drbd: fix potential kernel BUG (NULL deref)

BUG trace would look like:
 lc_find
 drbd_rs_complete_io
 got_OVResult
 drbd_asender

Could be triggered by explicit, or IO-error policy based,
detach during online-verify.

We may only dereference mdev->resync, if we first get_ldev(), as the
disk may break any time, causing mdev->resync to disappear once all
ldev references have been returned.
Already in flight online-verify requests or replies may still come in,
which we then need to ignore.
Signed-off-by: NPhilipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: NLars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_receiver.c

@@ -4241,10 +4241,13 @@ static int got_IsInSync(struct drbd_conf *mdev, struct p_header80 *h)
 
 	update_peer_seq(mdev, be32_to_cpu(p->seq_num));
 
+	if (get_ldev(mdev)) {
 		drbd_rs_complete_io(mdev, sector);
 		drbd_set_in_sync(mdev, sector, blksize);
 		/* rs_same_csums is supposed to count in units of BM_BLOCK_SIZE */
 		mdev->rs_same_csum += (blksize >> BM_BLOCK_SHIFT);
+		put_ldev(mdev);
+	}
 	dec_rs_pending(mdev);
 	atomic_add(blksize >> 9, &mdev->rs_sect_in);
 
@@ -4423,6 +4426,9 @@ static int got_OVResult(struct drbd_conf *mdev, struct p_header80 *h)
 	else
 		ov_oos_print(mdev);
 
+	if (!get_ldev(mdev))
+		return TRUE;
+
 	drbd_rs_complete_io(mdev, sector);
 	dec_rs_pending(mdev);
 
@@ -4437,6 +4443,7 @@ static int got_OVResult(struct drbd_conf *mdev, struct p_header80 *h)
 			drbd_resync_finished(mdev);
 		}
 	}
+	put_ldev(mdev);
 	return TRUE;
 }
 

drivers/block/drbd/drbd_worker.c

@@ -1027,7 +1027,10 @@ int w_e_end_csum_rs_req(struct drbd_conf *mdev, struct drbd_work *w, int cancel)
 		return 1;
 	}
 
+	if (get_ldev(mdev)) {
 		drbd_rs_complete_io(mdev, e->sector);
+		put_ldev(mdev);
+	}
 
 	di = e->digest;
 
@@ -1134,7 +1137,10 @@ int w_e_end_ov_reply(struct drbd_conf *mdev, struct drbd_work *w, int cancel)
 
 	/* after "cancel", because after drbd_disconnect/drbd_rs_cancel_all
 	 * the resync lru has been cleaned up already */
+	if (get_ldev(mdev)) {
 		drbd_rs_complete_io(mdev, e->sector);
+		put_ldev(mdev);
+	}
 
 	di = e->digest;