net: add proper RCU protection to /proc/net/ptype

Yin Fengwei reported an RCU stall in ptype_seq_show() and provided
a patch.

Real issue is that ptype_seq_next() and ptype_seq_show() violate
RCU rules.

ptype_seq_show() runs under rcu_read_lock(), and reads pt->dev
to get device name without any barrier.

At the same time, concurrent writers can remove a packet_type structure
(which is correctly freed after an RCU grace period) and clear pt->dev
without an RCU grace period.

Define ptype_iter_state to carry a dev pointer along seq_net_private:

struct ptype_iter_state {
struct seq_net_private p;
struct net_device *dev; // added in this patch
};

We need to record the device pointer in ptype_get_idx() and
ptype_seq_next() so that ptype_seq_show() is safe against
concurrent pt->dev changes.

We also need to add full RCU protection in ptype_seq_next().
(Missing READ_ONCE() when reading list.next values)

Many thanks to Dong Chenchen for providing a repro.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Fixes: 1d10f8a1f40b ("net-procfs: show net devices bound packet types")
Fixes: c353e8983e0d ("net: introduce per netns packet chains")
Reported-by: Yin Fengwei <[e-mail]>
Reported-by: Dong Chenchen <[e-mail]>
Closes: https://lore.kernel.org/netdev/CANn89iKRRKPnWjJmb-_3a=[e-mail]/T/#m7b80b9fc9b9267f90e0b7aad557595f686f9c50d

Signed-off-by: Eric Dumazet <[e-mail]>
Reviewed-by: Willem de Bruijn <[e-mail]>
Tested-by: Yin Fengwei <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/core/net-procfs.c

···
 	.show  = softnet_seq_show,
 };
 
+struct ptype_iter_state {
+	struct seq_net_private	p;
+	struct net_device	*dev;
+};
+
 static void *ptype_get_idx(struct seq_file *seq, loff_t pos)
 {
+	struct ptype_iter_state *iter = seq->private;
 	struct list_head *ptype_list = NULL;
 	struct packet_type *pt = NULL;
 	struct net_device *dev;
···
 	for_each_netdev_rcu(seq_file_net(seq), dev) {
 		ptype_list = &dev->ptype_all;
 		list_for_each_entry_rcu(pt, ptype_list, list) {
-			if (i == pos)
+			if (i == pos) {
+				iter->dev = dev;
 				return pt;
+			}
 			++i;
 		}
 	}
+
+	iter->dev = NULL;
 
 	list_for_each_entry_rcu(pt, &seq_file_net(seq)->ptype_all, list) {
 		if (i == pos)
···
 
 static void *ptype_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 {
+	struct ptype_iter_state *iter = seq->private;
 	struct net *net = seq_file_net(seq);
 	struct net_device *dev;
 	struct packet_type *pt;
···
 		return ptype_get_idx(seq, 0);
 
 	pt = v;
-	nxt = pt->list.next;
-	if (pt->dev) {
-		if (nxt != &pt->dev->ptype_all)
+	nxt = READ_ONCE(pt->list.next);
+	dev = iter->dev;
+	if (dev) {
+		if (nxt != &dev->ptype_all)
 			goto found;
 
-		dev = pt->dev;
 		for_each_netdev_continue_rcu(seq_file_net(seq), dev) {
-			if (!list_empty(&dev->ptype_all)) {
-				nxt = dev->ptype_all.next;
+			nxt = READ_ONCE(dev->ptype_all.next);
+			if (nxt != &dev->ptype_all) {
+				iter->dev = dev;
 				goto found;
 			}
 		}
-		nxt = net->ptype_all.next;
+		iter->dev = NULL;
+		nxt = READ_ONCE(net->ptype_all.next);
 		goto net_ptype_all;
 	}
 
···
 
 		if (nxt == &net->ptype_all) {
 			/* continue with ->ptype_specific if it's not empty */
-			nxt = net->ptype_specific.next;
+			nxt = READ_ONCE(net->ptype_specific.next);
 			if (nxt != &net->ptype_specific)
 				goto found;
 		}
 
 		hash = 0;
-		nxt = ptype_base[0].next;
+		nxt = READ_ONCE(ptype_base[0].next);
 	} else
 		hash = ntohs(pt->type) & PTYPE_HASH_MASK;
 
 	while (nxt == &ptype_base[hash]) {
 		if (++hash >= PTYPE_HASH_SIZE)
 			return NULL;
-		nxt = ptype_base[hash].next;
+		nxt = READ_ONCE(ptype_base[hash].next);
 	}
 found:
 	return list_entry(nxt, struct packet_type, list);
···
 
 static int ptype_seq_show(struct seq_file *seq, void *v)
 {
+	struct ptype_iter_state *iter = seq->private;
 	struct packet_type *pt = v;
+	struct net_device *dev;
 
-	if (v == SEQ_START_TOKEN)
+	if (v == SEQ_START_TOKEN) {
 		seq_puts(seq, "Type Device      Function\n");
-	else if ((!pt->af_packet_net || net_eq(pt->af_packet_net, seq_file_net(seq))) &&
-		 (!pt->dev || net_eq(dev_net(pt->dev), seq_file_net(seq)))) {
+		return 0;
+	}
+	dev = iter->dev;
+	if ((!pt->af_packet_net || net_eq(pt->af_packet_net, seq_file_net(seq))) &&
+		 (!dev || net_eq(dev_net(dev), seq_file_net(seq)))) {
 		if (pt->type == htons(ETH_P_ALL))
 			seq_puts(seq, "ALL ");
 		else
 			seq_printf(seq, "%04x", ntohs(pt->type));
 
 		seq_printf(seq, " %-8s %ps\n",
-			   pt->dev ? pt->dev->name : "", pt->func);
+			   dev ? dev->name : "", pt->func);
 	}
 
 	return 0;
···
 			 &softnet_seq_ops))
 		goto out_dev;
 	if (!proc_create_net("ptype", 0444, net->proc_net, &ptype_seq_ops,
-			sizeof(struct seq_net_private)))
+			sizeof(struct ptype_iter_state)))
 		goto out_softnet;
 
 	if (wext_proc_init(net))