net: do not send ICMP/NDISC Redirects when peer allocation fails

When inet_getpeer_v4() or inet_getpeer_v6() fails to allocate a peer entry
under memory pressure or tree size caps, redirect handlers previously fell
back to sending un-rate-limited ICMP/NDISC Redirect messages.

In IPv4, ip_rt_send_redirect() called icmp_send() directly when peer == NULL.
In IPv6, ip6_forward() and ndisc_send_redirect() passed a NULL peer into
inet_peer_xrlim_allow(), which returned true when peer == NULL.

Because ICMP/NDISC Redirects are not part of the default global rate limit
mask (sysctl_icmp_ratemask), sending redirects when peer == NULL creates
an un-rate-limited ICMP packet storm.

Fix this by failing closed in ip_rt_send_redirect(), ip6_forward(), and
ndisc_send_redirect() when peer is NULL.

Fixes: 92d8682926 ("inetpeer: Move ICMP rate limiting state into inet_peer entries.")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260724072901.1633601-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/route.c

@@ -892,8 +892,6 @@ void ip_rt_send_redirect(struct sk_buff *skb)
 	peer = inet_getpeer_v4(net->ipv4.peers, ip_hdr(skb)->saddr, vif);
 	if (!peer) {
 		rcu_read_unlock();
-		icmp_send(skb, ICMP_REDIRECT, ICMP_REDIR_HOST,
-			  rt_nexthop(rt, ip_hdr(skb)->daddr));
 		return;
 	}
 

net/ipv6/ip6_output.c

@@ -641,7 +641,7 @@ int ip6_forward(struct sk_buff *skb)
 		/* Limit redirects both by destination (here)
 		   and by source (inside ndisc_send_redirect)
 		 */
-		if (inet_peer_xrlim_allow(peer, 1*HZ))
+		if (peer && inet_peer_xrlim_allow(peer, 1*HZ))
 			ndisc_send_redirect(skb, target);
 		rcu_read_unlock();
 	} else {

net/ipv6/ndisc.c

@@ -1707,6 +1707,8 @@ void ndisc_send_redirect(struct sk_buff *skb, const struct in6_addr *target)
 	}
 
 	peer = inet_getpeer_v6(net->ipv6.peers, &ipv6_hdr(skb)->saddr);
+	if (!peer)
+		goto release;
 	ret = inet_peer_xrlim_allow(peer, 1*HZ);
 
 	if (!ret)