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

From: Jamal Hadi Salim <jhs@mojatatu.com>

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 bound is TC_HTB_MAXDEPTH, taken from HTB's own geometry 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
visit at most TC_HTB_MAXDEPTH classes. 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 consults two chains, 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@nebusec.ai
Co-developed-by: Victor Nogueira <redacted>
Signed-off-by: Victor Nogueira <redacted>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
 net/sched/sch_htb.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/sched/sch_htb.c b/net/sched/sch_htb.c
index fdac0dc8f35a..01bf7a564b15 100644
--- a/net/sched/sch_htb.c
+++ b/net/sched/sch_htb.c

@@ -225,6 +225,7 @@ static struct htb_class *htb_classify(struct sk_buff *skb, struct Qdisc *sch,
 	struct tcf_result res;
 	struct tcf_proto *tcf;
 	int result;
+	unsigned int hops = 0;
 
 	/* allow to select class by setting skb->priority to valid classid;
 	 * note that nfmark can be used too by attaching filter fw with no

@@ -244,6 +245,10 @@ static struct htb_class *htb_classify(struct sk_buff *skb, struct Qdisc *sch,
 
 	*qerr = NET_XMIT_SUCCESS | __NET_XMIT_BYPASS;
 	while (tcf && (result = tcf_classify_qdisc(skb, tcf, &res, false)) >= 0) {
+		if (++hops > TC_HTB_MAXDEPTH) {
+			pr_warn_ratelimited("htb: classify loop detected, dropping packet\n");
+			return NULL;
+		}
 #ifdef CONFIG_NET_CLS_ACT
 		switch (result) {
 		case TC_ACT_QUEUED:

@@ -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)) {

-- 
2.43.0