In the Linux kernel, the following vulnerability has been resolved:

cifs: fix handlecache and multiuser

In multiuser each individual user has their own tcon structure for the
share and thus their own handle for a cached directory.
When we umount such a share we much make sure to release the pinned down dentry
for each such tcon and not just the master tcon.

Otherwise we will get nasty warnings on umount that dentries are still in use:
[ 3459.590047] BUG: Dentry 00000000115c6f41{i=12000000019d95,n=/}  still in use\
 (2) [unmount of cifs cifs]
...
[ 3459.590492] Call Trace:
[ 3459.590500]  d_walk+0x61/0x2a0
[ 3459.590518]  ? shrink_lock_dentry.part.0+0xe0/0xe0
[ 3459.590526]  shrink_dcache_for_umount+0x49/0x110
[ 3459.590535]  generic_shutdown_super+0x1a/0x110
[ 3459.590542]  kill_anon_super+0x14/0x30
[ 3459.590549]  cifs_kill_sb+0xf5/0x104 [cifs]
[ 3459.590773]  deactivate_locked_super+0x36/0xa0
[ 3459.590782]  cleanup_mnt+0x131/0x190
[ 3459.590789]  task_work_run+0x5c/0x90
[ 3459.590798]  exit_to_user_mode_loop+0x151/0x160
[ 3459.590809]  exit_to_user_mode_prepare+0x83/0xd0
[ 3459.590818]  syscall_exit_to_user_mode+0x12/0x30
[ 3459.590828]  do_syscall_64+0x48/0x90
[ 3459.590833]  entry_SYSCALL_64_after_hwframe+0x44/0xae

https://git.kernel.org/stable/c/2fafbc198613823943c106d1ec9b516da692059f

https://git.kernel.org/stable/c/47178c7722ac528ea08aa82c3ef9ffa178962d7a

https://git.kernel.org/stable/c/e71f6a3a1d5c826a30384fec587ca7503ef6bde5

https://git.kernel.org/stable/c/ffa631c4bff59dde59b598011f570e27dfba3515