[PATCH 4.19 38/45] loop: access lo_backing_file only when the loop device is Lo_bound

[Greg Kroah-Hartman]

4.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Dongli Zhang <dongli.zhang@xxxxxxxxxx>

commit f7c8a4120eedf24c36090b7542b179ff7a649219 upstream.

Commit 758a58d0bc67 ("loop: set GENHD_FL_NO_PART_SCAN after
blkdev_reread_part()") separates "lo->lo_backing_file = NULL" and
"lo->lo_state = Lo_unbound" into different critical regions protected by
loop_ctl_mutex.

However, there is below race that the NULL lo->lo_backing_file would be
accessed when the backend of a loop is another loop device, e.g., loop0's
backend is a file, while loop1's backend is loop0.

loop0's backend is file            loop1's backend is loop0

__loop_clr_fd()
  mutex_lock(&loop_ctl_mutex);
  lo->lo_backing_file = NULL; --> set to NULL
  mutex_unlock(&loop_ctl_mutex);
                                   loop_set_fd()
                                     mutex_lock_killable(&loop_ctl_mutex);
                                     loop_validate_file()
                                       f = l->lo_backing_file; --> NULL
                                         access if loop0 is not Lo_unbound
  mutex_lock(&loop_ctl_mutex);
  lo->lo_state = Lo_unbound;
  mutex_unlock(&loop_ctl_mutex);

lo->lo_backing_file should be accessed only when the loop device is
Lo_bound.

In fact, the problem has been introduced already in commit 7ccd0791d985
("loop: Push loop_ctl_mutex down into loop_clr_fd()") after which
loop_validate_file() could see devices in Lo_rundown state with which it
did not count. It was harmless at that point but still.

Fixes: 7ccd0791d985 ("loop: Push loop_ctl_mutex down into loop_clr_fd()")
Reported-by: syzbot+9bdc1adc1c55e7fe765b@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/block/loop.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -657,7 +657,7 @@ static int loop_validate_file(struct fil
 			return -EBADF;
 
 		l = f->f_mapping->host->i_bdev->bd_disk->private_data;
-		if (l->lo_state == Lo_unbound) {
+		if (l->lo_state != Lo_bound) {
 			return -EINVAL;
 		}
 		f = l->lo_backing_file;