CVE-2026-72315 smb: client: fix busy dentry warning on unmount after DIO
A race condition in the Linux kernel SMB client can trigger a kernel warning and potential instability during unmount after direct I/O operations.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-72315?
CVE-2026-72315 is a high-severity denial of service vulnerability in Linux, published on 15 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
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.
Which versions are affected by CVE-2026-72315?
How do I fix CVE-2026-72315?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-72315 being exploited?
There is no known exploitation of CVE-2026-72315 in the wild and no public exploit code.
Sources
Also known as AZL-96039, BELL-CVE-2026-72315, DEBIAN-CVE-2026-72315, GHSA-3544-2m6j-7m5r, UBUNTU-CVE-2026-72315.