CVE-2024-56616 - Memory Corruption in Linux Kernel DRM DP MST Sideband Handling

Overview:
A newly disclosed vulnerability (CVE-2024-56616) was found and patched in the Linux kernel's DisplayPort Multi-Stream Transport (MST) subsystem. It concerns improper length checking in the DisplayPort sideband message handling logic. With a bad message sent by hardware or attacker-controlled devices, memory beyond the intended buffer could be accessed, potentially leading to system instability or malicious code execution. This post explains what happened, how it was fixed, and what it means for Linux users.

DRM (Direct Rendering Manager) is the Linux subsystem controlling graphics processing.

- DP MST (DisplayPort Multi-Stream Transport) allows a single cable to connect multiple displays in a daisy-chain.

Devices (“branches”) and the kernel talk using “sideband messages.” These are small packets of data, each consisting of a header and a body. The kernel must check the integrity and boundaries of these packets.

The Bug Explained

In the vulnerable code, the kernel received a message where the sideband body length was set to zero—not valid, but the header CRC (integrity check) passed. This confused the logic in drm_dp_sideband_append_payload() and allowed array indices to go out of range, writing way outside the intended message buffer.

Here’s a dmesg error trace from a real system hitting the bug

UBSAN: array-index-out-of-bounds in drivers/gpu/drm/display/drm_dp_mst_topology.c:786:25
index -1 is out of range for type 'u8 [48]'
Call Trace:
 drm_dp_sideband_append_payload+x33d/x350 [drm_display_helper]
 drm_dp_get_one_sb_msg+x3ce/x5f [drm_display_helper]
 drm_dp_mst_hpd_irq_handle_event+xc8/x158 [drm_display_helper]
memcpy: detected field-spanning write (size 18446744073709551615) of single field "&msg->msg[msg->curlen]" at drivers/gpu/drm/display/drm_dp_mst_topology.c:791 (size 256)
Call Trace:
 drm_dp_sideband_append_payload+x324/x350 [drm_display_helper]
 drm_dp_get_one_sb_msg+x3ce/x5f [drm_display_helper]
 drm_dp_mst_hpd_irq_handle_event+xc8/x158 [drm_display_helper]

Notice the huge value: 18446744073709551615 — this means an unsigned value of -1 used as a size, writing far outside the array.

Here’s a simplified snippet to illustrate the issue

// Before the fix, missing proper body length validation
if (header_crc_ok) {
    // body_len could be !
    // This is bad: later code assumes at least 1 byte valid
}

The sideband message must always have a body of at least 1 byte: a single CRC at minimum. If the body length is zero, something is wrong.

The patch adds a check

// Right before processing the message:
if (body_len < 1) {
    // Error: not a valid sideband message body
    return -EINVAL;
}

This ensures code never tries to process a zero-length (or negative) packet, which prevents accessing/writing off the end of the buffer.

How to Exploit (Conceptually)

An attacker who can control DisplayPort MST device communication could return deliberately malformed sideband messages with a header CRC passing but body_len = .

You’re fuzzing a system with a logic analyzer or programmable bridge

By sending a -length body, the Linux kernel would process the message and hit the out-of-bounds write—leading to memory corruption.

Creating a fake MST device requires hardware or a custom FPGA/USB-C board, but conceptually

# Pseudocode for MST sideband attack simulation
msg_header = make_valid_header(body_length=, correct_crc=True)
msg_body = b''

send_over_dp(msg_header + msg_body)

When this message triggers the bug, kernel memory corruption (and possibly a crash or remote code execution via DMA) could follow.

References and Further Reading

- CVE-2024-56616 entry at CVE.org
- Linux kernel mailing list patch discussion
- Git commit fixing the issue (kernel.org)
- drm_dp_mst_topology.c source

Conclusion

CVE-2024-56616 is a classic example of why strict boundary-checking is essential, especially when talking to devices over hardware buses. Thanks to the Linux DRM team for quick identification and remediation. This bug serves as both a cautionary tale and a reminder to keep your systems up to date.

*Stay safe, and keep your graphics stack patched!*

Timeline

Published on: 12/27/2024 15:15:21 UTC
Last modified on: 05/04/2025 09:59:57 UTC