xfrm: avoid RCU warnings around the per-netns netlink socket

net->xfrm.nlsk is used in 2 types of contexts:
 - fully under RCU, with rcu_read_lock + rcu_dereference and a NULL check
 - in the netlink handlers, with requests coming from a userspace socket

In the 2nd case, net->xfrm.nlsk is guaranteed to stay non-NULL and the
object is alive, since we can't enter the netns destruction path while
the user socket holds a reference on the netns.

After adding the __rcu annotation to netns_xfrm.nlsk (which silences
sparse warnings in the RCU users and __net_init code), we need to tell
sparse that the 2nd case is safe. Add a helper for that.

Signed-off-by: Sabrina Dubroca <[email protected]>
Reviewed-by: Simon Horman <[email protected]>
Signed-off-by: Steffen Klassert <[email protected]>

Author: qsn

include/net/netns/xfrm.h

@@ -59,7 +59,7 @@ 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

@@ -35,6 +35,15 @@
 #endif
 #include <linux/unaligned.h>
 
+static struct sock *xfrm_net_nlsk(const struct net *net, const struct sk_buff *skb)
+{
+	/* get the source of this request, see netlink_unicast_kernel */
+	const struct sock *sk = NETLINK_CB(skb).sk;
+
+	/* sk is refcounted, the netns stays alive and nlsk with it */
+	return rcu_dereference_protected(net->xfrm.nlsk, sk->sk_net_refcnt);
+}
+
 static int verify_one_alg(struct nlattr **attrs, enum xfrm_attr_type_t type,
 			  struct netlink_ext_ack *extack)
 {
@@ -1727,7 +1736,7 @@ 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 nlmsg_unicast(xfrm_net_nlsk(net, skb), r_skb, sportid);
 }
 
 static inline unsigned int xfrm_sadinfo_msgsize(void)
@@ -1787,7 +1796,7 @@ 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 nlmsg_unicast(xfrm_net_nlsk(net, skb), r_skb, sportid);
 }
 
 static int xfrm_get_sa(struct sk_buff *skb, struct nlmsghdr *nlh,
@@ -1807,7 +1816,7 @@ 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 = nlmsg_unicast(xfrm_net_nlsk(net, skb), resp_skb, NETLINK_CB(skb).portid);
 	}
 	xfrm_state_put(x);
 out_noput:
@@ -1898,7 +1907,7 @@ static int xfrm_alloc_userspi(struct sk_buff *skb, struct nlmsghdr *nlh,
 		}
 	}
 
-	err = nlmsg_unicast(net->xfrm.nlsk, resp_skb, NETLINK_CB(skb).portid);
+	err = nlmsg_unicast(xfrm_net_nlsk(net, skb), resp_skb, NETLINK_CB(skb).portid);
 
 out:
 	xfrm_state_put(x);
@@ -2543,7 +2552,7 @@ static int xfrm_get_default(struct sk_buff *skb, struct nlmsghdr *nlh,
 	r_up->out = net->xfrm.policy_default[XFRM_POLICY_OUT];
 	nlmsg_end(r_skb, r_nlh);
 
-	return nlmsg_unicast(net->xfrm.nlsk, r_skb, portid);
+	return nlmsg_unicast(xfrm_net_nlsk(net, skb), r_skb, portid);
 }
 
 static int xfrm_get_policy(struct sk_buff *skb, struct nlmsghdr *nlh,
@@ -2609,7 +2618,7 @@ 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 = nlmsg_unicast(xfrm_net_nlsk(net, skb), resp_skb,
 					    NETLINK_CB(skb).portid);
 		}
 	} else {
@@ -2782,7 +2791,7 @@ 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 = nlmsg_unicast(xfrm_net_nlsk(net, skb), r_skb, NETLINK_CB(skb).portid);
 	spin_unlock_bh(&x->lock);
 	xfrm_state_put(x);
 	return err;
@@ -3486,7 +3495,7 @@ static int xfrm_user_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
 			goto err;
 		}
 
-		err = netlink_dump_start(net->xfrm.nlsk, skb, nlh, &c);
+		err = netlink_dump_start(xfrm_net_nlsk(net, skb), skb, nlh, &c);
 		goto err;
 	}