CVE-2021-47108 is a vulnerability identified in the Linux kernel's Direct Rendering Manager (DRM) subsystem, specifically affecting the MediaTek HDMI driver. The issue arises in the function mtk_hdmi_bridge_mode_valid(), which is responsible for validating HDMI display modes. Recent commits introduced checks for CEA (Consumer Electronics Association) modes and limits on the maximal HDMI mode clock to address issues on certain MediaTek platforms such as MT8167. However, the MT8173 platform does not provide the mtk_hdmi_conf structure, which is expected by the function. As a result, when an HDMI cable is plugged in, the function attempts to dereference a NULL pointer (hdmi->conf), causing a kernel panic and system crash. This is a regression introduced by the new checks without proper NULL pointer validation. The fix involves adding a NULL pointer check for hdmi->conf in mtk_hdmi_bridge_mode_valid(), preventing the kernel panic and restoring HDMI functionality on affected devices. This vulnerability is specific to certain MediaTek platforms running Linux kernels containing the problematic commits. There are no known exploits in the wild, and no CVSS score has been assigned yet.

For European organizations using Linux-based systems on MediaTek hardware platforms such as MT8173, this vulnerability can lead to system instability and denial of service (DoS) conditions triggered by simply connecting an HDMI cable. This could affect devices in embedded systems, industrial control, or consumer electronics that rely on Linux with MediaTek chipsets. The kernel panic caused by the NULL pointer dereference results in a complete system crash, impacting availability. While confidentiality and integrity are not directly compromised, the disruption of service can affect operational continuity, especially in environments where uptime is critical. Organizations deploying Linux on MediaTek platforms in kiosks, digital signage, or IoT devices may experience unexpected reboots or failures, potentially leading to operational disruptions or increased maintenance costs. Given the specificity of the hardware and driver involved, the impact is limited to affected platforms but can be significant in those contexts.

Organizations should ensure that Linux kernel versions deployed on MediaTek MT8173 platforms include the patch that adds the NULL pointer check in mtk_hdmi_bridge_mode_valid(). This requires updating to a kernel version that contains the fix or applying the patch manually if using custom kernels. It is critical to verify the kernel source and commits to confirm the presence of the fix. Additionally, system integrators should test HDMI functionality after kernel updates to detect any regressions. For embedded or industrial devices, implementing watchdog timers can help recover from unexpected kernel panics. Monitoring kernel logs for HDMI-related errors can provide early warning signs. Where possible, restrict physical access to HDMI ports to prevent accidental triggering of the vulnerability. Finally, maintain close coordination with Linux kernel maintainers and MediaTek vendors for timely updates and advisories.