5.4-stable review patch. If anyone has any objections, please let me know.
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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/commi... [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/8DuRWwfqjoRDLDmBMlIfbrsZg9Gx50DHJc1ilxsEBNe2D...
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)