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, bdi_unregister gets called to stop further

writeback and wait for associated delayed work to complete. However,

wb_inode_writeback_end() 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->work_lock, and wb_inode_writeback_end() may get

called from interrupt, switch wb->work_lock to an irqsafe lock.