Re: [syzbot] [net?] general protection fault in qdisc_tree_reduce_backlog

[Hillf Danton <hdanton@sina.com>]

> > Date: Thu, 03 Jul 2025 22:20:37 -0700
> > syzbot found the following issue on:
> > 
> > HEAD commit:    bd475eeaaf3c Merge branch '200GbE' of git://git.kernel.org..
> > git tree:       net
> > console+strace: https://syzkaller.appspot.com/x/log.txt?x=12f0b3d4580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=36b0e72cad5298f8
> > dashboard link: https://syzkaller.appspot.com/bug?extid=1261670bbdefc5485a06
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=164d8c8c580000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14839ebc580000
> 
> Date: Fri, 4 Jul 2025 22:07:34 -0700 Cong Wang wrote:
> Maybe I didn't make it clear, I think Victor's patch also fixes this
> bug.
> 
> https://lore.kernel.org/netdev/20250704163422.160424-1-victor@mojatatu.com/
> 
> Can you check if you still see the crash with his fix?

#syz test

--- x/net/sched/sch_api.c
+++ y/net/sched/sch_api.c
@@ -336,17 +336,22 @@ out:
 	return q;
 }
 
-static struct Qdisc *qdisc_leaf(struct Qdisc *p, u32 classid)
+static struct Qdisc *qdisc_leaf(struct Qdisc *p, u32 classid,
+				struct netlink_ext_ack *extack)
 {
 	unsigned long cl;
 	const struct Qdisc_class_ops *cops = p->ops->cl_ops;
 
-	if (cops == NULL)
-		return NULL;
+	if (cops == NULL) {
+		NL_SET_ERR_MSG(extack, "Parent qdisc is not classful");
+		return ERR_PTR(-EOPNOTSUPP);
+	}
 	cl = cops->find(p, classid);
 
-	if (cl == 0)
-		return NULL;
+	if (cl == 0) {
+		NL_SET_ERR_MSG(extack, "Specified class not found");
+		return ERR_PTR(-ENOENT);
+	}
 	return cops->leaf(p, cl);
 }
 
@@ -1490,16 +1495,20 @@ static int __tc_get_qdisc(struct sk_buff
 					NL_SET_ERR_MSG(extack, "Failed to find qdisc with specified classid");
 					return -ENOENT;
 				}
-				q = qdisc_leaf(p, clid);
+				q = qdisc_leaf(p, clid, extack);
 			} else if (dev_ingress_queue(dev)) {
 				q = rtnl_dereference(dev_ingress_queue(dev)->qdisc_sleeping);
 			}
 		} else {
 			q = rtnl_dereference(dev->qdisc);
 		}
-		if (!q) {
-			NL_SET_ERR_MSG(extack, "Cannot find specified qdisc on specified device");
-			return -ENOENT;
+		if (IS_ERR_OR_NULL(q)) {
+			if (!q) {
+				NL_SET_ERR_MSG(extack,
+					       "Cannot find specified qdisc on specified device");
+				return -ENOENT;
+			}
+			return PTR_ERR(q);
 		}
 
 		if (tcm->tcm_handle && q->handle != tcm->tcm_handle) {
@@ -1602,7 +1611,9 @@ static int __tc_modify_qdisc(struct sk_b
 					NL_SET_ERR_MSG(extack, "Failed to find specified qdisc");
 					return -ENOENT;
 				}
-				q = qdisc_leaf(p, clid);
+				q = qdisc_leaf(p, clid, extack);
+				if (IS_ERR(q))
+					return PTR_ERR(q);
 			} else if (dev_ingress_queue_create(dev)) {
 				q = rtnl_dereference(dev_ingress_queue(dev)->qdisc_sleeping);
 			}
--