CVE-2026-72315 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
smb: client: fix busy dentry warning on unmount after DIO
Commit c68337442f03 ("cifs: Fix busy dentry used after unmounting") fixed the issue in cifs where deferred close of a file led to a dentry reference count not being released in umount, by flushing deferredclose_wq in cifs_kill_sb() to solve it.
However, the cifs DIO path suffers from the same busy-dentry problem caused by a delayed dentry reference-count release:
[dio] [cifsd] [close + umount]
netfs_unbuffered_write_iter_locked ... cifs_demultiplex_thread netfs_unbuffered_write cifs_issue_write netfs_wait_for_in_progress_stream [1]
... netfs_write_subrequest_terminated netfs_subreq_clear_in_progress netfs_wake_collector // wake [1]
netfs_put_subrequest netfs_put_request queue_work(system_dfl_wq, xxx) [2]
// dio write return cifs_close _cifsFileInfo_put // cfile->count 2->1 --cfile->count [3]
// umount cifs_kill_sb kill_anon_super // warning triggered! shrink_dcache_for_umount [4]
[system_dfl_wq] [5]
netfs_free_request ... _cifsFileInfo_put // cfile->count 1->0 --cfile->count queue_work(fileinfo_put_wq, xxx)
[fileinfo_put_wq] [6]
cifsFileInfo_put_work cifsFileInfo_put_final dput
If the umount path is triggered before [5], it results warning:
BUG: Dentry 00000000eab1f070{i=9a917b66ae404fec,n=test} still in use (1)
[unmount of cifs cifs]
The existing per-inode ictx->io_count wait in cifs_evict_inode() does not help: it lives in the inode eviction path, which runs after shrink_dcache_for_umount() has already warned about the busy dentries.
Fix it by adding a per-superblock outstanding-rreq counter that is incremented in cifs_init_request() and decremented in cifs_free_request(). In cifs_kill_sb(), before kill_anon_super(), wait for this counter to reach 0 - which guarantees that all cleanup_work for this sb have run and thus all relevant cfile puts are queued on fileinfo_put_wq or serverclose_wq. Then drain the workqueue so the dentry refs are dropped.
This is a targeted wait, not a flush of the system-wide system_dfl_wq.
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability described involves a race condition in the Linux kernel's CIFS (Common Internet File System) implementation that leads to busy dentry warnings during unmount operations. This issue specifically affects the direct I/O (DIO) path where deferred close operations create a scenario wherein dentry reference counts are not properly released before the file system is unmounted. The problem manifests when the cifs_kill_sb() function is called, which triggers shrink_dcache_for_umount(), resulting in warnings about dentries still being in use. This represents a classic synchronization issue in kernel file systems where cleanup operations do not complete before resource deallocation occurs.
The technical flaw stems from the asynchronous nature of DIO write operations and their associated cleanup workqueues. When a DIO write completes, the system queues cleanup work through system_dfl_wq which may not finish processing before umount is initiated. This creates a window where cifsFileInfo_put_final() cannot execute dput() on dentries because the workqueue items are still pending or in progress. The existing mechanism of waiting for per-inode ictx->io_count in cifs_evict_inode() fails because inode eviction occurs after the shrink_dcache_for_umount() warning has already been triggered, making the synchronization ineffective.
The operational impact of this vulnerability includes system stability issues during CIFS unmount operations, particularly when DIO writes are active. Users may encounter kernel warnings that indicate resource leaks or improper cleanup sequences, potentially leading to file system corruption or data loss in edge cases where multiple concurrent operations occur. The issue affects systems using CIFS client implementations that support direct I/O paths, making it relevant to enterprise storage environments and network file systems where high-performance I/O is required.
The mitigation strategy involves implementing a per-superblock outstanding request counter that tracks active requests during the DIO write process. This counter is incremented in cifs_init_request() and decremented in cifs_free_request(), providing precise tracking of in-flight operations. During cifs_kill_sb(), before calling kill_anon_super(), the system waits for this counter to reach zero, ensuring all cleanup work has completed. This targeted approach prevents the race condition by guaranteeing that all relevant cfile put operations are properly queued on either fileinfo_put_wq or serverclose_wq before proceeding with umount operations. The fix avoids flushing system-wide workqueues and instead implements a focused synchronization mechanism aligned with CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization) and addresses ATT&CK technique T1489 (Service Stop) through proper resource management during file system operations.
This solution directly addresses the root cause by ensuring that all cleanup work completes before unmount proceeds, eliminating the warning condition while maintaining system performance. The approach maintains backward compatibility while providing robust synchronization between I/O completion and resource deallocation phases. The implementation follows kernel best practices for managing asynchronous operations and ensures proper reference counting through explicit waiting mechanisms rather than relying on implicit ordering or global workqueue flushing that could impact overall system responsiveness.