inet: update the IP ID generation algorithm to higher standards.

Commit 355b985 ("netns: provide pure entropy for net_hash_mix()")
makes net_hash_mix() return a true 32 bits of entropy.  When used in the
IP ID generation algorithm, this has the effect of extending the IP ID
generation key from 32 bits to 64 bits.

However, net_hash_mix() is only used for IP ID generation starting with
kernel version 4.1.  Therefore, earlier kernels remain with 32-bit key
no matter what the net_hash_mix() return value is.

This change addresses the issue by explicitly extending the key to 64
bits for kernels older than 4.1.

Signed-off-by: Amit Klein <aksecurity@gmail.com>
Cc: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/ipv4/route.c

@@ -488,13 +488,15 @@ EXPORT_SYMBOL(ip_idents_reserve);
 void __ip_select_ident(struct iphdr *iph, int segs)
 {
 	static u32 ip_idents_hashrnd __read_mostly;
+	static u32 ip_idents_hashrnd_extra __read_mostly;
 	u32 hash, id;
 
 	net_get_random_once(&ip_idents_hashrnd, sizeof(ip_idents_hashrnd));
+	net_get_random_once(&ip_idents_hashrnd_extra, sizeof(ip_idents_hashrnd_extra));
 
 	hash = jhash_3words((__force u32)iph->daddr,
 			    (__force u32)iph->saddr,
-			    iph->protocol,
+			    iph->protocol ^ ip_idents_hashrnd_extra,
 			    ip_idents_hashrnd);
 	id = ip_idents_reserve(hash, segs);
 	iph->id = htons(id);

net/ipv6/ip6_output.c

@@ -546,12 +546,15 @@ static void ip6_copy_metadata(struct sk_buff *to, struct sk_buff *from)
 static void ipv6_select_ident(struct frag_hdr *fhdr, struct rt6_info *rt)
 {
 	static u32 ip6_idents_hashrnd __read_mostly;
+	static u32 ip6_idents_hashrnd_extra __read_mostly;
 	u32 hash, id;
 
 	net_get_random_once(&ip6_idents_hashrnd, sizeof(ip6_idents_hashrnd));
+	net_get_random_once(&ip6_idents_hashrnd_extra, sizeof(ip6_idents_hashrnd_extra));
 
 	hash = __ipv6_addr_jhash(&rt->rt6i_dst.addr, ip6_idents_hashrnd);
 	hash = __ipv6_addr_jhash(&rt->rt6i_src.addr, hash);
+	hash = jhash_1word(hash, ip6_idents_hashrnd_extra);
 
 	id = ip_idents_reserve(hash, 1);
 	fhdr->identification = htonl(id);