CVE-2024-26696: nilfs2: fix hang in nilfs_lookup_dirty_data_buffers()

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

nilfs2: fix hang in nilfslookupdirtydatabuffers()

Syzbot reported a hang issue in migratepagesbatch() called by mbind() and nilfslookupdirtydatabuffers() called in the log writer of nilfs2.

While migratepagesbatch() locks a folio and waits for the writeback to complete, the log writer thread that should bring the writeback to completion picks up the folio being written back in nilfslookupdirtydatabuffers() that it calls for subsequent log creation and was trying to lock the folio. Thus causing a deadlock.

In the first place, it is unexpected that folios/pages in the middle of writeback will be updated and become dirty. Nilfs2 adds a checksum to verify the validity of the log being written and uses it for recovery at mount, so data changes during writeback are suppressed. Since this is broken, an unclean shutdown could potentially cause recovery to fail.

Investigation revealed that the root cause is that the wait for writeback completion in nilfspagemkwrite() is conditional, and if the backing device does not require stable writes, data may be modified without waiting.

Fix these issues by making nilfspagemkwrite() wait for writeback to finish regardless of the stable write requirement of the backing device.