Merge branch 'ipv4-less-uses-of-shared-ip-generator'

Eric Dumazet says:

====================
ipv4: less uses of shared IP generator

From: Eric Dumazet <edumazet@google.com>

We keep receiving research reports based on linux IPID generation.

Before breaking part of the Internet by switching to pure
random generator, this series reduces the need for the
shared IP generator for TCP sockets.
====================

Link: https://lore.kernel.org/r/20220127011022.1274803-1-eric.dumazet@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/ip.h

@@ -525,19 +525,18 @@ static inline void ip_select_ident_segs(struct net *net, struct sk_buff *skb,
 {
 	struct iphdr *iph = ip_hdr(skb);
 
-	if ((iph->frag_off & htons(IP_DF)) && !skb->ignore_df) {
-		/* This is only to work around buggy Windows95/2000
-		 * VJ compression implementations.  If the ID field
-		 * does not change, they drop every other packet in
-		 * a TCP stream using header compression.
+	/* We had many attacks based on IPID, use the private
+	 * generator as much as we can.
 	 */
 	if (sk && inet_sk(sk)->inet_daddr) {
 		iph->id = htons(inet_sk(sk)->inet_id);
 		inet_sk(sk)->inet_id += segs;
-		} else {
-			iph->id = 0;
+		return;
 	}
+	if ((iph->frag_off & htons(IP_DF)) && !skb->ignore_df) {
+		iph->id = 0;
 	} else {
+		/* Unfortunately we need the big hammer to get a suitable IPID */
 		__ip_select_ident(net, iph, segs);
 	}
 }

net/ipv4/ip_output.c

@@ -162,11 +162,18 @@ int ip_build_and_send_pkt(struct sk_buff *skb, const struct sock *sk,
 	iph->daddr    = (opt && opt->opt.srr ? opt->opt.faddr : daddr);
 	iph->saddr    = saddr;
 	iph->protocol = sk->sk_protocol;
-	if (ip_dont_fragment(sk, &rt->dst)) {
+	/* Do not bother generating IPID for small packets (eg SYNACK) */
+	if (skb->len <= IPV4_MIN_MTU || ip_dont_fragment(sk, &rt->dst)) {
 		iph->frag_off = htons(IP_DF);
 		iph->id = 0;
 	} else {
 		iph->frag_off = 0;
+		/* TCP packets here are SYNACK with fat IPv4/TCP options.
+		 * Avoid using the hashed IP ident generator.
+		 */
+		if (sk->sk_protocol == IPPROTO_TCP)
+			iph->id = (__force __be16)prandom_u32();
+		else
 			__ip_select_ident(net, iph, 1);
 	}