[SRU][J][PATCH 1/1] ipv6: mcast: extend RCU protection in igmp6_send()

[Cengiz Can]

From: Eric Dumazet <edumazet at google.com>

igmp6_send() can be called without RTNL or RCU being held.

Extend RCU protection so that we can safely fetch the net pointer
and avoid a potential UAF.

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: b8ad0cbc58f7 ("[NETNS][IPV6] mcast - handle several network namespace")
Signed-off-by: Eric Dumazet <edumazet at google.com>
Reviewed-by: David Ahern <dsahern at kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu at amazon.com>
Link: https://patch.msgid.link/20250207135841.1948589-9-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba at kernel.org>
(backported from commit 087c1faa594fa07a66933d750c0b2610aa1a2946)
[bot_kybele: Kept this tree's OUTREQUESTS-equivalent stat: the moved-under-RCU
 counter is IP6_UPD_PO_STATS(net, idev, IPSTATS_MIB_OUT, full_len) here, not
 upstream's IP6_INC_STATS(..., IPSTATS_MIB_OUTREQUESTS).]
CVE-2025-21759
Assisted-by: kybele:claude-opus-4.8
Signed-off-by: Cengiz Can <cengiz.can at canonical.com>
---
 net/ipv6/mcast.c | 32 +++++++++++++++-----------------
 1 file changed, 15 insertions(+), 17 deletions(-)

diff --git a/net/ipv6/mcast.c b/net/ipv6/mcast.c
index 4f2030877a0b..37327041e0e1 100644
--- a/net/ipv6/mcast.c
+++ b/net/ipv6/mcast.c
@@ -2154,21 +2154,21 @@ static void mld_send_cr(struct inet6_dev *idev)
 
 static void igmp6_send(struct in6_addr *addr, struct net_device *dev, int type)
 {
-	struct net *net = dev_net(dev);
-	struct sock *sk = net->ipv6.igmp_sk;
+	const struct in6_addr *snd_addr, *saddr;
+	int err, len, payload_len, full_len;
+	struct in6_addr addr_buf;
 	struct inet6_dev *idev;
 	struct sk_buff *skb;
 	struct mld_msg *hdr;
-	const struct in6_addr *snd_addr, *saddr;
-	struct in6_addr addr_buf;
 	int hlen = LL_RESERVED_SPACE(dev);
 	int tlen = dev->needed_tailroom;
-	int err, len, payload_len, full_len;
 	u8 ra[8] = { IPPROTO_ICMPV6, 0,
 		     IPV6_TLV_ROUTERALERT, 2, 0, 0,
 		     IPV6_TLV_PADN, 0 };
-	struct flowi6 fl6;
 	struct dst_entry *dst;
+	struct flowi6 fl6;
+	struct net *net;
+	struct sock *sk;
 
 	if (type == ICMPV6_MGM_REDUCTION)
 		snd_addr = &in6addr_linklocal_allrouters;
@@ -2179,20 +2179,21 @@ static void igmp6_send(struct in6_addr *addr, struct net_device *dev, int type)
 	payload_len = len + sizeof(ra);
 	full_len = sizeof(struct ipv6hdr) + payload_len;
 
-	rcu_read_lock();
-	IP6_UPD_PO_STATS(net, __in6_dev_get(dev),
-		      IPSTATS_MIB_OUT, full_len);
-	rcu_read_unlock();
+	skb = alloc_skb(hlen + tlen + full_len, GFP_KERNEL);
 
-	skb = sock_alloc_send_skb(sk, hlen + tlen + full_len, 1, &err);
+	rcu_read_lock();
 
+	net = dev_net_rcu(dev);
+	idev = __in6_dev_get(dev);
+	IP6_UPD_PO_STATS(net, idev, IPSTATS_MIB_OUT, full_len);
 	if (!skb) {
-		rcu_read_lock();
-		IP6_INC_STATS(net, __in6_dev_get(dev),
-			      IPSTATS_MIB_OUTDISCARDS);
+		IP6_INC_STATS(net, idev, IPSTATS_MIB_OUTDISCARDS);
 		rcu_read_unlock();
 		return;
 	}
+	sk = net->ipv6.igmp_sk;
+	skb_set_owner_w(skb, sk);
+
 	skb->priority = TC_PRIO_CONTROL;
 	skb_reserve(skb, hlen);
 
@@ -2217,9 +2218,6 @@ static void igmp6_send(struct in6_addr *addr, struct net_device *dev, int type)
 					 IPPROTO_ICMPV6,
 					 csum_partial(hdr, len, 0));
 
-	rcu_read_lock();
-	idev = __in6_dev_get(skb->dev);
-
 	icmpv6_flow_init(sk, &fl6, type,
 			 &ipv6_hdr(skb)->saddr, &ipv6_hdr(skb)->daddr,
 			 skb->dev->ifindex);
-- 
2.53.0