CVE-2024-40916 in Linux
Summary
by MITRE • 07/12/2024
In the Linux kernel, the following vulnerability has been resolved:
drm/exynos: hdmi: report safe 640x480 mode as a fallback when no EDID found
When reading EDID fails and driver reports no modes available, the DRM core adds an artificial 1024x786 mode to the connector. Unfortunately some variants of the Exynos HDMI (like the one in Exynos4 SoCs) are not able to drive such mode, so report a safe 640x480 mode instead of nothing in case of the EDID reading failure.
This fixes the following issue observed on Trats2 board since commit 13d5b040363c ("drm/exynos: do not return negative values from .get_modes()"):
[drm] Exynos DRM: using 11c00000.fimd device for DMA mapping operations
exynos-drm exynos-drm: bound 11c00000.fimd (ops fimd_component_ops) exynos-drm exynos-drm: bound 12c10000.mixer (ops mixer_component_ops) exynos-dsi 11c80000.dsi: [drm:samsung_dsim_host_attach] Attached s6e8aa0 device (lanes:4 bpp:24 mode-flags:0x10b)
exynos-drm exynos-drm: bound 11c80000.dsi (ops exynos_dsi_component_ops) exynos-drm exynos-drm: bound 12d00000.hdmi (ops hdmi_component_ops) [drm] Initialized exynos 1.1.0 20180330 for exynos-drm on minor 1
exynos-hdmi 12d00000.hdmi: [drm:hdmiphy_enable.part.0] *ERROR* PLL could not reach steady state
panel-samsung-s6e8aa0 11c80000.dsi.0: ID: 0xa2, 0x20, 0x8c exynos-mixer 12c10000.mixer: timeout waiting for VSYNC ------------[ cut here ]------------
WARNING: CPU: 1 PID: 11 at drivers/gpu/drm/drm_atomic_helper.c:1682 drm_atomic_helper_wait_for_vblanks.part.0+0x2b0/0x2b8 [CRTC:70:crtc-1] vblank wait timed out
Modules linked in: CPU: 1 PID: 11 Comm: kworker/u16:0 Not tainted 6.9.0-rc5-next-20240424 #14913 Hardware name: Samsung Exynos (Flattened Device Tree) Workqueue: events_unbound deferred_probe_work_func Call trace: unwind_backtrace from show_stack+0x10/0x14 show_stack from dump_stack_lvl+0x68/0x88 dump_stack_lvl from __warn+0x7c/0x1c4 __warn from warn_slowpath_fmt+0x11c/0x1a8 warn_slowpath_fmt from drm_atomic_helper_wait_for_vblanks.part.0+0x2b0/0x2b8 drm_atomic_helper_wait_for_vblanks.part.0 from drm_atomic_helper_commit_tail_rpm+0x7c/0x8c drm_atomic_helper_commit_tail_rpm from commit_tail+0x9c/0x184 commit_tail from drm_atomic_helper_commit+0x168/0x190 drm_atomic_helper_commit from drm_atomic_commit+0xb4/0xe0 drm_atomic_commit from drm_client_modeset_commit_atomic+0x23c/0x27c drm_client_modeset_commit_atomic from drm_client_modeset_commit_locked+0x60/0x1cc drm_client_modeset_commit_locked from drm_client_modeset_commit+0x24/0x40 drm_client_modeset_commit from __drm_fb_helper_restore_fbdev_mode_unlocked+0x9c/0xc4 __drm_fb_helper_restore_fbdev_mode_unlocked from drm_fb_helper_set_par+0x2c/0x3c drm_fb_helper_set_par from fbcon_init+0x3d8/0x550 fbcon_init from visual_init+0xc0/0x108 visual_init from do_bind_con_driver+0x1b8/0x3a4 do_bind_con_driver from do_take_over_console+0x140/0x1ec do_take_over_console from do_fbcon_takeover+0x70/0xd0 do_fbcon_takeover from fbcon_fb_registered+0x19c/0x1ac fbcon_fb_registered from register_framebuffer+0x190/0x21c register_framebuffer from __drm_fb_helper_initial_config_and_unlock+0x350/0x574 __drm_fb_helper_initial_config_and_unlock from exynos_drm_fbdev_client_hotplug+0x6c/0xb0 exynos_drm_fbdev_client_hotplug from drm_client_register+0x58/0x94 drm_client_register from exynos_drm_bind+0x160/0x190 exynos_drm_bind from try_to_bring_up_aggregate_device+0x200/0x2d8 try_to_bring_up_aggregate_device from __component_add+0xb0/0x170 __component_add from mixer_probe+0x74/0xcc mixer_probe from platform_probe+0x5c/0xb8 platform_probe from really_probe+0xe0/0x3d8 really_probe from __driver_probe_device+0x9c/0x1e4 __driver_probe_device from driver_probe_device+0x30/0xc0 driver_probe_device from __device_attach_driver+0xa8/0x120 __device_attach_driver from bus_for_each_drv+0x80/0xcc bus_for_each_drv from __device_attach+0xac/0x1fc __device_attach from bus_probe_device+0x8c/0x90 bus_probe_device from deferred_probe_work_func+0 ---truncated---
