loop: access lo_backing_file only when the loop device is Lo_bound
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@syzkaller.appspotmail.com
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 1e6edd5..bf1c61c 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -656,7 +656,7 @@ static int loop_validate_file(struct file *file, struct block_device *bdev)
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;