f2fs: fix to avoid UAF in f2fs_write_end_io()

As syzbot reported an use-after-free issue in f2fs_write_end_io().

It is caused by below race condition:

loop device				umount
- worker_thread
 - loop_process_work
  - do_req_filebacked
   - lo_rw_aio
    - lo_rw_aio_complete
     - blk_mq_end_request
      - blk_update_request
       - f2fs_write_end_io
        - dec_page_count
        - folio_end_writeback
					- kill_f2fs_super
					 - kill_block_super
					  - f2fs_put_super
					 : free(sbi)
       : get_pages(, F2FS_WB_CP_DATA)
         accessed sbi which is freed

In kill_f2fs_super(), we will drop all page caches of f2fs inodes before
call free(sbi), it guarantee that all folios should end its writeback, so
it should be safe to access sbi before last folio_end_writeback().

Let's relocate ckpt thread wakeup flow before folio_end_writeback() to
resolve this issue.

Cc: stable@kernel.org
Fixes: e234088758fc ("f2fs: avoid wait if IO end up when do_checkpoint for better performance")
Reported-by: syzbot+b4444e3c972a7a124187@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=b4444e3c972a7a124187
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index 2e133a723b99dd68ac97bc7308be9c1682a772b1..f461f1318b4cc05e0f324e30c368d8c3e672dc51 100644 (file)
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -378,14 +378,20 @@ static void f2fs_write_end_io(struct bio *bio)
                                folio->index != nid_of_node(folio));
 
                dec_page_count(sbi, type);
+
+               /*
+                * we should access sbi before folio_end_writeback() to
+                * avoid racing w/ kill_f2fs_super()
+                */
+               if (type == F2FS_WB_CP_DATA && !get_pages(sbi, type) &&
+                               wq_has_sleeper(&sbi->cp_wait))
+                       wake_up(&sbi->cp_wait);
+
                if (f2fs_in_warm_node_list(sbi, folio))
                        f2fs_del_fsync_node_entry(sbi, folio);
                folio_clear_f2fs_gcing(folio);
                folio_end_writeback(folio);
        }
-       if (!get_pages(sbi, F2FS_WB_CP_DATA) &&
-                               wq_has_sleeper(&sbi->cp_wait))
-               wake_up(&sbi->cp_wait);
 
        bio_put(bio);
 }