net: xfrm: Add '_rcu' tag for rcu protected pointer in netns_xfrm

For rcu protected pointers, we'd better add '__rcu' for them.

Once added '__rcu' tag for rcu protected pointer, the sparse tool reports
warnings.

net/xfrm/xfrm_user.c:1198:39: sparse:    expected struct sock *sk
net/xfrm/xfrm_user.c:1198:39: sparse:    got struct sock [noderef] <asn:4> *nlsk
[...]

So introduce a new wrapper function of nlmsg_unicast  to handle type
conversions.

This patch also fixes a direct access of a rcu protected socket.

Fixes: be33690d("[XFRM]: Fix aevent related crash")
Signed-off-by: Su Yanjun <suyj.fnst@cn.fujitsu.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

include/net/netns/xfrm.h

@@ -57,7 +57,7 @@ struct netns_xfrm {
 	struct list_head	inexact_bins;
 
 
-	struct sock		*nlsk;
+	struct sock		__rcu *nlsk;
 	struct sock		*nlsk_stash;
 
 	u32			sysctl_aevent_etime;

net/xfrm/xfrm_user.c

@@ -1071,6 +1071,22 @@ static inline int xfrm_nlmsg_multicast(struct net *net, struct sk_buff *skb,
 	return nlmsg_multicast(nlsk, skb, pid, group, GFP_ATOMIC);
 }
 
+/* A similar wrapper like xfrm_nlmsg_multicast checking that nlsk is still
+ * available.
+ */
+static inline int xfrm_nlmsg_unicast(struct net *net, struct sk_buff *skb,
+				     u32 pid)
+{
+	struct sock *nlsk = rcu_dereference(net->xfrm.nlsk);
+
+	if (!nlsk) {
+		kfree_skb(skb);
+		return -EPIPE;
+	}
+
+	return nlmsg_unicast(nlsk, skb, pid);
+}
+
 static inline unsigned int xfrm_spdinfo_msgsize(void)
 {
 	return NLMSG_ALIGN(4)
@@ -1195,7 +1211,7 @@ static int xfrm_get_spdinfo(struct sk_buff *skb, struct nlmsghdr *nlh,
 	err = build_spdinfo(r_skb, net, sportid, seq, *flags);
 	BUG_ON(err < 0);
 
-	return nlmsg_unicast(net->xfrm.nlsk, r_skb, sportid);
+	return xfrm_nlmsg_unicast(net, r_skb, sportid);
 }
 
 static inline unsigned int xfrm_sadinfo_msgsize(void)
@@ -1254,7 +1270,7 @@ static int xfrm_get_sadinfo(struct sk_buff *skb, struct nlmsghdr *nlh,
 	err = build_sadinfo(r_skb, net, sportid, seq, *flags);
 	BUG_ON(err < 0);
 
-	return nlmsg_unicast(net->xfrm.nlsk, r_skb, sportid);
+	return xfrm_nlmsg_unicast(net, r_skb, sportid);
 }
 
 static int xfrm_get_sa(struct sk_buff *skb, struct nlmsghdr *nlh,
@@ -1274,7 +1290,7 @@ static int xfrm_get_sa(struct sk_buff *skb, struct nlmsghdr *nlh,
 	if (IS_ERR(resp_skb)) {
 		err = PTR_ERR(resp_skb);
 	} else {
-		err = nlmsg_unicast(net->xfrm.nlsk, resp_skb, NETLINK_CB(skb).portid);
+		err = xfrm_nlmsg_unicast(net, resp_skb, NETLINK_CB(skb).portid);
 	}
 	xfrm_state_put(x);
 out_noput:
@@ -1337,7 +1353,7 @@ static int xfrm_alloc_userspi(struct sk_buff *skb, struct nlmsghdr *nlh,
 		goto out;
 	}
 
-	err = nlmsg_unicast(net->xfrm.nlsk, resp_skb, NETLINK_CB(skb).portid);
+	err = xfrm_nlmsg_unicast(net, resp_skb, NETLINK_CB(skb).portid);
 
 out:
 	xfrm_state_put(x);
@@ -1903,7 +1919,7 @@ static int xfrm_get_policy(struct sk_buff *skb, struct nlmsghdr *nlh,
 		if (IS_ERR(resp_skb)) {
 			err = PTR_ERR(resp_skb);
 		} else {
-			err = nlmsg_unicast(net->xfrm.nlsk, resp_skb,
+			err = xfrm_nlmsg_unicast(net, resp_skb,
 						 NETLINK_CB(skb).portid);
 		}
 	} else {
@@ -2062,7 +2078,7 @@ static int xfrm_get_ae(struct sk_buff *skb, struct nlmsghdr *nlh,
 	err = build_aevent(r_skb, x, &c);
 	BUG_ON(err < 0);
 
-	err = nlmsg_unicast(net->xfrm.nlsk, r_skb, NETLINK_CB(skb).portid);
+	err = xfrm_nlmsg_unicast(net, r_skb, NETLINK_CB(skb).portid);
 	spin_unlock_bh(&x->lock);
 	xfrm_state_put(x);
 	return err;