ext4: fix an use-after-free issue about data=journal writeback mode

hulk inclusion
category: bugfix
bugzilla: 185944, https://gitee.com/openeuler/kernel/issues/I4OP92


CVE: NA
---------------------------

Our syzkaller report an use-after-free issue that accessing the freed
buffer_head on the writeback page in __ext4_journalled_writepage(). The
problem is that if there was a truncate racing with the data=journalled
writeback procedure, the writeback length could become zero and
bget_one() refuse to get buffer_head's refcount, then the truncate
procedure release buffer once we drop page lock, finally, the last
ext4_walk_page_buffers() trigger the use-after-free problem.

sync                               truncate
ext4_sync_file()
 file_write_and_wait_range()
                                   ext4_setattr(0)
                                    inode->i_size = 0
  ext4_writepage()
   len = 0
   __ext4_journalled_writepage()
    page_bufs = page_buffers(page)
    ext4_walk_page_buffers(bget_one) <- does not get refcount
                                    do_invalidatepage()
                                      free_buffer_head()
    ext4_walk_page_buffers(page_bufs) <- trigger use-after-free

After commit bdf96838 ("ext4: fix race between truncate and
__ext4_journalled_writepage()"), we have already handled the racing
case, so the bget_one() and bput_one() are not needed. So this patch
simply remove these hunk, and recheck the i_size to make it safe.

Fixes: bdf96838 ("ext4: fix race between truncate and __ext4_journalled_writepage()")
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: yangerkun <yangerkun@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>