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Analysis
by VulDB Data Team • 10/03/2025
The vulnerability described in CVE-2024-40916 affects the Linux kernel's display subsystem, specifically within the drm/exynos driver for HDMI functionality. This issue manifests when the system fails to read Extended Display Identification Data (EDID) from an HDMI display, which typically occurs when no display is connected or when communication fails. The problem arises from how the DRM core handles such failures, particularly on Exynos SoC platforms including the Exynos4 series. When EDID reading fails, the DRM core automatically adds a default 1024x786 mode to the connector, but certain Exynos HDMI implementations are unable to drive this resolution, leading to system instability and display failures.
The technical flaw stems from an insufficient fallback mechanism in the HDMI driver logic. When EDID data cannot be retrieved, the system should provide a safe, compatible resolution that the hardware can actually support. Instead, it defaults to a resolution that may exceed the capabilities of the specific HDMI controller variant. This mismatch causes hardware errors such as PLL (Phase-Locked Loop) instability and timeout conditions during VSYNC synchronization, as evidenced by the kernel log showing "PLL could not reach steady state" and "timeout waiting for VSYNC". These errors occur because the hardware cannot handle the artificial 1024x786 mode that the DRM core attempts to configure, resulting in system instability during display initialization.
The operational impact of this vulnerability is significant for embedded systems running on Exynos platforms, particularly those using the Trats2 board as referenced in the issue. The failure to properly handle EDID reading errors leads to complete display initialization failures, preventing normal system operation and potentially causing system hangs or crashes. The kernel logs show the system attempting to configure the display but failing with hardware-level timeouts and warnings, indicating that the driver cannot properly manage the fallback scenario. This vulnerability affects the fundamental display subsystem and impacts any application or user interaction that depends on proper display initialization, making it a critical issue for embedded Linux systems.
The fix implemented addresses this by modifying the HDMI driver to report a safe 640x480 mode as a fallback instead of leaving the connector with no valid modes when EDID reading fails. This approach aligns with industry best practices for robust error handling in display drivers and follows the principle of providing minimal, hardware-compatible configurations when higher-level information is unavailable. The solution prevents the system from attempting to configure unsupported resolutions and ensures that basic display functionality remains available even in the absence of proper EDID data. This change resolves the immediate hardware compatibility issues and provides a stable fallback that allows the system to continue operating while maintaining display capabilities.
This vulnerability demonstrates a classic example of inadequate fallback handling in embedded systems, where the system fails to gracefully degrade to a safe state when primary functionality is unavailable. It relates to CWE-248, which covers "Uncaught Exception" in software systems, and aligns with ATT&CK technique T1490, "Inhibit System Recovery", as the vulnerability can cause system instability and prevent normal operation. The fix represents a proper defensive programming approach that ensures system resilience and proper error recovery, which is essential for embedded systems where hardware limitations and communication failures are common occurrences.