net_sched: hfsc: Fix a UAF vulnerability in class with netem as child qdisc

jira VULN-68295
cve CVE-2025-37890
commit-author Victor Nogueira <[email protected]>
commit 141d343

As described in Gerrard's report [1], we have a UAF case when an hfsc class
has a netem child qdisc. The crux of the issue is that hfsc is assuming
that checking for cl->qdisc->q.qlen == 0 guarantees that it hasn't inserted
the class in the vttree or eltree (which is not true for the netem
duplicate case).

This patch checks the n_active class variable to make sure that the code
won't insert the class in the vttree or eltree twice, catering for the
reentrant case.

[1] https://lore.kernel.org/netdev/CAHcdcOm+03OD2j6R0=YHKqmy=VgJ8xEOKuP6c7mSgnp-TEJJbw@mail.gmail.com/

Fixes: 37d9cf1 ("sched: Fix detection of empty queues in child qdiscs")
	Reported-by: Gerrard Tai <[email protected]>
	Acked-by: Jamal Hadi Salim <[email protected]>
	Signed-off-by: Victor Nogueira <[email protected]>
Link: https://patch.msgid.link/[email protected]
	Signed-off-by: Jakub Kicinski <[email protected]>
(cherry picked from commit 141d343)
	Signed-off-by: Marcin Wcisło <[email protected]>

Author: pvts-mat

net/sched/sch_hfsc.c

@@ -1568,7 +1568,7 @@ hfsc_enqueue(struct sk_buff *skb, struct Qdisc *sch, struct sk_buff **to_free)
 		return err;
 	}
 
-	if (first) {
+	if (first && !cl->cl_nactive) {
 		if (cl->cl_flags & HFSC_RSC)
 			init_ed(cl, len);
 		if (cl->cl_flags & HFSC_FSC)