inet: switch IP ID generator to siphash

commit df453700 upstream.

According to Amit Klein and Benny Pinkas, IP ID generation is too weak
and might be used by attackers.

Even with recent net_hash_mix() fix (netns: provide pure entropy for net_hash_mix())
having 64bit key and Jenkins hash is risky.

It is time to switch to siphash and its 128bit keys.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Amit Klein <aksecurity@gmail.com>
Reported-by: Benny Pinkas <benny@pinkas.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 4.9: adjust context]
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/siphash.h

@@ -21,6 +21,11 @@ typedef struct {
 	u64 key[2];
 } siphash_key_t;
 
+static inline bool siphash_key_is_zero(const siphash_key_t *key)
+{
+	return !(key->key[0] | key->key[1]);
+}
+
 u64 __siphash_aligned(const void *data, size_t len, const siphash_key_t *key);
 #ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
 u64 __siphash_unaligned(const void *data, size_t len, const siphash_key_t *key);

include/net/netns/ipv4.h

@@ -8,6 +8,7 @@
 #include <linux/uidgid.h>
 #include <net/inet_frag.h>
 #include <linux/rcupdate.h>
+#include <linux/siphash.h>
 
 struct tcpm_hash_bucket;
 struct ctl_table_header;
@@ -137,5 +138,6 @@ struct netns_ipv4 {
 	int sysctl_fib_multipath_use_neigh;
 #endif
 	atomic_t	rt_genid;
+	siphash_key_t	ip_id_key;
 };
 #endif

net/ipv4/route.c

@@ -496,15 +496,17 @@ EXPORT_SYMBOL(ip_idents_reserve);
 
 void __ip_select_ident(struct net *net, struct iphdr *iph, int segs)
 {
-	static u32 ip_idents_hashrnd __read_mostly;
 	u32 hash, id;
 
-	net_get_random_once(&ip_idents_hashrnd, sizeof(ip_idents_hashrnd));
+	/* Note the following code is not safe, but this is okay. */
+	if (unlikely(siphash_key_is_zero(&net->ipv4.ip_id_key)))
+		get_random_bytes(&net->ipv4.ip_id_key,
+				 sizeof(net->ipv4.ip_id_key));
 
-	hash = jhash_3words((__force u32)iph->daddr,
+	hash = siphash_3u32((__force u32)iph->daddr,
 			    (__force u32)iph->saddr,
-			    iph->protocol ^ net_hash_mix(net),
-			    ip_idents_hashrnd);
+			    iph->protocol,
+			    &net->ipv4.ip_id_key);
 	id = ip_idents_reserve(hash, segs);
 	iph->id = htons(id);
 }

net/ipv6/output_core.c

@@ -10,15 +10,25 @@
 #include <net/secure_seq.h>
 #include <linux/netfilter.h>
 
-static u32 __ipv6_select_ident(struct net *net, u32 hashrnd,
+static u32 __ipv6_select_ident(struct net *net,
 			       const struct in6_addr *dst,
 			       const struct in6_addr *src)
 {
+	const struct {
+		struct in6_addr dst;
+		struct in6_addr src;
+	} __aligned(SIPHASH_ALIGNMENT) combined = {
+		.dst = *dst,
+		.src = *src,
+	};
 	u32 hash, id;
 
-	hash = __ipv6_addr_jhash(dst, hashrnd);
-	hash = __ipv6_addr_jhash(src, hash);
-	hash ^= net_hash_mix(net);
+	/* Note the following code is not safe, but this is okay. */
+	if (unlikely(siphash_key_is_zero(&net->ipv4.ip_id_key)))
+		get_random_bytes(&net->ipv4.ip_id_key,
+				 sizeof(net->ipv4.ip_id_key));
+
+	hash = siphash(&combined, sizeof(combined), &net->ipv4.ip_id_key);
 
 	/* Treat id of 0 as unset and if we get 0 back from ip_idents_reserve,
 	 * set the hight order instead thus minimizing possible future
@@ -41,7 +51,6 @@ static u32 __ipv6_select_ident(struct net *net, u32 hashrnd,
  */
 void ipv6_proxy_select_ident(struct net *net, struct sk_buff *skb)
 {
-	static u32 ip6_proxy_idents_hashrnd __read_mostly;
 	struct in6_addr buf[2];
 	struct in6_addr *addrs;
 	u32 id;
@@ -53,11 +62,7 @@ void ipv6_proxy_select_ident(struct net *net, struct sk_buff *skb)
 	if (!addrs)
 		return;
 
-	net_get_random_once(&ip6_proxy_idents_hashrnd,
-			    sizeof(ip6_proxy_idents_hashrnd));
-
-	id = __ipv6_select_ident(net, ip6_proxy_idents_hashrnd,
-				 &addrs[1], &addrs[0]);
+	id = __ipv6_select_ident(net, &addrs[1], &addrs[0]);
 	skb_shinfo(skb)->ip6_frag_id = htonl(id);
 }
 EXPORT_SYMBOL_GPL(ipv6_proxy_select_ident);
@@ -66,12 +71,9 @@ __be32 ipv6_select_ident(struct net *net,
 			 const struct in6_addr *daddr,
 			 const struct in6_addr *saddr)
 {
-	static u32 ip6_idents_hashrnd __read_mostly;
 	u32 id;
 
-	net_get_random_once(&ip6_idents_hashrnd, sizeof(ip6_idents_hashrnd));
-
-	id = __ipv6_select_ident(net, ip6_idents_hashrnd, daddr, saddr);
+	id = __ipv6_select_ident(net, daddr, saddr);
 	return htonl(id);
 }
 EXPORT_SYMBOL(ipv6_select_ident);