cifs: Fix use-after-free in rdata->read_into_pages()

cifs: Fix use-after-free in rdata->read_into_pages()

When the network status is unstable, use-after-free may occur when
read data from the server.

  BUG: KASAN: use-after-free in readpages_fill_pages+0x14c/0x7e0

  Call Trace:
   <TASK>
   dump_stack_lvl+0x38/0x4c
   print_report+0x16f/0x4a6
   kasan_report+0xb7/0x130
   readpages_fill_pages+0x14c/0x7e0
   cifs_readv_receive+0x46d/0xa40
   cifs_demultiplex_thread+0x121c/0x1490
   kthread+0x16b/0x1a0
   ret_from_fork+0x2c/0x50
   </TASK>

  Allocated by task 2535:
   kasan_save_stack+0x22/0x50
   kasan_set_track+0x25/0x30
   __kasan_kmalloc+0x82/0x90
   cifs_readdata_direct_alloc+0x2c/0x110
   cifs_readdata_alloc+0x2d/0x60
   cifs_readahead+0x393/0xfe0
   read_pages+0x12f/0x470
   page_cache_ra_unbounded+0x1b1/0x240
   filemap_get_pages+0x1c8/0x9a0
   filemap_read+0x1c0/0x540
   cifs_strict_readv+0x21b/0x240
   vfs_read+0x395/0x4b0
   ksys_read+0xb8/0x150
   do_syscall_64+0x3f/0x90
   entry_SYSCALL_64_after_hwframe+0x72/0xdc

  Freed by task 79:
   kasan_save_stack+0x22/0x50
   kasan_set_track+0x25/0x30
   kasan_save_free_info+0x2e/0x50
   __kasan_slab_free+0x10e/0x1a0
   __kmem_cache_free+0x7a/0x1a0
   cifs_readdata_release+0x49/0x60
   process_one_work+0x46c/0x760
   worker_thread+0x2a4/0x6f0
   kthread+0x16b/0x1a0
   ret_from_fork+0x2c/0x50

  Last potentially related work creation:
   kasan_save_stack+0x22/0x50
   __kasan_record_aux_stack+0x95/0xb0
   insert_work+0x2b/0x130
   __queue_work+0x1fe/0x660
   queue_work_on+0x4b/0x60
   smb2_readv_callback+0x396/0x800
   cifs_abort_connection+0x474/0x6a0
   cifs_reconnect+0x5cb/0xa50
   cifs_readv_from_socket.cold+0x22/0x6c
   cifs_read_page_from_socket+0xc1/0x100
   readpages_fill_pages.cold+0x2f/0x46
   cifs_readv_receive+0x46d/0xa40
   cifs_demultiplex_thread+0x121c/0x1490
   kthread+0x16b/0x1a0
   ret_from_fork+0x2c/0x50

The following function calls will cause UAF of the rdata pointer.

readpages_fill_pages
 cifs_read_page_from_socket
  cifs_readv_from_socket
   cifs_reconnect
    __cifs_reconnect
     cifs_abort_connection
      mid->callback() --> smb2_readv_callback
       queue_work(&rdata->work)  # if the worker completes first,
                                 # the rdata is freed
          cifs_readv_complete
            kref_put
              cifs_readdata_release
                kfree(rdata)
 return rdata->...               # UAF in readpages_fill_pages()

Similarly, this problem also occurs in the uncache_fill_pages().

Fix this by adjusts the order of condition judgment in the return
statement.

Signed-off-by: ZhaoLong Wang <wangzhaolong1@huawei.com>
Cc: stable@vger.kernel.org
Acked-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>