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

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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: 37d9cf1a3c ("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>
This commit is contained in:
Pedro Tammela 2025-05-22 15:14:47 -03:00 committed by Greg Kroah-Hartman
parent a5a9c42cd7
commit a0ec22fa20
1 changed files with 8 additions and 1 deletions
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9
net/sched/sch_hfsc.c
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@ -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.
@ -1041,6 +1046,8 @@ hfsc_change_class(struct Qdisc *sch, u32 classid, u32 parentid,
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);
@ -1571,7 +1578,7 @@ hfsc_enqueue(struct sk_buff *skb, struct Qdisc *sch, struct sk_buff **to_free)
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)