f2fs: fix UAF caused by decrementing sbi->nr_pages[] in f2fs_write_end_io()

The xfstests case "generic/107" and syzbot have both reported a NULL
pointer dereference.

The concurrent scenario that triggers the panic is as follows:

F2FS_WB_CP_DATA write callback          umount
                                        - f2fs_write_checkpoint
                                         - f2fs_wait_on_all_pages(sbi, F2FS_WB_CP_DATA)
- blk_mq_end_request
 - bio_endio
  - f2fs_write_end_io
   : dec_page_count(sbi, F2FS_WB_CP_DATA)
   : wake_up(&sbi->cp_wait)
                                        - kill_f2fs_super
                                         - kill_block_super
                                          - f2fs_put_super
                                           : iput(sbi->node_inode)
                                           : sbi->node_inode = NULL
   : f2fs_in_warm_node_list
    - is_node_folio // sbi->node_inode is NULL and panic

The root cause is that f2fs_put_super() calls iput(sbi->node_inode) and
sets sbi->node_inode to NULL after sbi->nr_pages[F2FS_WB_CP_DATA] is
decremented to zero. As a result, f2fs_in_warm_node_list() may
dereference a NULL node_inode when checking whether a folio belongs to
the node inode, leading to a panic.

This patch fixes the issue by calling f2fs_in_warm_node_list() before
decrementing sbi->nr_pages[F2FS_WB_CP_DATA], thus preventing the
use-after-free condition.

Cc: stable@kernel.org
Fixes: 50fa53eccf9f ("f2fs: fix to avoid broken of dnode block list")
Reported-by: syzbot+6e4cb1cac5efc96ea0ca@syzkaller.appspotmail.com
Signed-off-by: Yongpeng Yang <yangyongpeng@xiaomi.com>
Reviewed-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 400f0400e13d3ee0e4e625661d059817913f9102..57fc9bad31bf658e05098df0692cc52c4070f27b 100644 (file)
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -386,6 +386,8 @@ static void f2fs_write_end_io(struct bio *bio)
                                folio->index, NODE_TYPE_REGULAR, true);
                        f2fs_bug_on(sbi, folio->index != nid_of_node(folio));
                }
+               if (f2fs_in_warm_node_list(sbi, folio))
+                       f2fs_del_fsync_node_entry(sbi, folio);
 
                dec_page_count(sbi, type);
 
@@ -397,8 +399,6 @@ static void f2fs_write_end_io(struct bio *bio)
                                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);
        }