In the Linux kernel, the following vulnerability has been resolved:
smb: During unmount, ensure all cached dir instances drop their dentry
The unmount process (cifs_kill_sb() calling close_all_cached_dirs()) can race with various cached directory operations, which ultimately results in dentries not being dropped and these kernel BUGs:
BUG: Dentry ffff88814f37e358{i=1000000000080,n=/} still in use (2) [unmount of cifs cifs] VFS: Busy inodes after unmount of cifs (cifs) ------------[ cut here ]------------ kernel BUG at fs/super.c:661!
This happens when a cfid is in the process of being cleaned up when, and has been removed from the cfids->entries list, including:
- Receiving a lease break from the server
- Server reconnection triggers invalidate_all_cached_dirs(), which
- The laundromat thread decides to expire an old cfid.
To solve these problems, dropping the dentry is done in queued work done in a newly-added cfid_put_wq workqueue, and close_all_cached_dirs() flushes that workqueue after it drops all the dentries of which it's aware. This is a global workqueue (rather than scoped to a mount), but the queued work is minimal.
The final cleanup work for cleaning up a cfid is performed via work queued in the serverclose_wq workqueue; this is done separate from dropping the dentries so that close_all_cached_dirs() doesn't block on any server operations.
Both of these queued works expect to invoked with a cfid reference and a tcon reference to avoid those objects from being freed while the work is ongoing.
While we're here, add proper locking to close_all_cached_dirs(), and locking around the freeing of cfid->dentry.