ipv6: use siphash in rt6_exception_hash()

A group of security researchers brought to our attention
the weakness of hash function used in rt6_exception_hash()

Lets use siphash instead of Jenkins Hash, to considerably
reduce security risks.

Following patch deals with IPv4.

Fixes: 35732d0 ("ipv6: introduce a hash table to store dst cache")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Keyu Man <kman001@ucr.edu>
Cc: Wei Wang <weiwan@google.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Acked-by: Wei Wang <weiwan@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: edumazet

net/ipv6/route.c

@@ -41,6 +41,7 @@
 #include <linux/nsproxy.h>
 #include <linux/slab.h>
 #include <linux/jhash.h>
+#include <linux/siphash.h>
 #include <net/net_namespace.h>
 #include <net/snmp.h>
 #include <net/ipv6.h>
@@ -1484,17 +1485,24 @@ static void rt6_exception_remove_oldest(struct rt6_exception_bucket *bucket)
 static u32 rt6_exception_hash(const struct in6_addr *dst,
 			      const struct in6_addr *src)
 {
-	static u32 seed __read_mostly;
-	u32 val;
+	static siphash_key_t rt6_exception_key __read_mostly;
+	struct {
+		struct in6_addr dst;
+		struct in6_addr src;
+	} __aligned(SIPHASH_ALIGNMENT) combined = {
+		.dst = *dst,
+	};
+	u64 val;
 
-	net_get_random_once(&seed, sizeof(seed));
-	val = jhash2((const u32 *)dst, sizeof(*dst)/sizeof(u32), seed);
+	net_get_random_once(&rt6_exception_key, sizeof(rt6_exception_key));
 
 #ifdef CONFIG_IPV6_SUBTREES
 	if (src)
-		val = jhash2((const u32 *)src, sizeof(*src)/sizeof(u32), val);
+		combined.src = *src;
 #endif
-	return hash_32(val, FIB6_EXCEPTION_BUCKET_SIZE_SHIFT);
+	val = siphash(&combined, sizeof(combined), &rt6_exception_key);
+
+	return hash_64(val, FIB6_EXCEPTION_BUCKET_SIZE_SHIFT);
 }
 
 /* Helper function to find the cached rt in the hash table