ipmr: fix tables suspicious RCU usage

Similar to the previous patch, plumb the RCU lock inside
the ipmr_get_table(), provided a lockless variant and apply
the latter in the few spots were the lock is already held.

Fixes: 709b46e ("net: Add compat ioctl support for the ipv4 multicast ioctl SIOCGETSGCNT")
Fixes: f0ad086 ("ipv4: ipmr: support multiple tables")
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/ipv4/ipmr.c

@@ -142,7 +142,7 @@ static struct mr_table *ipmr_mr_table_iter(struct net *net,
 	return ret;
 }
 
-static struct mr_table *ipmr_get_table(struct net *net, u32 id)
+static struct mr_table *__ipmr_get_table(struct net *net, u32 id)
 {
 	struct mr_table *mrt;
 
@@ -153,6 +153,16 @@ static struct mr_table *ipmr_get_table(struct net *net, u32 id)
 	return NULL;
 }
 
+static struct mr_table *ipmr_get_table(struct net *net, u32 id)
+{
+	struct mr_table *mrt;
+
+	rcu_read_lock();
+	mrt = __ipmr_get_table(net, id);
+	rcu_read_unlock();
+	return mrt;
+}
+
 static int ipmr_fib_lookup(struct net *net, struct flowi4 *flp4,
 			   struct mr_table **mrt)
 {
@@ -194,7 +204,7 @@ static int ipmr_rule_action(struct fib_rule *rule, struct flowi *flp,
 
 	arg->table = fib_rule_get_table(rule, arg);
 
-	mrt = ipmr_get_table(rule->fr_net, arg->table);
+	mrt = __ipmr_get_table(rule->fr_net, arg->table);
 	if (!mrt)
 		return -EAGAIN;
 	res->mrt = mrt;
@@ -325,6 +335,8 @@ static struct mr_table *ipmr_get_table(struct net *net, u32 id)
 	return net->ipv4.mrt;
 }
 
+#define __ipmr_get_table ipmr_get_table
+
 static int ipmr_fib_lookup(struct net *net, struct flowi4 *flp4,
 			   struct mr_table **mrt)
 {
@@ -413,7 +425,7 @@ static struct mr_table *ipmr_new_table(struct net *net, u32 id)
 	if (id != RT_TABLE_DEFAULT && id >= 1000000000)
 		return ERR_PTR(-EINVAL);
 
-	mrt = ipmr_get_table(net, id);
+	mrt = __ipmr_get_table(net, id);
 	if (mrt)
 		return mrt;
 
@@ -1388,7 +1400,7 @@ int ip_mroute_setsockopt(struct sock *sk, int optname, sockptr_t optval,
 		goto out_unlock;
 	}
 
-	mrt = ipmr_get_table(net, raw_sk(sk)->ipmr_table ? : RT_TABLE_DEFAULT);
+	mrt = __ipmr_get_table(net, raw_sk(sk)->ipmr_table ? : RT_TABLE_DEFAULT);
 	if (!mrt) {
 		ret = -ENOENT;
 		goto out_unlock;
@@ -2276,11 +2288,13 @@ int ipmr_get_route(struct net *net, struct sk_buff *skb,
 	struct mr_table *mrt;
 	int err;
 
-	mrt = ipmr_get_table(net, RT_TABLE_DEFAULT);
-	if (!mrt)
+	rcu_read_lock();
+	mrt = __ipmr_get_table(net, RT_TABLE_DEFAULT);
+	if (!mrt) {
+		rcu_read_unlock();
 		return -ENOENT;
+	}
 
-	rcu_read_lock();
 	cache = ipmr_cache_find(mrt, saddr, daddr);
 	if (!cache && skb->dev) {
 		int vif = ipmr_find_vif(mrt, skb->dev);
@@ -2564,7 +2578,7 @@ static int ipmr_rtm_getroute(struct sk_buff *in_skb, struct nlmsghdr *nlh,
 	grp = nla_get_in_addr_default(tb[RTA_DST], 0);
 	tableid = nla_get_u32_default(tb[RTA_TABLE], 0);
 
-	mrt = ipmr_get_table(net, tableid ? tableid : RT_TABLE_DEFAULT);
+	mrt = __ipmr_get_table(net, tableid ? tableid : RT_TABLE_DEFAULT);
 	if (!mrt) {
 		err = -ENOENT;
 		goto errout_free;
@@ -2618,7 +2632,7 @@ static int ipmr_rtm_dumproute(struct sk_buff *skb, struct netlink_callback *cb)
 	if (filter.table_id) {
 		struct mr_table *mrt;
 
-		mrt = ipmr_get_table(sock_net(skb->sk), filter.table_id);
+		mrt = __ipmr_get_table(sock_net(skb->sk), filter.table_id);
 		if (!mrt) {
 			if (rtnl_msg_family(cb->nlh) != RTNL_FAMILY_IPMR)
 				return skb->len;
@@ -2726,7 +2740,7 @@ static int rtm_to_ipmr_mfcc(struct net *net, struct nlmsghdr *nlh,
 			break;
 		}
 	}
-	mrt = ipmr_get_table(net, tblid);
+	mrt = __ipmr_get_table(net, tblid);
 	if (!mrt) {
 		ret = -ENOENT;
 		goto out;
@@ -2934,13 +2948,15 @@ static void *ipmr_vif_seq_start(struct seq_file *seq, loff_t *pos)
 	struct net *net = seq_file_net(seq);
 	struct mr_table *mrt;
 
-	mrt = ipmr_get_table(net, RT_TABLE_DEFAULT);
-	if (!mrt)
+	rcu_read_lock();
+	mrt = __ipmr_get_table(net, RT_TABLE_DEFAULT);
+	if (!mrt) {
+		rcu_read_unlock();
 		return ERR_PTR(-ENOENT);
+	}
 
 	iter->mrt = mrt;
 
-	rcu_read_lock();
 	return mr_vif_seq_start(seq, pos);
 }