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net/sched: sch_hfsc: bound the classify inner-filter walk with a drift budget

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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 729c4896ab ("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: a2f7922713 ("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>
Link: https://patch.msgid.link/QDISC-CTUU.v3.20260916184908@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Jamal Hadi Salim 2026-09-17 06:57:32 -04:00 committed by Jakub Kicinski
parent ab888242fc
commit 8a60ade227
1 changed files with 22 additions and 0 deletions
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22
net/sched/sch_hfsc.c
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@ -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;