[SRU][N/O/P][PATCH v2 3/3] net_sched: hfsc: Address reentrant enqueue adding class to eltree twice

[Emil Renner Berthing]

Ian Whitfield wrote:
> From: Pedro Tammela <pctammela at mojatatu.com>
>
> 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 at syst3mfailure.io>
> Reported-by: William Liu <will at willsroot.io>
> Acked-by: Jamal Hadi Salim <jhs at mojatatu.com>
> Tested-by: Victor Nogueira <victor at mojatatu.com>
> Signed-off-by: Pedro Tammela <pctammela at mojatatu.com>
> Link: https://patch.msgid.link/20250522181448.1439717-2-pctammela@mojatatu.com
> Signed-off-by: Paolo Abeni <pabeni at redhat.com>
>

nit: extra empty line here too

> (cherry picked from commit ac9fe7dd8e730a103ae4481147395cc73492d786)
> CVE-2025-37890
> Signed-off-by: Ian Whitfield <ian.whitfield at canonical.com>
> ---
>  net/sched/sch_hfsc.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/net/sched/sch_hfsc.c b/net/sched/sch_hfsc.c
> index cbfcee7e8524..362999e11ca4 100644
> --- a/net/sched/sch_hfsc.c
> +++ b/net/sched/sch_hfsc.c
> @@ -175,6 +175,11 @@ 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.
> @@ -1032,6 +1037,8 @@ 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);
> @@ -1564,7 +1571,7 @@ 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)
> --
> 2.43.0
>
>
> --
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