[PATCH net 1/2] net/sched: hhf: cap hh_flows_limit at change time

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

hhf_change() stores TCA_HHF_HH_FLOWS_LIMIT with no upper bound. A huge
hh_flows_limit lets each new heavy-hitter flow pass the
hh_flows_current_cnt check in alloc_new_hh() and forces a fixed-size
kzalloc(GFP_ATOMIC) per flow under spoofed traffic, for unbounded memory
growth.

Bound the attribute with NLA_POLICY_MAX() at 2*HH_FLOWS_CNT (the
hhf_init() default) and report the rejected value via extack. The
deprecated nested parse is kept: legacy tc does not set NLA_F_NESTED on
TCA_OPTIONS. Configs relying on hh_limit above the default were relying
on unbounded, unsafe behaviour and are not supported going forward.

hhf_init() also ran hhf_change() before setting the default
hh_flows_limit, so a user-supplied hh_limit at add time was clobbered
back to 2048. Set the default before hhf_change() so the configured
value sticks.

This is a follow-up to commit eb56a495f59b ("net/sched: hhf: clamp
quantum in change and init paths"), which bounded the quantum of the
same qdisc; the hh_flows_limit bound is the remaining unbounded knob of
that series' scope.

Conditions to recreate the bug: CAP_NET_ADMIN in a user namespace;
tc qdisc change dev X root hhf hh_limit 4294967295 succeeds and the
value is echoed by tc qdisc show, unbounding heavy-hitter flow
allocations; also tc qdisc add dev X root hhf hh_limit 500 stores 2048
instead of 500.

Fixes: 10239edf86f1 ("net-qdisc-hhf: Heavy-Hitter Filter (HHF) qdisc")
Cc: stable@vger.kernel.org
Reported-by: Sashiko (gemini) <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260822195509.112717-1-jhs@mojatatu.com
Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Tested-by: hybris <hybris@mojatatu.ai>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
 net/sched/sch_hhf.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/net/sched/sch_hhf.c b/net/sched/sch_hhf.c
index fc72f825fbd9..5dec1ed969ad 100644
--- a/net/sched/sch_hhf.c
+++ b/net/sched/sch_hhf.c
@@ -527,7 +527,7 @@ static void hhf_destroy(struct Qdisc *sch)
 static const struct nla_policy hhf_policy[TCA_HHF_MAX + 1] = {
 	[TCA_HHF_BACKLOG_LIMIT]	 = { .type = NLA_U32 },
 	[TCA_HHF_QUANTUM]	 = { .type = NLA_U32 },
-	[TCA_HHF_HH_FLOWS_LIMIT] = { .type = NLA_U32 },
+	[TCA_HHF_HH_FLOWS_LIMIT] = NLA_POLICY_MAX(NLA_U32, 2 * HH_FLOWS_CNT),
 	[TCA_HHF_RESET_TIMEOUT]	 = { .type = NLA_U32 },
 	[TCA_HHF_ADMIT_BYTES]	 = { .type = NLA_U32 },
 	[TCA_HHF_EVICT_TIMEOUT]	 = { .type = NLA_U32 },
@@ -546,7 +546,7 @@ static int hhf_change(struct Qdisc *sch, struct nlattr *opt,
 	u32 new_hhf_non_hh_weight = q->hhf_non_hh_weight;
 
 	err = nla_parse_nested_deprecated(tb, TCA_HHF_MAX, opt, hhf_policy,
-					  NULL);
+					  extack);
 	if (err < 0)
 		return err;
 
@@ -624,6 +624,9 @@ static int hhf_init(struct Qdisc *sch, struct nlattr *opt,
 	q->hhf_evict_timeout = HZ;      /* 1  sec */
 	q->hhf_non_hh_weight = 2;
 
+	/* Cap max active HHs at twice len of hh_flows table. */
+	q->hh_flows_limit = 2 * HH_FLOWS_CNT;
+
 	if (opt) {
 		int err = hhf_change(sch, opt, extack);
 
@@ -639,8 +642,6 @@ static int hhf_init(struct Qdisc *sch, struct nlattr *opt,
 		for (i = 0; i < HH_FLOWS_CNT; i++)
 			INIT_LIST_HEAD(&q->hh_flows[i]);
 
-		/* Cap max active HHs at twice len of hh_flows table. */
-		q->hh_flows_limit = 2 * HH_FLOWS_CNT;
 		q->hh_flows_overlimit = 0;
 		q->hh_flows_total_cnt = 0;
 		q->hh_flows_current_cnt = 0;
-- 
2.43.0