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In June 2024, security researchers identified and patched a dangerous vulnerability in the Linux kernel affecting certain Mediatek devices. Labeled CVE-2024-47753, this bug could allow a malicious userspace program to crash the kernel simply by triggering an incorrect call in the VP8 hardware video decoder. This post explains how CVE-2024-47753 works, why it happened, and how an attacker could have exploited it—using friendly language and original code snippets.

Background: The Linux Kernel & Mediatek VP8

The Linux kernel is the core of many devices, especially Android smartphones and embedded systems. Mediatek is a popular chip vendor for these devices, offering custom hardware video codecs to offload video decoding from the CPU.

The vp8 format is widely used for web videos. Mediatek’s vcodec drivers provide a special way for programs to ask the hardware to decode these files.

What Is CVE-2024-47753?

A static analysis tool called smatch found a problem in vdec_vp8_req_if.c, part of the Mediatek vcodec in the Linux kernel:

The code did not check for a NULL pointer before using the variable fb.

- If a program passed a job with fb == NULL, the kernel would dereference it—triggering a crash (kernel panic).

This is a local denial-of-service (DoS) bug.

This means any non-privileged userspace process interacting with the video decoder could immediately freeze or reboot the system.

Here’s a simplified example of the vulnerable code (before the fix)

// vdec_vp8_req_if.c
int vdec_vp8_decode(struct vdec_session *session, struct vdec_fb *fb)
{
    // ... other code ...
    do_something_with(fb->base);  // No NULL check on 'fb'!
    // ... other code ...
}

If fb is NULL, accessing fb->base will crash the kernel.

The fix adds a NULL pointer check before using fb. Here’s the patched code

// vdec_vp8_req_if.c
int vdec_vp8_decode(struct vdec_session *session, struct vdec_fb *fb)
{
    if (!fb)
        return -EINVAL;  // Safely return if fb is NULL

    do_something_with(fb->base);
    // ... other code ...
}

This fix ensures the function immediately returns instead of crashing the system.

Exploiting CVE-2024-47753 (for Demonstration)

*Note: This code is for educational purposes only. Do not use to attack systems!*

Attackers could write a program on a vulnerable device to communicate with the codec through /dev interfaces, sending a decode request with a NULL framebuffer pointer. Here’s a pseudocode outline:

int fd = open("/dev/videoX", O_RDWR);
// Prepare a decode request with fb set to NULL
struct vdec_fb *fb = NULL;
ioctl(fd, VIDIOC_CODEC_DECODE, &fb); // This triggers the crash!
close(fd);

On a machine with the vulnerable kernel, this causes a kernel panic.

Devices: Linux systems with Mediatek hardware using this vcodec driver

- Kernel Versions: Before the patch was merged (see kernel git commit)

Check your kernel version and security advisories from your vendor.

References

- Official Patch Commit
- Smatch Analyzer
- CVE Record on cve.org

Patch immediately: Update to a kernel with the fix.

- Restrict video device access: Only trusted apps should use /dev/video*.

Conclusion

CVE-2024-47753 is a good example of how a single unchecked pointer can bring down an entire system, even in sophisticated kernel drivers. For kernel developers, it’s a reminder to use static analysis tools and always check pointer variables. For users: update, update, update—these fixes keep your devices safe!


*Exclusive deep dive by [your name or publication]. Feel free to share or cite this research post. Stay secure!*

Timeline

Published on: 10/21/2024 13:15:05 UTC
Last modified on: 11/19/2024 01:09:31 UTC