Published on: 12/04/2024 15:15:12 UTC
Last modified on: 12/19/2024 09:39:55 UTC