CVE-2022-49995

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

writeback: avoid use-after-free after removing device

When a disk is removed, bdiunregister gets called to stop further writeback and wait for associated delayed work to complete. However, wbinodewritebackend() may schedule bandwidth estimation dwork after this has completed, which can result in the timer attempting to access the just freed bdi_writeback.

Fix this by checking if the bdi_writeback is alive, similar to when scheduling writeback work.

Since this requires wb->worklock, and wbinodewritebackend() may get called from interrupt, switch wb->work_lock to an irqsafe lock.