writeback: avoid use-after-free after removing device

Vulnerability Description

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.

References

• https://git.kernel.org/stable/c/f96b9f7c1676923bce871e728bb49c0dfa5013cc
• https://git.kernel.org/stable/c/9a6c710f3bc10bc9cc23e1c080b53245b7f9d5b7
• https://git.kernel.org/stable/c/f87904c075515f3e1d8f4a7115869d3b914674fd