[PATCH net v3 1/2] net/sched: sch_hfsc: bound the classify inner-filter walk with a drift budget

[Jamal Hadi Salim <jhs@mojatatu.com>]

hfsc_classify() applies the "filter may only point downwards" level check
only when the filter result carries no bound class. A filter created with
a flowid gets res.class set once at bind time, so the check never runs for
it during classification. hfsc_adjust_levels() can later raise a class's
level without revalidating existing bindings, leaving two binds that were
each legal at bind time pointing at each other; the classify walk then
bounces between two interior classes forever with the qdisc lock held and
BH disabled — a soft lockup from a single packet. The stuck walk trips
the watchdog:

  watchdog: BUG: soft lockup - CPU#3 stuck for 13s! [ping:444]
  RIP: 0010:u32_classify+0x542/0x17f0
  ...
  tcf_classify+0x66/0xa0
  hfsc_enqueue+0x166/0xdf0

Bound the traversal with a budget of non-descending hops, the only way a
configured walk can move without descending the class tree once levels
drift after bind time. The budget is cumulative over the whole walk and
is deliberately not reset on a descending hop: a chain that alternates a
descent with a lateral hop would return the budget every lap and never
trip. Descending hops never decrement it, so legitimately deep trees are
unaffected and a terminating lateral chain still classifies normally.
Drop the packet with a rate-limited warning once the budget is exhausted,
mirroring the merged HTB fix.

This is a follow-up to commit 729c4896ab82 ("net/sched: sch_htb: limit
htb_classify inner-class filter hops"), which bounded the same classify
loop on the HTB side but left the HFSC walk unbounded.

Conditions to recreate the bug:
- CONFIG_NET_SCHED, CONFIG_NET_SCH_HFSC, CONFIG_NET_CLS_U32,
  CONFIG_LOCKUP_DETECTOR.
- Build a cycle with two legal-at-bind-time flowid binds and a level
  drift: class X 1:1 (child of root) with leaf child 1:10; class Y 1:2
  (sibling of X) with children 1:20 and 1:200; root u32 filter flowid
  1:1; filter on X flowid 1:2 (legal when Y is a leaf); after Y's level
  rises to 2, filter on Y flowid 1:1 (legal then). Send one packet (ping
  on the device). Unfixed kernel: classify spins with the qdisc lock
  held; with softlockup_panic=1 it panics.
- Reachable from unprivileged user via unshare -Urn (CAP_NET_ADMIN).

Fixes: a2f79227138c ("net_sched: sch_hfsc: fix classification loops")
Reported-by: Sashiko (gemini + nipa) <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/netdev/QDISC-CTUU.v2.20260913192614@mojatatu.com/
Link: https://sashiko.dev/#/patchset/QDISC-CTUU.v2.20260913192614@mojatatu.com
Link: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/QDISC-CTUU.v2.20260913192614%40mojatatu.com
Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Tested-by: hybris <hybris@mojatatu.ai>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
v3:
   -  Reword the budget comment and the warning to describe the
      cumulative non-descending hop budget that is implemented, and
      explain why it is deliberately not reset on a descending hop.
      (Sashiko: gemini, nipa)
v2:
   -  Replace the absolute hop bound (root->level) with a budget that
      counts only non-descending hops, so a legal lateral chain still
      classifies. (Sashiko)
   -  Reword the "strict descent" changelog claim (Sashiko).
---
 net/sched/sch_hfsc.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/net/sched/sch_hfsc.c b/net/sched/sch_hfsc.c
index e87f5021a199..284490fd6ca9 100644
--- a/net/sched/sch_hfsc.c
+++ b/net/sched/sch_hfsc.c
@@ -386,6 +386,15 @@ cftree_update(struct hfsc_class *cl)
 #define	SM_MASK		((1ULL << SM_SHIFT) - 1)
 #define	ISM_MASK	((1ULL << ISM_SHIFT) - 1)

+/*
+ * Cap on the non-descending hops a classify walk may take before its
+ * filter chain is treated as misconfigured. A flowid binding that was
+ * legal at bind time can become lateral once hfsc_adjust_levels()
+ * raises a class level; a few such hops are legitimate, an unbounded
+ * run means the chain cycles.
+ */
+#define	HFSC_CLASSIFY_MAX_DRIFT	8
+
 static inline u64
 seg_x2y(u64 x, u64 sm)
 {
@@ -1133,6 +1142,7 @@ hfsc_classify(struct sk_buff *skb, struct Qdisc *sch, int *qerr)
 	struct hfsc_class *head, *cl;
 	struct tcf_result res;
 	struct tcf_proto *tcf;
+	unsigned int drift;
 	int result;

 	if (TC_H_MAJ(skb->priority ^ sch->handle) == 0 &&
@@ -1142,6 +1152,7 @@ hfsc_classify(struct sk_buff *skb, struct Qdisc *sch, int *qerr)

 	*qerr = NET_XMIT_SUCCESS | __NET_XMIT_BYPASS;
 	head = &q->root;
+	drift = HFSC_CLASSIFY_MAX_DRIFT;
 	tcf = rcu_dereference_bh(q->root.filter_list);
 	while (tcf && (result = tcf_classify_qdisc(skb, tcf, &res, false)) >= 0) {
 #ifdef CONFIG_NET_CLS_ACT
@@ -1167,6 +1178,17 @@ hfsc_classify(struct sk_buff *skb, struct Qdisc *sch, int *qerr)
 		if (cl->level == 0)
 			return cl; /* hit leaf class */

+		/*
+		 * flowid binds skip the level check above (res.class is set
+		 * at bind time and levels drift after), so a walk can follow
+		 * lateral hops without descending; a bounded number of them
+		 * is legal, more means the chain cycles.
+		 */
+		if (cl->level >= head->level && drift-- == 0) {
+			pr_warn_ratelimited("hfsc: classify hop budget exhausted, dropping packet\n");
+			return NULL;
+		}
+
 		/* apply inner filter chain */
 		tcf = rcu_dereference_bh(cl->filter_list);
 		head = cl;
--
2.43.0