ipv6: mcast: add RCU protection to mld_newpack()

mld_newpack() can be called without RTNL or RCU being held.

Note that we no longer can use sock_alloc_send_skb() because
ipv6.igmp_sk uses GFP_KERNEL allocations which can sleep.

Instead use alloc_skb() and charge the net->ipv6.igmp_sk
socket under RCU protection.

Fixes: b8ad0cb ("[NETNS][IPV6] mcast - handle several network namespace")
Signed-off-by: Eric Dumazet <[email protected]>
Reviewed-by: David Ahern <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: edumazet

net/ipv6/mcast.c

@@ -1773,28 +1773,32 @@ static struct sk_buff *mld_newpack(struct inet6_dev *idev, unsigned int mtu)
 	struct net_device *dev = idev->dev;
 	int hlen = LL_RESERVED_SPACE(dev);
 	int tlen = dev->needed_tailroom;
-	struct net *net = dev_net(dev);
 	const struct in6_addr *saddr;
 	struct in6_addr addr_buf;
 	struct mld2_report *pmr;
 	struct sk_buff *skb;
 	unsigned int size;
 	struct sock *sk;
-	int err;
+	struct net *net;
 
-	sk = net->ipv6.igmp_sk;
 	/* we assume size > sizeof(ra) here
 	 * Also try to not allocate high-order pages for big MTU
 	 */
 	size = min_t(int, mtu, PAGE_SIZE / 2) + hlen + tlen;
-	skb = sock_alloc_send_skb(sk, size, 1, &err);
+	skb = alloc_skb(size, GFP_KERNEL);
 	if (!skb)
 		return NULL;
 
 	skb->priority = TC_PRIO_CONTROL;
 	skb_reserve(skb, hlen);
 	skb_tailroom_reserve(skb, mtu, tlen);
 
+	rcu_read_lock();
+
+	net = dev_net_rcu(dev);
+	sk = net->ipv6.igmp_sk;
+	skb_set_owner_w(skb, sk);
+
 	if (ipv6_get_lladdr(dev, &addr_buf, IFA_F_TENTATIVE)) {
 		/* <draft-ietf-magma-mld-source-05.txt>:
 		 * use unspecified address as the source address
@@ -1806,6 +1810,8 @@ static struct sk_buff *mld_newpack(struct inet6_dev *idev, unsigned int mtu)
 
 	ip6_mc_hdr(sk, skb, dev, saddr, &mld2_all_mcr, NEXTHDR_HOP, 0);
 
+	rcu_read_unlock();
+
 	skb_put_data(skb, ra, sizeof(ra));
 
 	skb_set_transport_header(skb, skb_tail_pointer(skb) - skb->data);