net/flow_dissector: switch to siphash

UDP IPv6 packets auto flowlabels are using a 32bit secret
(static u32 hashrnd in net/core/flow_dissector.c) and
apply jhash() over fields known by the receivers.

Attackers can easily infer the 32bit secret and use this information
to identify a device and/or user, since this 32bit secret is only
set at boot time.

Really, using jhash() to generate cookies sent on the wire
is a serious security concern.

Trying to change the rol32(hash, 16) in ip6_make_flowlabel() would be
a dead end. Trying to periodically change the secret (like in sch_sfq.c)
could change paths taken in the network for long lived flows.

Let's switch to siphash, as we did in commit df453700e8
("inet: switch IP ID generator to siphash")

Using a cryptographically strong pseudo random function will solve this
privacy issue and more generally remove other weak points in the stack.

Packet schedulers using skb_get_hash_perturb() benefit from this change.

Fixes: b56774163f ("ipv6: Enable auto flow labels by default")
Fixes: 42240901f7 ("ipv6: Implement different admin modes for automatic flow labels")
Fixes: 67800f9b1f ("ipv6: Call skb_get_hash_flowi6 to get skb->hash in ip6_make_flowlabel")
Fixes: cb1ce2ef38 ("ipv6: Implement automatic flow label generation on transmit")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Jonathan Berger <jonathann1@walla.com>
Reported-by: Amit Klein <aksecurity@gmail.com>
Reported-by: Benny Pinkas <benny@pinkas.net>
Cc: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_sfb.c

@@ -18,7 +18,7 @@
 #include <linux/errno.h>
 #include <linux/skbuff.h>
 #include <linux/random.h>
-#include <linux/jhash.h>
+#include <linux/siphash.h>
 #include <net/ip.h>
 #include <net/pkt_sched.h>
 #include <net/pkt_cls.h>
@@ -45,7 +45,7 @@ struct sfb_bucket {
  * (Section 4.4 of SFB reference : moving hash functions)
  */
 struct sfb_bins {
-	u32		  perturbation; /* jhash perturbation */
+	siphash_key_t	  perturbation; /* siphash key */
 	struct sfb_bucket bins[SFB_LEVELS][SFB_NUMBUCKETS];
 };
 
@@ -217,7 +217,8 @@ static u32 sfb_compute_qlen(u32 *prob_r, u32 *avgpm_r, const struct sfb_sched_da
 
 static void sfb_init_perturbation(u32 slot, struct sfb_sched_data *q)
 {
-	q->bins[slot].perturbation = prandom_u32();
+	get_random_bytes(&q->bins[slot].perturbation,
+			 sizeof(q->bins[slot].perturbation));
 }
 
 static void sfb_swap_slot(struct sfb_sched_data *q)
@@ -314,9 +315,9 @@ static int sfb_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		/* If using external classifiers, get result and record it. */
 		if (!sfb_classify(skb, fl, &ret, &salt))
 			goto other_drop;
-		sfbhash = jhash_1word(salt, q->bins[slot].perturbation);
+		sfbhash = siphash_1u32(salt, &q->bins[slot].perturbation);
 	} else {
-		sfbhash = skb_get_hash_perturb(skb, q->bins[slot].perturbation);
+		sfbhash = skb_get_hash_perturb(skb, &q->bins[slot].perturbation);
 	}
 
 
@@ -352,7 +353,7 @@ static int sfb_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		/* Inelastic flow */
 		if (q->double_buffering) {
 			sfbhash = skb_get_hash_perturb(skb,
-			    q->bins[slot].perturbation);
+			    &q->bins[slot].perturbation);
 			if (!sfbhash)
 				sfbhash = 1;
 			sfb_skb_cb(skb)->hashes[slot] = sfbhash;