Improper Update of Reference Count Affecting kernel-rt-kvm package, versions *

Note: Versions mentioned in the description apply only to the upstream kernel-rt-kvm package and not the kernel-rt-kvm package as distributed by Centos. See How to fix? for Centos:10 relevant fixed versions and status.

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.

• https://bugzilla.redhat.com/show_bug.cgi?id=2516711
• https://access.redhat.com/security/cve/CVE-2026-72315
• https://git.kernel.org/stable/c/75f5c412fa867efa0bf9b646bffe0d912109e84a
• https://git.kernel.org/stable/c/f0eac9c3c3711f24efc2aaf12b8ec3e54a38c214