f2fs: fix data loss caused by incorrect use of nat_entry flag

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

f2fs: fix data loss caused by incorrect use of nat_entry flag

Data loss can occur when fsync is performed on a newly created file (before any checkpoint has been written) concurrently with a checkpoint operation. The scenario is as follows:

create & write & fsync 'file A' write checkpoint - f2fs_do_sync_file // inline inode - f2fs_write_inode // inode folio is dirty - f2fs_write_checkpoint - f2fs_flush_merged_writes - f2fs_sync_node_pages - f2fs_flush_nat_entries - f2fs_fsync_node_pages // no dirty node - f2fs_need_inode_block_update // return false SPO and lost 'file A'

f2fs_flush_nat_entries() sets the IS_CHECKPOINTED and HAS_LAST_FSYNC flags for the nat_entry, but this does not mean that the checkpoint has actually completed successfully. However, f2fs_need_inode_block_update() checks these flags and incorrectly assumes that the checkpoint has finished.

The root cause is that the semantics of IS_CHECKPOINTED and HAS_LAST_FSYNC are only guaranteed after the checkpoint write fully completes.

This patch modifies f2fs_need_inode_block_update() to acquire the sbi->node_write lock before reading the nat_entry flags, ensuring that once IS_CHECKPOINTED and HAS_LAST_FSYNC are observed to be set, the checkpoint operation has already completed.