CVE-2026-89712 in Linux
Summary
by MITRE • 09/11/2026
In the Linux kernel, the following vulnerability has been resolved:
NFSD: restart ssc_expire_umount walk after dropping nfsd_ssc_lock
nfsd4_ssc_expire_umount() walks nn->nfsd_ssc_mount_list with list_for_each_entry_safe(ni, tmp, ...). For each expired entry it sets nsui_busy = true, drops nfsd_ssc_lock to run mntput() on the source vfsmount, then reacquires the lock to list_del + kfree the entry and continue iterating via the macro's saved tmp pointer.
The nsui_busy flag protects the current ni from concurrent nfsd4_ssc_setup_dul() finders during the lock-drop window, but it does not pin tmp. Another nfsd RPC thread that fails its source- server mount and reaches nfsd4_ssc_cancel_dul() will, during that same window, take nfsd_ssc_lock, list_del + kfree its own ssc_umount item, and release the lock. If that item is the saved tmp of the expire walk, the next iteration dereferences a freed nfsd4_ssc_umount_item.
Restart the walk from the head after the mntput() unlock window so no saved next pointer survives the lock-drop. The list is bounded by the number of active inter-server source mounts (typically small) and the expire delayed-work runs periodically rather than per-IO, so the restart is cheap.
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Analysis
by VulDB Data Team • 09/11/2026
The vulnerability identified in the Linux kernel's Network File System server component involves a use-after-free condition within the nfsd4_ssc_expire_umount function. This routine is responsible for managing expired stateful copy (SSC) mount operations by iterating through a list of pending unmount tasks. The core technical flaw arises from an unsafe concurrency pattern during lock management. Specifically, the code iterates over nn->nfsd_ssc_mount_list using the list_for_each_entry_safe macro, which relies on saving a pointer to the next element in a temporary variable named tmp before processing the current entry. When handling each expired entry, the function sets a busy flag and then drops the nfsd_ssc_lock to perform mntput operations on the source virtual file system mount structure. This lock drop is necessary to prevent deadlocks but creates a critical window of vulnerability where shared data structures are unprotected.
During this unlocked interval, another concurrent NFS daemon RPC thread may encounter a failure in its own source server mount operation and invoke nfsd4_ssc_cancel_dul. This function acquires the same nfsd_ssc_lock, removes an ssc_umount item from the list via list_del, frees it with kfree, and then releases the lock. If this concurrently executed cancellation targets the specific entry stored in the tmp pointer of the expiring walk, the saved next pointer becomes a dangling reference to freed memory. When the original iteration resumes after reacquiring the lock, it attempts to dereference this invalid pointer for the subsequent list traversal step. This results in a use-after-free vulnerability that can lead to kernel crashes, data corruption, or potentially arbitrary code execution if an attacker can influence the allocation patterns to exploit the corrupted memory state.
The operational impact of this flaw is significant as it affects the stability and security of NFS server deployments handling inter-server copy operations. The race condition depends on precise timing between expiration processing and cancellation events, which may occur under heavy load or during network instability causing mount failures. Exploitation could allow a local user with access to NFS services or potentially remote attackers triggering specific error conditions to destabilize the kernel service. This aligns with CWE-416, Use After Free, where memory is accessed after it has been freed due to improper handling of pointers across lock boundaries. The vulnerability highlights the complexity of managing shared data structures in concurrent environments and the risks associated with dropping locks during critical list iterations without ensuring pointer validity.
Mitigation strategies primarily involve applying the kernel patch that addresses this race condition by restarting the walk from the head after releasing the lock for mntput operations. This approach ensures that no saved next pointers survive the unlock window, thereby eliminating the possibility of dereferencing freed memory. Since the list is bounded by the number of active inter-server source mounts and the expiration work runs periodically rather than per I/O operation, restarting the iteration incurs minimal performance overhead. Administrators should ensure their systems are updated with the latest kernel versions containing this fix to prevent potential exploitation. Additionally, reviewing concurrent lock management practices in other parts of the NFS server code can help identify similar patterns that might be susceptible to race conditions under different circumstances.