CVE-2026-90155 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
ksmbd: detach blocked lock requests before freeing
A file_lock retained by ksmbd for byte-range lock bookkeeping can still be part of the VFS blocked-request graph. In particular, the VFS can chain a new waiter below an already blocked request through flc_blocked_requests. The ksmbd_file reference count does not cover that graph.
Both __ksmbd_close_fd() and the cross-request unlock path free these retained file_lock objects directly. If a dependent waiter is still attached, locks_release_private() hits BUG_ON(!list_empty(&flc->flc_blocked_requests)). The same lifetime mismatch can leave a freed ksmbd_lock reachable through its request-local llist.
Detach the file_lock from the blocked-request graph before freeing it in the close, cross-request unlock, and rollback paths. locks_delete_block() also wakes requests chained below the object. Remove llist when a completed lock is published so a globally visible ksmbd_lock no longer points into the submitting worker's stack.
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Analysis
by VulDB Data Team • 09/17/2026
The vulnerability identified within the Linux kernel component known as ksmbd stems from an improper handling of file lock lifecycles, specifically regarding byte-range locks managed by the server daemon. The core technical flaw lies in a mismatch between the reference counting mechanisms used by ksmbd and those enforced by the Virtual File System VFS layer. When ksmbd retains a file_lock object for bookkeeping purposes during its operational lifetime, it assumes exclusive control over that structure's memory management. However, the VFS maintains an independent graph of blocked requests through the flc_blocked_requests field, which allows new waiters to be chained below already blocked requests. This dependency creates a situation where the ksmbd_file reference count does not account for these additional references held by the VFS blocking chain. Consequently, when ksmbd proceeds to free or release these retained file_lock objects without first detaching them from the VFS graph, it violates the expected lifetime constraints of the underlying kernel structures.
The operational impact of this flaw is severe and manifests primarily as a kernel panic due to assertion failures within critical locking subsystems. Specifically, when both __ksmbd_close_fd() and the cross-request unlock path attempt to free these retained file_lock objects directly, they may leave dependent waiters still attached to the blocked request graph. This triggers a BUG_ON condition inside locks_release_private(), which explicitly checks that the flc_blocked_requests list is empty before proceeding with deallocation. If this check fails because the ksmbd code has freed the lock while it remains part of an active VFS chain, the kernel will panic, resulting in a denial of service for all users and services relying on the affected system. Furthermore, this lifetime mismatch can also leave a freed ksmbd_lock object reachable through its request-local linked list, potentially leading to use-after-free scenarios if those stale pointers are dereferenced by other parts of the subsystem or subsequent operations.
From a classification perspective, this vulnerability aligns with CWE-416 Use After Free and CWE-362 Concurrent Execution Using Shared Resource with Improper Synchronization, as it involves incorrect management of shared kernel data structures across different execution contexts and lifecycles. The attack vector typically requires an authenticated user to interact with the SMB server in a way that triggers lock contention or rapid file descriptor closure while locks are pending, allowing them to manipulate the timing of deallocation relative to VFS state updates. In terms of MITRE ATT&CK mapping, this falls under T1499 Endpoint Denial of Service and potentially T1053 Scheduled Task/Job if exploited in a manner that causes persistent system instability or requires specific scheduling conditions to trigger the race condition effectively.
To mitigate this vulnerability, it is imperative to ensure that all file_lock objects are properly detached from the VFS blocked-request graph prior to any deallocation attempts by ksmbd. This involves calling locks_delete_block() which not only removes the lock from the chain but also wakes up any requests chained below it, ensuring a clean state before memory release. Additionally, developers must remove linked lists when a completed lock is published so that globally visible ksmbd_lock structures no longer point into the stack space of submitting workers, thereby preventing access to freed or invalid memory regions. System administrators should apply kernel updates provided by their distribution vendors as soon as they become available, and until then, monitoring for unusual system crashes related to SMB operations can help in early detection of exploitation attempts targeting this specific race condition.