CVE-2026-89874 in Linux
Summary
by MITRE • 09/16/2026
In the Linux kernel, the following vulnerability has been resolved:
media: v4l2-async: avoid deleting unlinked ASC entry on link error
v4l2_async_match_notify() creates ancillary media links before adding asc->asc_subdev_entry to sd->asc_list.
If ancillary link creation fails, the function jumps to err_call_unbind while asc_subdev_entry has not been linked yet. Async connections are zero-allocated, so the list entry still has NULL next and prev pointers on this path. Calling list_del() on it can therefore dereference NULL instead of returning the original link creation error.
Do not delete asc_subdev_entry from err_call_unbind. There is no list insertion to undo on this path; the bound callback and sub-device registration are the operations that need to be rolled back.
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Analysis
by VulDB Data Team • 09/16/2026
The vulnerability identified in the Linux kernel's video for Linux two (v4l2) asynchronous subsystem represents a critical logic error within the media framework, specifically affecting how ancillary component connections are managed during device initialization and binding processes. This issue arises from an incorrect sequence of operations when establishing links between sub-devices. The core flaw lies in the v4l2_async_match_notify function, which is responsible for coordinating the association of different media components. In this routine, the code attempts to create ancillary media links before formally adding the asynchronous connection structure entry to the device's list of active connections. This ordering creates a race condition-like scenario where if the link creation step fails due to hardware constraints or configuration errors, the system enters an error handling path that incorrectly assumes the structural linkage has already occurred.
The technical root cause is a null pointer dereference triggered by improper cleanup logic. When ancillary link creation fails, the execution flow jumps to an error handler labeled err_call_unbind. At this specific point in the code execution, the asc_subdev_entry structure has not yet been inserted into the sd->asc_list because that insertion happens after successful link establishment. Since async connections are zero-allocated structures, their list pointers next and prev remain NULL if they have never been added to a linked list. The buggy implementation attempts to call list_del on this unlinked entry during error recovery. Because the entry is not part of any active list, calling list_del results in dereferencing these NULL pointers rather than performing a valid removal operation. This leads to a kernel panic or crash, effectively causing a denial of service for the media subsystem and potentially destabilizing the entire operating system depending on the context in which this code path is triggered.
From an operational impact perspective, this vulnerability allows local attackers with sufficient privileges to trigger a system crash by manipulating media device configurations that result in link creation failures during initialization. While it does not directly provide privilege escalation or remote code execution capabilities, its availability as a denial-of-service vector poses significant risks for systems relying on stable video processing pipelines, such as embedded devices, servers handling multimedia streams, or industrial control systems where continuous operation is critical. The instability introduced by this null pointer dereference can disrupt media capture and playback services, leading to service outages that may cascade into broader system unavailability if the kernel fails to recover gracefully from the exception.
This vulnerability aligns with CWE-476, which describes a NULL Pointer Dereference, as the primary technical flaw involves accessing memory through an uninitialized or null pointer due to incorrect state management. Furthermore, it relates to CWE-823, Use of Out-of-range Pointer Offset, in the broader sense that the code operates on data structures outside their expected valid bounds for list manipulation operations. In terms of attack patterns, this scenario can be mapped to ATT&CK technique T1059, Command and Scripting Interpreter, if an attacker uses scripting to repeatedly trigger the condition, or more accurately to T1499, Endpoint Denial of Service, as the ultimate goal is system instability rather than data exfiltration. The failure lies in not validating whether a resource has been successfully acquired before attempting to release it, a common pattern in systems programming errors that leads to inconsistent state transitions.
The resolution involves correcting the error handling logic within v4l2_async_match_notify by ensuring that list deletion operations are only performed on entries that have actually been inserted into the linked list. The fix explicitly avoids deleting asc_subdev_entry from the err_call_unbind path because no list insertion occurred prior to the failure. Instead, the rollback process focuses solely on reversing the bound callback and sub-device registration steps, which were successfully executed before the link creation attempt failed. This ensures that resource cleanup is accurate and does not trigger invalid memory accesses. To mitigate this issue in environments where kernel updates may be delayed, administrators should monitor media subsystem logs for initialization failures involving async connections and ensure that hardware configurations are validated against supported profiles to minimize the likelihood of triggering the specific code path associated with link creation errors during device probing phases.