[PATCH 4.14 27/28] block: fix use-after-free in disk_part_iter_next

[Greg Kroah-Hartman]

From: Ming Lei <ming....@redhat.com>

commit aebf5db917055b38f4945ed6d621d9f07a44ff30 upstream.

Make sure that bdgrab() is done on the 'block_device' instance before
referring to it for avoiding use-after-free.

Cc: <sta...@vger.kernel.org>
Reported-by: syzbot+825f0f9657d4e5280...@syzkaller.appspotmail.com
Signed-off-by: Ming Lei <ming....@redhat.com>
Reviewed-by: Christoph Hellwig <h...@lst.de>
Signed-off-by: Jens Axboe <ax...@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

---
 block/genhd.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

--- a/block/genhd.c
+++ b/block/genhd.c
@@ -208,14 +208,17 @@ struct hd_struct *disk_part_iter_next(st
                part = rcu_dereference(ptbl->part[piter->idx]);
                if (!part)
                        continue;
+               get_device(part_to_dev(part));
+               piter->part = part;
                if (!part_nr_sects_read(part) &&
                    !(piter->flags & DISK_PITER_INCL_EMPTY) &&
                    !(piter->flags & DISK_PITER_INCL_EMPTY_PART0 &&
-                     piter->idx == 0))
+                     piter->idx == 0)) {
+                       put_device(part_to_dev(part));
+                       piter->part = NULL;
                        continue;
+               }
 
-               get_device(part_to_dev(part));
-               piter->part = part;
                piter->idx += inc;
                break;
        }