CVE-2026-89660
Summary
by MITRE • 09/11/2026
In the Linux kernel, the following vulnerability has been resolved:
NFSD: Prevent client use-after-free during admin state revocation
A stateid holds only a bare pointer to its nfs4_client; a stateid reference does not pin it. The client survives only because __destroy_client() drains its stateids before free_client() runs.
nfsd4_revoke_states() drops nn->client_lock across revoke_one_stid(), which dereferences the client to revoke a stateid and read clp->cl_minorversion. A teardown racing the dropped lock can free the client first.
Pinning cl_rpc_users under client_lock blocks the DESTROY_CLIENTID and EXCHANGE_ID teardown, which refuses while cl_rpc_users is non-zero. force_expire_client() ignores it: once its wait for cl_rpc_users to reach zero has passed, a later pin goes unnoticed.
Under client_lock, skip a client whose cl_time is already zero -- force_expire_client() clears it there before waiting -- otherwise pin cl_rpc_users before dropping the lock. The walk then either sees the expiry and skips, or pins in time for that wait to cover the revoke.
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Analysis
by VulDB Data Team • 09/11/2026
The Linux kernel vulnerability identified as NFSD: Prevent client use-after-free during admin state revocation represents a critical race condition within the Network File System server implementation. This flaw arises from an improper handling of reference counting and locking mechanisms when managing NFSv4 client states, specifically during the administrative revocation of those states. The core issue stems from the design where a stateid contains only a bare pointer to its associated nfs4_client structure without holding a direct reference that would pin the client in memory. Consequently, the survival of the client object relies entirely on the guarantee that __destroy_client() will drain all active stateids before invoking free_client(). This assumption is violated when concurrent operations race against each other due to lock release intervals, leading to premature deallocation of the client structure while it is still being accessed by another thread.
The technical flaw manifests in nfsd4_revoke_states(), which drops nn->client_lock across calls to revoke_one_stid(). During this window, the code dereferences the client pointer to revoke a specific stateid and read clp->cl_minorversion. If a teardown process races with this operation and acquires the lock first or completes its cleanup sequence before the revocation logic finishes accessing the data, it can free the client object prematurely. This creates a classic use-after-free scenario where subsequent access to the now-freed memory results in undefined behavior, potentially leading to kernel crashes, privilege escalation, or information disclosure depending on how the freed memory is reused by other subsystems.
The root cause involves complex interactions between force_expire_client() and normal teardown paths like DESTROY_CLIENTID and EXCHANGE_ID. While pinning cl_rpc_users under client_lock was intended to block these teardown operations while an operation is in progress, force_expire_client() ignores this protection mechanism. Once its wait for cl_rpc_users to reach zero has passed, a later pin goes unnoticed because the expiry logic does not re-check the reference count after waking up from sleep. This allows a race condition where the client can be freed even though there are still active references that should have kept it alive, particularly when administrative state revocation is occurring simultaneously with client teardown procedures.
The operational impact of this vulnerability includes severe stability issues for NFS servers handling high volumes of concurrent connections or frequent administrative changes to client states. Attackers who can trigger rapid state revocations and teardowns in parallel may exploit the race condition to cause denial-of-service through kernel panics or potentially achieve arbitrary code execution if they can control the contents of the freed memory region. This is particularly dangerous in multi-tenant environments where untrusted users might influence NFS client behavior, making this a significant security risk for enterprise deployments relying on Linux-based NFS servers.
Mitigation strategies involve applying the upstream kernel patch that addresses the locking and reference counting logic. The fix ensures that under client_lock, any client whose cl_time is already zero is skipped immediately since force_expire_client() clears it there before waiting. For other clients, the solution requires pinning cl_rpc_users before dropping the lock to ensure that either the expiry check sees the correct state or the pin remains valid for the duration of the wait period in force_expire_client(). Administrators should update their Linux kernels to versions containing this fix and monitor NFS server logs for signs of instability related to client disconnections. Additionally, minimizing unnecessary administrative revocations during peak load periods can reduce the window of exposure until patches are fully deployed across all infrastructure components.
This vulnerability aligns with CWE-416 Use After Free, which describes situations where a program continues to use a pointer after it has been freed, leading to unpredictable behavior and potential security breaches. From an ATT&CK perspective, this flaw could be leveraged in the context of Defense Evasion or Privilege Escalation if exploited successfully by a local attacker with access to NFS services. The lack of proper synchronization between reference counting and lock management highlights the importance of rigorous concurrency control in kernel-level network service implementations.