--- /dev/null
+From ac9fe7dd8e730a103ae4481147395cc73492d786 Mon Sep 17 00:00:00 2001
+From: Pedro Tammela <pctammela@mojatatu.com>
+Date: Thu, 22 May 2025 15:14:47 -0300
+Subject: net_sched: hfsc: Address reentrant enqueue adding class to eltree twice
+
+From: Pedro Tammela <pctammela@mojatatu.com>
+
+commit ac9fe7dd8e730a103ae4481147395cc73492d786 upstream.
+
+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 <savy@syst3mfailure.io>
+Reported-by: William Liu <will@willsroot.io>
+Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
+Tested-by: Victor Nogueira <victor@mojatatu.com>
+Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
+Link: https://patch.msgid.link/20250522181448.1439717-2-pctammela@mojatatu.com
+Signed-off-by: Paolo Abeni <pabeni@redhat.com>
+Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
+---
+ net/sched/sch_hfsc.c | 9 ++++++++-
+ 1 file changed, 8 insertions(+), 1 deletion(-)
+
+--- a/net/sched/sch_hfsc.c
++++ b/net/sched/sch_hfsc.c
+@@ -176,6 +176,11 @@ struct hfsc_sched {
+
+ #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.
+@@ -1038,6 +1043,8 @@ hfsc_change_class(struct Qdisc *sch, u32
+ 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);
+@@ -1576,7 +1583,7 @@ hfsc_enqueue(struct sk_buff *skb, struct
+ 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)