rtnetlink: fix double call of rtnl_link_get_net_ifla()

Currently rtnl_link_get_net_ifla() gets called twice when we create
peer devices, once in rtnl_add_peer_net() and once in each ->newlink()
implementation.

This looks safer, however, it leads to a classic Time-of-Check to
Time-of-Use (TOCTOU) bug since IFLA_NET_NS_PID is very dynamic. And
because of the lack of checking error pointer of the second call, it
also leads to a kernel crash as reported by syzbot.

Fix this by getting rid of the second call, which already becomes
redudant after Kuniyuki's work. We have to propagate the result of the
first rtnl_link_get_net_ifla() down to each ->newlink().

Reported-by: syzbot+21ba4d5adff0b6a7cfc6@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=21ba4d5adff0b6a7cfc6
Fixes: 0eb87b02a705 ("veth: Set VETH_INFO_PEER to veth_link_ops.peer_type.")
Fixes: 6b84e558e95d ("vxcan: Set VXCAN_INFO_PEER to vxcan_link_ops.peer_type.")
Fixes: fefd5d082172 ("netkit: Set IFLA_NETKIT_PEER_INFO to netkit_link_ops.peer_type.")
Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20241129212519.825567-1-xiyou.wangcong@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

1 files changed, 23 insertions, 21 deletions

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 58df76fe408a..ab5f201bf0ab 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -3746,6 +3746,7 @@ static int rtnl_group_changelink(const struct sk_buff *skb,
 static int rtnl_newlink_create(struct sk_buff *skb, struct ifinfomsg *ifm,
 			       const struct rtnl_link_ops *ops,
 			       struct net *tgt_net, struct net *link_net,
+			       struct net *peer_net,
 			       const struct nlmsghdr *nlh,
 			       struct nlattr **tb, struct nlattr **data,
 			       struct netlink_ext_ack *extack)
@@ -3776,8 +3777,13 @@ static int rtnl_newlink_create(struct sk_buff *skb, struct ifinfomsg *ifm,
 
 	dev->ifindex = ifm->ifi_index;
 
+	if (link_net)
+		net = link_net;
+	if (peer_net)
+		net = peer_net;
+
 	if (ops->newlink)
-		err = ops->newlink(link_net ? : net, dev, tb, data, extack);
+		err = ops->newlink(net, dev, tb, data, extack);
 	else
 		err = register_netdevice(dev);
 	if (err < 0) {
@@ -3812,40 +3818,33 @@ out_unregister:
 	goto out;
 }
 
-static int rtnl_add_peer_net(struct rtnl_nets *rtnl_nets,
-			     const struct rtnl_link_ops *ops,
-			     struct nlattr *data[],
-			     struct netlink_ext_ack *extack)
+static struct net *rtnl_get_peer_net(const struct rtnl_link_ops *ops,
+				     struct nlattr *data[],
+				     struct netlink_ext_ack *extack)
 {
 	struct nlattr *tb[IFLA_MAX + 1];
-	struct net *net;
 	int err;
 
 	if (!data || !data[ops->peer_type])
-		return 0;
+		return NULL;
 
 	err = rtnl_nla_parse_ifinfomsg(tb, data[ops->peer_type], extack);
 	if (err < 0)
-		return err;
+		return ERR_PTR(err);
 
 	if (ops->validate) {
 		err = ops->validate(tb, NULL, extack);
 		if (err < 0)
-			return err;
+			return ERR_PTR(err);
 	}
 
-	net = rtnl_link_get_net_ifla(tb);
-	if (IS_ERR(net))
-		return PTR_ERR(net);
-	if (net)
-		rtnl_nets_add(rtnl_nets, net);
-
-	return 0;
+	return rtnl_link_get_net_ifla(tb);
 }
 
 static int __rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 			  const struct rtnl_link_ops *ops,
 			  struct net *tgt_net, struct net *link_net,
+			  struct net *peer_net,
 			  struct rtnl_newlink_tbs *tbs,
 			  struct nlattr **data,
 			  struct netlink_ext_ack *extack)
@@ -3894,14 +3893,15 @@ static int __rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 		return -EOPNOTSUPP;
 	}
 
-	return rtnl_newlink_create(skb, ifm, ops, tgt_net, link_net, nlh, tb, data, extack);
+	return rtnl_newlink_create(skb, ifm, ops, tgt_net, link_net, peer_net, nlh,
+				   tb, data, extack);
 }
 
 static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 			struct netlink_ext_ack *extack)
 {
+	struct net *tgt_net, *link_net = NULL, *peer_net = NULL;
 	struct nlattr **tb, **linkinfo, **data = NULL;
-	struct net *tgt_net, *link_net = NULL;
 	struct rtnl_link_ops *ops = NULL;
 	struct rtnl_newlink_tbs *tbs;
 	struct rtnl_nets rtnl_nets;
@@ -3971,9 +3971,11 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 		}
 
 		if (ops->peer_type) {
-			ret = rtnl_add_peer_net(&rtnl_nets, ops, data, extack);
-			if (ret < 0)
+			peer_net = rtnl_get_peer_net(ops, data, extack);
+			if (IS_ERR(peer_net))
 				goto put_ops;
+			if (peer_net)
+				rtnl_nets_add(&rtnl_nets, peer_net);
 		}
 	}
 
@@ -4004,7 +4006,7 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 	}
 
 	rtnl_nets_lock(&rtnl_nets);
-	ret = __rtnl_newlink(skb, nlh, ops, tgt_net, link_net, tbs, data, extack);
+	ret = __rtnl_newlink(skb, nlh, ops, tgt_net, link_net, peer_net, tbs, data, extack);
 	rtnl_nets_unlock(&rtnl_nets);
 
 put_net: