CVE-2026-22989 in Linux
Summary
by MITRE • 01/23/2026
In the Linux kernel, the following vulnerability has been resolved:
nfsd: check that server is running in unlock_filesystem
If we are trying to unlock the filesystem via an administrative interface and nfsd isn't running, it crashes the server. This happens currently because nfsd4_revoke_states() access state structures (eg., conf_id_hashtbl) that has been freed as a part of the server shutdown.
[ 59.465072] Call trace:
[ 59.465308] nfsd4_revoke_states+0x1b4/0x898 [nfsd] (P)
[ 59.465830] write_unlock_fs+0x258/0x440 [nfsd]
[ 59.466278] nfsctl_transaction_write+0xb0/0x120 [nfsd]
[ 59.466780] vfs_write+0x1f0/0x938
[ 59.467088] ksys_write+0xfc/0x1f8
[ 59.467395] __arm64_sys_write+0x74/0xb8
[ 59.467746] invoke_syscall.constprop.0+0xdc/0x1e8
[ 59.468177] do_el0_svc+0x154/0x1d8
[ 59.468489] el0_svc+0x40/0xe0
[ 59.468767] el0t_64_sync_handler+0xa0/0xe8
[ 59.469138] el0t_64_sync+0x1ac/0x1b0
Ensure this can't happen by taking the nfsd_mutex and checking that the server is still up, and then holding the mutex across the call to nfsd4_revoke_states().
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Analysis
by VulDB Data Team • 05/18/2026
The vulnerability identified as CVE-2026-22989 represents a critical race condition and null pointer dereference issue within the Linux kernel's NFS daemon implementation. This flaw exists in the nfsd subsystem where administrative operations attempt to unlock filesystem resources while the NFS server is in the process of shutting down. The vulnerability stems from insufficient synchronization mechanisms that fail to verify the operational status of the NFS daemon before attempting critical state management operations. When an administrator attempts to unlock the filesystem through the administrative interface, the system crashes if nfsd is not actively running, creating a denial of service condition that can be exploited to disrupt NFS services.
The technical implementation of this vulnerability manifests in the nfsd4_revoke_states() function which attempts to access state structures such as conf_id_hashtbl that have already been freed during the server shutdown sequence. This occurs because the administrative interface does not properly validate whether the NFS server is still operational before proceeding with state cleanup operations. The call trace demonstrates the execution path leading to the crash, showing that the failure originates from the nfsd4_revoke_states function which is invoked through write_unlock_fs, ultimately triggered by nfsctl_transaction_write operations. This sequence reveals a fundamental flaw in the kernel's state management where cleanup operations are performed without proper checks for server operational status, resulting in access to freed memory structures.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise system stability and availability of network file services. An attacker or unauthorized user with access to administrative interfaces could exploit this condition to cause system crashes, leading to denial of service for legitimate NFS clients and potentially disrupting critical file sharing operations. The vulnerability particularly affects systems relying heavily on NFS for distributed file access, where the crash could result in extended downtime and data accessibility issues. From a cybersecurity perspective, this represents a privilege escalation vector that could be leveraged to create persistent denial of service conditions, impacting availability as defined by the CIA triad.
The mitigation strategy for this vulnerability involves implementing proper synchronization mechanisms using the nfsd_mutex to ensure thread safety during critical operations. The fix requires checking that the NFS server is still running before proceeding with state cleanup operations and maintaining the mutex lock across the entire nfsd4_revoke_states() call sequence. This approach aligns with established security practices for preventing race conditions and ensures proper resource management during system shutdown sequences. The solution addresses the underlying CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization) vulnerability pattern and follows ATT&CK technique T1499.004 (Endpoint Denial of Service) by preventing unauthorized or unintended system crashes through proper state management and synchronization protocols. The fix ensures that administrative operations cannot proceed when the underlying service is not properly initialized or is in transition, thereby preventing access to freed memory structures and maintaining system stability.