CVE-2026-89662 in Linux
Summary
by MITRE • 09/12/2026
In the Linux kernel, the following vulnerability has been resolved:
NFSD: Prevent lock owner use-after-free during client teardown
__destroy_client() releases a client's open owners, but a lock owner whose only reference is a blocked lock (nbl) stays on cl_ownerstr_hashtbl. client_has_state() does not count a bare owner, so DESTROY_CLIENTID can reach __destroy_client() with such owners present.
__destroy_client() then walks the table, calling remove_blocked_locks() on each owner without a reference. Freeing a blocked lock drops the owner reference held via flc_owner. The per-net laundromat reaps blocked locks from nn->blocked_locks_lru independently of client state. The two paths share blocked_locks_lock only for the list splice, not the owner's lifetime. The laundromat therefore frees the owner as __destroy_client() dereferences it, a NULL dereference in remove_blocked_locks().
nfsd4_release_lockowner() holds a reference across the same call; __destroy_client() does not. Hold cl_lock across the walk, taking a reference and unhashing each owner, then drop it before remove_blocked_locks() and nfs4_put_stateowner(), which take blocked_locks_lock and cl_lock.
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Analysis
by VulDB Data Team • 09/12/2026
The vulnerability identified in the Linux kernel involves a critical use-after-free condition within the Network File System daemon (NFSD) subsystem during client teardown operations. This flaw specifically affects how lock owners are managed when a NFSv4 client disconnects or is destroyed. The core issue arises from an inconsistency in reference counting and locking mechanisms between different code paths that handle blocked locks and owner lifecycle management. When a client session ends, the kernel invokes __destroy_client() to release resources associated with open owners. However, if there exists a lock owner whose only remaining reference is tied to a blocked lock, this owner remains present in the cl_ownerstr_hashtbl hash table despite being logically detached from active use. The function client_has_state(), which determines whether stateful operations are still pending for a client, fails to account for these bare owners that lack direct references other than their association with blocked locks. Consequently, the system proceeds to send a DESTROY_CLIENTID RPC message and triggers __destroy_client() even though such vulnerable owner structures are still present in memory.
The technical flaw manifests when __destroy_client() iterates through the hash table of lock owners. For each owner found without an active reference count, it attempts to call remove_blocked_locks(). This function interacts with the blocked locks list and ultimately leads to freeing resources associated with flc_owner. Simultaneously, a separate kernel mechanism known as the per-net laundromat operates independently on nn->blocked_locks_lru to reap and free expired or orphaned blocked locks. While both paths interact with the same underlying data structures, they only share the blocked_locks_lock mutex for list splicing operations, not for protecting the lifetime of the owner structure itself. This lack of synchronized protection allows a race condition where the laundromat may free the lock owner object at the exact moment __destroy_client() is attempting to dereference it during its cleanup walk. The result is a NULL pointer dereference within remove_blocked_locks(), leading to kernel panic or potential exploitation for privilege escalation if an attacker can control the memory layout and timing of these concurrent operations.
From a security taxonomy perspective, this vulnerability aligns with CWE-416: Use After Free, as it involves accessing memory after it has been freed due to improper synchronization between parallel execution paths. In terms of MITRE ATT&CK mapping, this falls under Tactic Execution (TA0002) and specifically Technique T1059 Command and Scripting Interpreter if exploited via local privilege escalation through kernel exploitation techniques like CVE-2023-something patterns often seen in NFS daemon bugs. The operational impact is severe because it compromises the stability of the Linux kernel on systems running NFS servers, potentially causing denial-of-service conditions or allowing unauthorized access to privileged memory regions depending on how the NULL dereference is handled by surrounding code and whether additional exploitation primitives exist nearby.
Mitigation strategies primarily involve applying vendor-provided kernel patches that address this specific race condition in NFSD implementations. Administrators should ensure their systems are updated with the latest stable kernels containing fixes for NFSv4 client teardown logic. Additionally, hardening measures such as enabling Kernel Self-Protection Project (KSP) features like KASLR and stack canaries may mitigate exploitation success rates by randomizing memory addresses and detecting corruption attempts early. For environments where NFS is not required, disabling the NFSD service reduces the attack surface significantly. Monitoring logs for kernel oops or panic messages related to nfsd components can also aid in rapid detection of attempted exploits before they cause widespread system instability.