nilfs2: make block erasure safe in nilfs_finish_roll_forward()

The implementation of writing a zero-fill block in
nilfs_finish_roll_forward() is not safe.  The buffer is being cleared
without acquiring a lock or setting the uptodate flag, so theoretically,
between the time the buffer's data is cleared and the time it is written
back to the block device using sync_dirty_buffer(), that zero data can be
undone by concurrent block device reads.

Since this buffer points to a location that has been read from disk once,
the uptodate flag will most likely remain, but since it was obtained with
__getblk(), that is not guaranteed.  In other words, this is exceptional,
and this function itself is not normally called (only once when mounting
after a specific pattern of unclean shutdown), so it is highly unlikely
that this will actually cause a problem.

Anyway, eliminate this potential race issue by protecting the clearing of
buffer data with a buffer lock and setting the buffer's uptodate flag
within the protected section.

Link: https://lkml.kernel.org/r/20240511002942.9608-1-konishi.ryusuke@gmail.com
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/nilfs2/recovery.c

@@ -702,8 +702,12 @@ static void nilfs_finish_roll_forward(struct the_nilfs *nilfs,
 	if (WARN_ON(!bh))
 		return;  /* should never happen */
 
+	lock_buffer(bh);
 	memset(bh->b_data, 0, bh->b_size);
+	set_buffer_uptodate(bh);
 	set_buffer_dirty(bh);
+	unlock_buffer(bh);
+
 	err = sync_dirty_buffer(bh);
 	if (unlikely(err))
 		nilfs_warn(nilfs->ns_sb,