net, sched: fix panic when updating miniq {b,q}stats

While working on fixing another bug, I ran into the following panic
on arm64 by simply attaching clsact qdisc, adding a filter and running
traffic on ingress to it:

[...]
[ 178.188591] Unable to handle kernel read from unreadable memory at virtual address 810fb501f000
[ 178.197314] Mem abort info:
[ 178.200121] ESR = 0x96000004
[ 178.203168] Exception class = DABT (current EL), IL = 32 bits
[ 178.209095] SET = 0, FnV = 0
[ 178.212157] EA = 0, S1PTW = 0
[ 178.215288] Data abort info:
[ 178.218175] ISV = 0, ISS = 0x00000004
[ 178.222019] CM = 0, WnR = 0
[ 178.224997] user pgtable: 4k pages, 48-bit VAs, pgd = 0000000023cb3f33
[ 178.231531] [0000810fb501f000] *pgd=0000000000000000
[ 178.236508] Internal error: Oops: 96000004 [#1] SMP
[...]
[ 178.311855] CPU: 73 PID: 2497 Comm: ping Tainted: G W 4.15.0-rc7+ #5
[ 178.319413] Hardware name: FOXCONN R2-1221R-A4/C2U4N_MB, BIOS G31FB18A 03/31/2017
[ 178.326887] pstate: 60400005 (nZCv daif +PAN -UAO)
[ 178.331685] pc : __netif_receive_skb_core+0x49c/0xac8
[ 178.336728] lr : __netif_receive_skb+0x28/0x78
[ 178.341161] sp : ffff00002344b750
[ 178.344465] x29: ffff00002344b750 x28: ffff810fbdfd0580
[ 178.349769] x27: 0000000000000000 x26: ffff000009378000
[...]
[ 178.418715] x1 : 0000000000000054 x0 : 0000000000000000
[ 178.424020] Process ping (pid: 2497, stack limit = 0x000000009f0a3ff4)
[ 178.430537] Call trace:
[ 178.432976] __netif_receive_skb_core+0x49c/0xac8
[ 178.437670] __netif_receive_skb+0x28/0x78
[ 178.441757] process_backlog+0x9c/0x160
[ 178.445584] net_rx_action+0x2f8/0x3f0
[...]

Reason is that sch_ingress and sch_clsact are doing mini_qdisc_pair_init()
which sets up miniq pointers to cpu_{b,q}stats from the underlying qdisc.
Problem is that this cannot work since they are actually set up right after
the qdisc ->init() callback in qdisc_create(), so first packet going into
sch_handle_ingress() tries to call mini_qdisc_bstats_cpu_update() and we
therefore panic.

In order to fix this, allocation of {b,q}stats needs to happen before we
call into ->init(). In net-next, there's already such option through commit
d59f5ffa59d8 ("net: sched: a dflt qdisc may be used with per cpu stats").
However, the bug needs to be fixed in net still for 4.15. Thus, include
these bits to reduce any merge churn and reuse the static_flags field to
set TCQ_F_CPUSTATS, and remove the allocation from qdisc_create() since
there is no other user left. Prashant Bhole ran into the same issue but
for net-next, thus adding him below as well as co-author. Same issue was
also reported by Sandipan Das when using bcc.

Fixes: 46209401f8f6 ("net: core: introduce mini_Qdisc and eliminate usage of tp->q for clsact fastpath")
Reference: https://lists.iovisor.org/pipermail/iovisor-dev/2018-January/001190.html
Reported-by: Sandipan Das <[e-mail]>
Co-authored-by: Prashant Bhole <[e-mail]>
Co-authored-by: John Fastabend <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Cc: Jiri Pirko <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/sch_generic.h

···
 	const struct Qdisc_class_ops	*cl_ops;
 	char			id[IFNAMSIZ];
 	int			priv_size;
+	unsigned int		static_flags;
 
 	int 			(*enqueue)(struct sk_buff *skb,
 					   struct Qdisc *sch,
···
 			       unsigned int len);
 struct Qdisc *qdisc_alloc(struct netdev_queue *dev_queue,
 			  const struct Qdisc_ops *ops);
+void qdisc_free(struct Qdisc *qdisc);
 struct Qdisc *qdisc_create_dflt(struct netdev_queue *dev_queue,
 				const struct Qdisc_ops *ops, u32 parentid);
 void __qdisc_calculate_pkt_len(struct sk_buff *skb,

