CVE-2026-68173: ublk: wait on ublk_dev_ready() instead of ub->completion

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

ublk: wait on ublkdevready() instead of ub->completion

ub->completion is only re-armed by a successful STARTUSERRECOVERY. If the ublk server sends ENDUSERRECOVERY without one - e.g. its START failed with -EBUSY and the error was ignored - the wait is satisfied by the stale completion of the previous recovery cycle, and the device is marked LIVE and the requeue list kicked while the FETCH stream is still running and ubq->canceling is still set. The kick redispatches a previously requeued request, ublkqueuerqcommon() sees ->canceling and parks it again via ublkabortrq(), and after the last FETCH clears ->canceling nothing ever kicks the requeue list again: the request is stranded there while holding its tag. If it is the flush machinery's flushrq, every subsequent fsync piles up in uninterruptible sleep and teardown hangs on tag draining. This matches a report of a lost PREFLUSH with ext4 on top of ublk after daemon crash recovery.

ub->completion is an edge-triggered latch used as a proxy for the level condition "every queue has fetched all I/O commands", which can regress (FBATCH's UNPREP, daemon death) and whose re-arm can be skipped. Drop it and wait on the real condition instead: the new helper ublkwaitdevreadyandlock() waits on ublkdevready() via waitvareventinterruptible(), woken from ublkmarkioready(), then re-checks it under ub->mutex, waiting again on regression, and returns with the mutex held and readiness guaranteed.

Readiness becomes true in the same ub->mutex critical section that clears the last queue's ->canceling, so ENDUSERRECOVERY marks the device LIVE and kicks the requeue list strictly after ->canceling clears. The wait stays interruptible, so a server whose daemon died can still be signalled out. For ublkctrlstartdev() this replaces the fail-fast -EINVAL on an FBATCH ready->UNPREP regression with waiting until the device is ready again.