CVE-2024-53176

Summary

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 removes all the cfids from the list
• 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.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/73934e535cffbda1490fa97d82690a0f9aa73e94
• https://git.kernel.org/stable/c/ff4528bbc82d0d90073751f7b49e7b9e9c7e5638
• https://git.kernel.org/stable/c/548812afd96982a76a93ba76c0582ea670c40d9e
• https://git.kernel.org/stable/c/3fa640d035e5ae526769615c35cb9ed4be6e3662

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