On 3/18/19 6:23 AM, Dongli Zhang wrote:
> 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.
Thanks, applied.
--
Jens Axboe
