drbd: validate resync_after dependency on attach already

We validated resync_after dependencies, if changed via disk-options.
But we did not validate them when first created via attach.
We also did not check or cleanup dependencies that used to be correct,
but now point to meanwhile removed minor devices.

If the drbd_resync_after_valid() validation in disk-options tried to
follow a dependency chain in this way, this could lead to NULL pointer
dereference.

Validate resync_after settings in drbd_adm_attach() already, as well as
in drbd_adm_disk_opts(), and and only reject dependency loops.
Depending on non-existing disks is allowed and equivalent to no dependency.

Signed-off-by: Philipp Reisner <[e-mail]>
Signed-off-by: Lars Ellenberg <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/drbd/drbd_nl.c

···
 		goto fail;
 	}
 
+	write_lock_irq(&global_state_lock);
+	retcode = drbd_resync_after_valid(mdev, new_disk_conf->resync_after);
+	write_unlock_irq(&global_state_lock);
+	if (retcode != NO_ERROR)
+		goto fail;
+
 	rcu_read_lock();
 	nc = rcu_dereference(mdev->tconn->net_conf);
 	if (nc) {

drivers/block/drbd/drbd_worker.c

···
 	int resync_after;
 
 	while (1) {
-		if (!odev->ldev)
+		if (!odev->ldev || odev->state.disk == D_DISKLESS)
 			return 1;
 		rcu_read_lock();
 		resync_after = rcu_dereference(odev->ldev->disk_conf)->resync_after;
···
 		if (resync_after == -1)
 			return 1;
 		odev = minor_to_mdev(resync_after);
-		if (!expect(odev))
+		if (!odev)
 			return 1;
 		if ((odev->state.conn >= C_SYNC_SOURCE &&
 		     odev->state.conn <= C_PAUSED_SYNC_T) ||
···
 
 	if (o_minor == -1)
 		return NO_ERROR;
-	if (o_minor < -1 || minor_to_mdev(o_minor) == NULL)
+	if (o_minor < -1 || o_minor > MINORMASK)
 		return ERR_RESYNC_AFTER;
 
 	/* check for loops */
···
 	while (1) {
 		if (odev == mdev)
 			return ERR_RESYNC_AFTER_CYCLE;
+
+		/* You are free to depend on diskless, non-existing,
+		 * or not yet/no longer existing minors.
+		 * We only reject dependency loops.
+		 * We cannot follow the dependency chain beyond a detached or
+		 * missing minor.
+		 */
+		if (!odev || !odev->ldev || odev->state.disk == D_DISKLESS)
+			return NO_ERROR;
 
 		rcu_read_lock();
 		resync_after = rcu_dereference(odev->ldev->disk_conf)->resync_after;