netdev - [PATCH net 2/2] net/sched: sch_hfsc: upgrade 'rt' to 'sc' when it becomes a inner curve

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Message-Id: <20231013151057.2611860-3-pctammela@mojatatu.com>
Date: Fri, 13 Oct 2023 12:10:57 -0300
From: Pedro Tammela <pctammela@...atatu.com>
To: netdev@...r.kernel.org
Cc: jhs@...atatu.com,
	xiyou.wangcong@...il.com,
	jiri@...nulli.us,
	davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	Pedro Tammela <pctammela@...atatu.com>,
	Christian Theune <ct@...ingcircus.io>,
	Budimir Markovic <markovicbudimir@...il.com>
Subject: [PATCH net 2/2] net/sched: sch_hfsc: upgrade 'rt' to 'sc' when it becomes a inner curve

Christian Theune says:
   I upgraded from 6.1.38 to 6.1.55 this morning and it broke my traffic shaping script,
   leaving me with a non-functional uplink on a remote router.

A 'rt' curve cannot be used as a inner curve (parent class), but we were
allowing such configurations since the qdisc was introduced. Such
configurations would trigger a UAF as Budimir explains:
   The parent will have vttree_insert() called on it in init_vf(),
   but will not have vttree_remove() called on it in update_vf()
   because it does not have the HFSC_FSC flag set.

The qdisc always assumes that inner classes have the HFSC_FSC flag set.
This is by design as it doesn't make sense 'qdisc wise' for an 'rt'
curve to be an inner curve.

Budimir's original patch disallows users to add classes with a 'rt'
parent, but this is too strict as it breaks users that have been using
'rt' as a inner class. Another approach, taken by this patch, is to
upgrade the inner 'rt' into a 'sc', warning the user in the process.
It avoids the UAF reported by Budimir while also being more permissive
to bad scripts/users/code using 'rt' as a inner class.

Users checking the `tc class ls [...]` or `tc class get [...]` dumps would
observe the curve change and are potentially breaking with this change.

Cc: Christian Theune <ct@...ingcircus.io>
Cc: Budimir Markovic <markovicbudimir@...il.com>
Fixes: 0c9570eeed69 ("net/sched: sch_hfsc: upgrade 'rt' to 'sc' when it becomes a inner curve")
Signed-off-by: Pedro Tammela <pctammela@...atatu.com>
---
 net/sched/sch_hfsc.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/net/sched/sch_hfsc.c b/net/sched/sch_hfsc.c
index 98805303218d..880c5f16b29c 100644
--- a/net/sched/sch_hfsc.c
+++ b/net/sched/sch_hfsc.c
@@ -902,6 +902,14 @@ hfsc_change_usc(struct hfsc_class *cl, struct tc_service_curve *usc,
 	cl->cl_flags |= HFSC_USC;
 }

+static void
+hfsc_upgrade_rt(struct hfsc_class *cl)
+{
+	cl->cl_fsc = cl->cl_rsc;
+	rtsc_init(&cl->cl_virtual, &cl->cl_fsc, cl->cl_vt, cl->cl_total);
+	cl->cl_flags |= HFSC_FSC;
+}
+
 static const struct nla_policy hfsc_policy[TCA_HFSC_MAX + 1] = {
 	[TCA_HFSC_RSC]	= { .len = sizeof(struct tc_service_curve) },
 	[TCA_HFSC_FSC]	= { .len = sizeof(struct tc_service_curve) },
@@ -1061,6 +1069,12 @@ hfsc_change_class(struct Qdisc *sch, u32 classid, u32 parentid,
 	cl->cf_tree = RB_ROOT;

 	sch_tree_lock(sch);
+	/* Check if the inner class is a misconfigured 'rt' */
+	if (!(parent->cl_flags & HFSC_FSC) && parent != &q->root) {
+		NL_SET_ERR_MSG(extack,
+			       "Forced curve change on parent 'rt' to 'sc'");
+		hfsc_upgrade_rt(parent);
+	}
 	qdisc_class_hash_insert(&q->clhash, &cl->cl_common);
 	list_add_tail(&cl->siblings, &parent->children);
 	if (parent->level == 0)
-- 
2.39.2