Commit 729c4896ab82 for kernel

commit 729c4896ab829169f95915d65edd530325910b37
Author: Jamal Hadi Salim <jhs@mojatatu.com>
Date:   Wed Aug 26 11:33:39 2026 -0300

    net/sched: sch_htb: limit htb_classify inner-class filter hops

    htb_classify() follows each filter-selected inner class by switching
    to cl->filter_list, but never bounds the number of hops. A filter on
    an inner class can point back to itself or to another inner class that
    points back, creating an infinite loop in the packet classification
    path with the qdisc lock held and BH disabled — a soft lockup / panic
    from a single packet.

    Bound the traversal with a hop counter and drop the packet with a
    rate-limited warning once the bound is exceeded. The counter is
    incremented at the point the inner filter chain is picked up, after the
    TC_ACT_* switch has consumed the classifier verdict, so a terminal
    TC_ACT_QUEUED/STOLEN/TRAP on the last permitted chain still sets *qerr
    to __NET_XMIT_STOLEN and the packet is not charged as a drop by this
    qdisc or its parent.

    The bound is TC_HTB_MAXDEPTH, taken from HTB's own parameters rather than
    from the qdisc hierarchy depth limit. Class levels run from 0 to
    TC_HTB_MAXDEPTH - 1, so a traversal that strictly descends in level can
    take at most TC_HTB_MAXDEPTH hops. That descent is what a sane
    configuration does, but it is assumed here rather than enforced:
    htb_find() resolves a classid against every class in the qdisc, so a
    filter may equally select a sibling or an ancestor. The normal
    root -> inner -> leaf path takes a single hop, so the bound does not
    affect legitimate classification.

    htb_classify() can now return NULL irrespective of CONFIG_NET_CLS_ACT,
    whereas previously every NULL return sat inside that ifdef. The NULL
    handler in htb_enqueue() therefore cannot stay conditional either, so
    drop the ifdef around it. This matches hfsc_enqueue(), which has always
    handled a NULL class unconditionally. Without it, a kernel built
    without actions would dereference a NULL class instead of dropping.

    Conditions to recreate the bug:
    - CONFIG_NET_SCHED, CONFIG_NET_SCH_HTB, CONFIG_NET_CLS_U32,
      CONFIG_LOCKUP_DETECTOR.
    - Create an HTB qdisc on a device (e.g. lo), add an inner class
      1:1 with a leaf child 1:10, install a root u32 filter selecting
      1:1, and an inner-class u32 filter on 1:1 also selecting 1:1.
    - Send one packet (ping). On the unfixed kernel the classify loop
      spins with the qdisc lock held; with softlockup_panic=1 it panics.
    - Reachable from unprivileged user via unshare -Urn (CAP_NET_ADMIN).

    Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
    Reported-by: Vega <vega@nebusec.ai>
    Co-developed-by: Victor Nogueira <victor@mojatatu.com>
    Signed-off-by: Victor Nogueira <victor@mojatatu.com>
    Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
    Reviewed-by: Eric Dumazet <edumazet@google.com>
    Link: https://patch.msgid.link/20260826143339.271935-1-victor@mojatatu.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/sched/sch_htb.c b/net/sched/sch_htb.c
index fdac0dc8f35a..1ba67b121de4 100644
--- a/net/sched/sch_htb.c
+++ b/net/sched/sch_htb.c
@@ -224,6 +224,7 @@ static struct htb_class *htb_classify(struct sk_buff *skb, struct Qdisc *sch,
 	struct htb_class *cl;
 	struct tcf_result res;
 	struct tcf_proto *tcf;
+	unsigned int hops = 0;
 	int result;

 	/* allow to select class by setting skb->priority to valid classid;
@@ -266,6 +267,10 @@ static struct htb_class *htb_classify(struct sk_buff *skb, struct Qdisc *sch,
 		if (!cl->level)
 			return cl;	/* we hit leaf; return it */

+		if (++hops > TC_HTB_MAXDEPTH) {
+			pr_warn_ratelimited("htb: classify loop detected, dropping packet\n");
+			return NULL;
+		}
 		/* we have got inner class; apply inner filter chain */
 		tcf = rcu_dereference_bh(cl->filter_list);
 	}
@@ -633,13 +638,11 @@ static int htb_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		} else {
 			return qdisc_drop(skb, sch, to_free);
 		}
-#ifdef CONFIG_NET_CLS_ACT
 	} else if (!cl) {
 		if (ret & __NET_XMIT_BYPASS)
 			qdisc_qstats_drop(sch);
 		__qdisc_drop(skb, to_free);
 		return ret;
-#endif
 	} else if ((ret = qdisc_enqueue(skb, cl->leaf.q,
 					to_free)) != NET_XMIT_SUCCESS) {
 		if (net_xmit_drop_count(ret)) {