CVE-2026-98523 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
drm/client: fix restore of partially initialized client
I got a null-ptr-deref report when closing a DRM file descriptor:
WARNING: drivers/gpu/drm/drm_atomic.c:2031 at __drm_atomic_helper_set_config+0x18e/0x1b0 [drm]
Call Trace: drm_client_modeset_commit_atomic+0x16b/0x220 [drm]
drm_client_modeset_commit_locked+0x56/0x160 [drm]
drm_client_modeset_commit+0x21/0x40 [drm]
__drm_fb_helper_restore_fbdev_mode_unlocked.part.0+0x7b/0x80 drm_fbdev_client_restore+0xe/0x20 [drm_client_lib]
drm_client_dev_restore+0x9f/0xc0 [drm]
drm_release+0xc5/0xe0 [drm]
The warning is followed by a NULL pointer dereference:
BUG: kernel NULL pointer dereference, address: 0000000000000008
RIP: __drm_fb_helper_restore_fbdev_mode_unlocked.part.0+0x41/0x80 [drm_kms_helper]
Call Trace: drm_fbdev_client_restore+0xe/0x20 [drm_client_lib]
drm_client_dev_restore+0x9f/0xc0 [drm]
drm_release+0xc5/0xe0 [drm]
__fput+0xdc/0x2b0 __x64_sys_close+0x39/0x80 do_syscall_64+0x8d/0x460 entry_SYSCALL_64_after_hwframe+0x76/0x7e
drm_client_register() adds the DRM client to the device client list before invoking the initial hotplug callback. If the hotplug callback fails, the client remains registered.
For the fbdev client, a failure during drm_fb_helper_initial_config() causes the partially initialized fbdev helper to be cleaned up. drm_fb_helper_fini() releases fb_helper->info and leaves it NULL.
The fbdev client therefore remains registered even though there is no fully initialized framebuffer device.
Later, when userspace closes the DRM file descriptor, drm_release() can invoke the restore callbacks of registered DRM clients:
drm_release() drm_client_dev_restore() drm_fbdev_client_restore() drm_fb_helper_restore_fbdev_mode_unlocked()
drm_fbdev_client_restore() currently restores the fbdev state unconditionally. For a partially initialized fbdev client this can submit an incomplete modeset state and subsequently access fbdev state which has not been initialized, resulting in the warning and NULL pointer dereference above.
drm_fbdev_client_unregister() already uses fb_helper->info to distinguish a fully probed framebuffer device from a partially initialized client.
Use the same condition in drm_fbdev_client_restore() and skip restore if no framebuffer device has been successfully initialized.
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Analysis
by VulDB Data Team • 10/18/2026
The Linux kernel contains a vulnerability within the Direct Rendering Manager subsystem that results in a null pointer dereference during file descriptor closure, specifically affecting systems utilizing fbdev emulation. This issue arises from an inconsistency in how partially initialized DRM clients are handled during cleanup and restoration phases. When a user space application closes a DRM file descriptor, the kernel invokes release callbacks which may trigger state restoration routines for registered client devices. In this specific scenario, the vulnerability manifests as a critical null pointer dereference at address 0x8 within the function __drm_fb_helper_restore_fbdev_mode_unlocked, leading to a kernel panic or system instability depending on execution context and memory layout.
The root cause lies in the lifecycle management of DRM clients during initialization. The drm_client_register function adds a client to the device's internal list before invoking its initial hotplug callback. If this hotplug callback fails, such as when drm_fb_helper_initial_config encounters an error for the fbdev helper, the client remains registered despite not being fully initialized. During failure handling, drm_fb_helper_fini is called which releases resources including the framebuffer info structure and sets it to NULL. Consequently, a partially initialized state persists in memory while the client object remains active within the global registry of DRM clients.
When userspace subsequently closes the associated file descriptor, the kernel executes drm_release which calls into drm_client_dev_restore. This function iterates through registered clients and invokes their respective restore callbacks without verifying whether the client was successfully probed or initialized. For fbdev clients, this leads to a call to drm_fbdev_client_restore which unconditionally attempts to access members of the framebuffer helper structure that were cleared during the failed initialization phase. Accessing these uninitialized fields results in the observed null pointer dereference because the info field is NULL rather than pointing to valid memory allocated for display mode settings.
This vulnerability aligns with CWE-476, Null Pointer Dereference, as it involves accessing a memory location through a pointer that has not been properly initialized or has been explicitly set to zero due to error handling paths. From an offensive security perspective, this behavior can be mapped to MITRE ATT&CK technique T1059, Command and Scripting Interpreter, if exploited for denial of service against the kernel subsystem, although it primarily represents a stability issue rather than direct code execution unless specific heap grooming conditions are met. The flaw highlights a lack of state validation before executing restoration logic on dynamically allocated driver structures.
The resolution involves modifying drm_fbdev_client_restore to check whether the framebuffer info structure is valid before attempting any restore operations. By reusing the existing condition found in drm_fbdev_client_unregister, which checks if fb_helper->info is non-NULL, the kernel can correctly distinguish between fully probed devices and those that failed initialization. This ensures that restoration logic is skipped for partially initialized clients, preventing access to freed or null memory regions. Developers should apply this patch immediately to prevent potential denial of service attacks triggered by closing DRM file descriptors on affected systems. System administrators monitoring logs for WARNING messages in drm_atomic.c related to set_config operations are advised to verify kernel versions and update accordingly to mitigate this risk.