net/sched/sch_api.c

···
 	}
 
 	if (!ops->init || (err = ops->init(sch, tca[TCA_OPTIONS])) == 0) {
-		if (qdisc_is_percpu_stats(sch)) {
-			sch->cpu_bstats =
-				netdev_alloc_pcpu_stats(struct gnet_stats_basic_cpu);
-			if (!sch->cpu_bstats)
-				goto err_out4;
-
-			sch->cpu_qstats = alloc_percpu(struct gnet_stats_queue);
-			if (!sch->cpu_qstats)
-				goto err_out4;
-		}
-
 		if (tca[TCA_STAB]) {
 			stab = qdisc_get_stab(tca[TCA_STAB]);
 			if (IS_ERR(stab)) {
···
 		ops->destroy(sch);
 err_out3:
 	dev_put(dev);
-	kfree((char *) sch - sch->padded);
+	qdisc_free(sch);
 err_out2:
 	module_put(ops->owner);
 err_out:
···
 	return NULL;
 
 err_out4:
-	free_percpu(sch->cpu_bstats);
-	free_percpu(sch->cpu_qstats);
 	/*
 	 * Any broken qdiscs that would require a ops->reset() here?
 	 * The qdisc was never in action so it shouldn't be necessary.

net/sched/sch_generic.c

···
 	qdisc_skb_head_init(&sch->q);
 	spin_lock_init(&sch->q.lock);
 
+	if (ops->static_flags & TCQ_F_CPUSTATS) {
+		sch->cpu_bstats =
+			netdev_alloc_pcpu_stats(struct gnet_stats_basic_cpu);
+		if (!sch->cpu_bstats)
+			goto errout1;
+
+		sch->cpu_qstats = alloc_percpu(struct gnet_stats_queue);
+		if (!sch->cpu_qstats) {
+			free_percpu(sch->cpu_bstats);
+			goto errout1;
+		}
+	}
+
 	spin_lock_init(&sch->busylock);
 	lockdep_set_class(&sch->busylock,
 			  dev->qdisc_tx_busylock ?: &qdisc_tx_busylock);
···
 			  dev->qdisc_running_key ?: &qdisc_running_key);
 
 	sch->ops = ops;
+	sch->flags = ops->static_flags;
 	sch->enqueue = ops->enqueue;
 	sch->dequeue = ops->dequeue;
 	sch->dev_queue = dev_queue;
···
 	refcount_set(&sch->refcnt, 1);
 
 	return sch;
+errout1:
+	kfree(p);
 errout:
 	return ERR_PTR(err);
 }
···
 }
 EXPORT_SYMBOL(qdisc_reset);
 
-static void qdisc_free(struct Qdisc *qdisc)
+void qdisc_free(struct Qdisc *qdisc)
 {
 	if (qdisc_is_percpu_stats(qdisc)) {
 		free_percpu(qdisc->cpu_bstats);

net/sched/sch_ingress.c

···
 {
 	struct ingress_sched_data *q = qdisc_priv(sch);
 	struct net_device *dev = qdisc_dev(sch);
-	int err;
 
 	net_inc_ingress_queue();
 
···
 	q->block_info.chain_head_change = clsact_chain_head_change;
 	q->block_info.chain_head_change_priv = &q->miniqp;
 
-	err = tcf_block_get_ext(&q->block, sch, &q->block_info);
-	if (err)
-		return err;
-
-	sch->flags |= TCQ_F_CPUSTATS;
-
-	return 0;
+	return tcf_block_get_ext(&q->block, sch, &q->block_info);
 }
 
 static void ingress_destroy(struct Qdisc *sch)
···
 	.cl_ops		=	&ingress_class_ops,
 	.id		=	"ingress",
 	.priv_size	=	sizeof(struct ingress_sched_data),
+	.static_flags	=	TCQ_F_CPUSTATS,
 	.init		=	ingress_init,
 	.destroy	=	ingress_destroy,
 	.dump		=	ingress_dump,
···
 	q->egress_block_info.chain_head_change = clsact_chain_head_change;
 	q->egress_block_info.chain_head_change_priv = &q->miniqp_egress;
 
-	err = tcf_block_get_ext(&q->egress_block, sch, &q->egress_block_info);
-	if (err)
-		return err;
-
-	sch->flags |= TCQ_F_CPUSTATS;
-
-	return 0;
+	return tcf_block_get_ext(&q->egress_block, sch, &q->egress_block_info);
 }
 
 static void clsact_destroy(struct Qdisc *sch)
···
 	.cl_ops		=	&clsact_class_ops,
 	.id		=	"clsact",
 	.priv_size	=	sizeof(struct clsact_sched_data),
+	.static_flags	=	TCQ_F_CPUSTATS,
 	.init		=	clsact_init,
 	.destroy	=	clsact_destroy,
 	.dump		=	ingress_dump,