CVE-2026-53198: ksmbd: fix use-after-free of a deferred file_lock on double SMB2_CANCEL

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

ksmbd: fix use-after-free of a deferred filelock on double SMB2CANCEL

A deferred byte-range lock (an SMB2LOCK that blocks) registers an async work on conn->asyncrequests via setupasyncwork(), with cancelfn = smb2removeblockedlock and cancelargv[0] pointing at the struct filelock.

When the request is cancelled, the worker frees the filelock with locksfreelock() and takes the cancelled early-exit, which "goto out"s and never reaches releaseasyncwork() -- the only site that unlinks the work from conn->asyncrequests and clears cancelfn/cancelargv. The work therefore stays matchable on asyncrequests with a live cancelfn pointing at the freed filelock, until connection teardown finally runs releaseasyncwork().

smb2cancel() fires cancelfn unconditionally with no state guard, so a second SMB2CANCEL for the same AsyncId, arriving in that window, re-runs smb2removeblockedlock() on the freed filelock -- a slab use-after-free:

BUG: KASAN: slab-use-after-free in locksdeleteblock locksdeleteblock locksdeleteblock ksmbdvfsposixlockunblock smb2removeblockedlock smb2cancel <- 2nd SMB2CANCEL fires cancelfn handleksmbdwork Allocated by ...: locksalloclock <- smb2lock Freed by ...: locksfreelock <- smb2lock (cancelled branch) ... cache filelockcache of size 192

Reproduced on mainline with KASAN by an authenticated SMB client.

Skip a work whose state is already KSMBDWORKCANCELLED so its cancel callback cannot be fired a second time.