net_sched: hfsc: Address reentrant enqueue adding class to eltree twice

Savino says:
"We are writing to report that this recent patch
(141d34391abbb315d68556b7c67ad97885407547) [1]
can be bypassed, and a UAF can still occur when HFSC is utilized with
NETEM.

The patch only checks the cl->cl_nactive field to determine whether
it is the first insertion or not [2], but this field is only
incremented by init_vf [3].

By using HFSC_RSC (which uses init_ed) [4], it is possible to bypass the
check and insert the class twice in the eltree.
Under normal conditions, this would lead to an infinite loop in
hfsc_dequeue for the reasons we already explained in this report [5].

However, if TBF is added as root qdisc and it is configured with a
very low rate,
it can be utilized to prevent packets from being dequeued.
This behavior can be exploited to perform subsequent insertions in the
HFSC eltree and cause a UAF."

To fix both the UAF and the infinite loop, with netem as an hfsc child,
check explicitly in hfsc_enqueue whether the class is already in the eltree
whenever the HFSC_RSC flag is set.

[1] https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=141d34391abbb315d68556b7c67ad97885407547
[2] https://elixir.bootlin.com/linux/v6.15-rc5/source/net/sched/sch_hfsc.c#L1572
[3] https://elixir.bootlin.com/linux/v6.15-rc5/source/net/sched/sch_hfsc.c#L677
[4] https://elixir.bootlin.com/linux/v6.15-rc5/source/net/sched/sch_hfsc.c#L1574
[5] https://lore.kernel.org/netdev/8DuRWwfqjoRDLDmBMlIfbrsZg9Gx50DHJc1ilxsEBNe2D6NMoigR_eIRIG0LOjMc3r10nUUZtArXx4oZBIdUfZQrwjcQhdinnMis_0G7VEk=@willsroot.io/T/#u

Fixes: 37d9cf1a3ce3 ("sched: Fix detection of empty queues in child qdiscs")
Reported-by: Savino Dicanosa <[e-mail]>
Reported-by: William Liu <[e-mail]>
Acked-by: Jamal Hadi Salim <[e-mail]>
Tested-by: Victor Nogueira <[e-mail]>
Signed-off-by: Pedro Tammela <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

net/sched/sch_hfsc.c

···
 
 #define	HT_INFINITY	0xffffffffffffffffULL	/* infinite time value */
 
+static bool cl_in_el_or_vttree(struct hfsc_class *cl)
+{
+	return ((cl->cl_flags & HFSC_FSC) && cl->cl_nactive) ||
+		((cl->cl_flags & HFSC_RSC) && !RB_EMPTY_NODE(&cl->el_node));
+}
 
 /*
  * eligible tree holds backlogged classes being sorted by their eligible times.
···
 	if (cl == NULL)
 		return -ENOBUFS;
 
+	RB_CLEAR_NODE(&cl->el_node);
+
 	err = tcf_block_get(&cl->block, &cl->filter_list, sch, extack);
 	if (err) {
 		kfree(cl);
···
 	sch->qstats.backlog += len;
 	sch->q.qlen++;
 
-	if (first && !cl->cl_nactive) {
+	if (first && !cl_in_el_or_vttree(cl)) {
 		if (cl->cl_flags & HFSC_RSC)
 			init_ed(cl, len);
 		if (cl->cl_flags & HFSC_FSC)