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 df453700e8d8
("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: b56774163f99 ("ipv6: Enable auto flow labels by default")
Fixes: 42240901f7c4 ("ipv6: Implement different admin modes for automatic flow labels")
Fixes: 67800f9b1f4e ("ipv6: Call skb_get_hash_flowi6 to get skb->hash in ip6_make_flowlabel")
Fixes: cb1ce2ef387b ("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>

1 files changed, 8 insertions, 6 deletions

diff --git a/net/sched/sch_sfq.c b/net/sched/sch_sfq.c
index 68404a9d2ce4..c787d4d46017 100644
--- a/net/sched/sch_sfq.c
+++ b/net/sched/sch_sfq.c
@@ -14,7 +14,7 @@
 #include <linux/errno.h>
 #include <linux/init.h>
 #include <linux/skbuff.h>
-#include <linux/jhash.h>
+#include <linux/siphash.h>
 #include <linux/slab.h>
 #include <linux/vmalloc.h>
 #include <net/netlink.h>
@@ -117,7 +117,7 @@ struct sfq_sched_data {
 	u8		headdrop;
 	u8		maxdepth;	/* limit of packets per flow */
 
-	u32		perturbation;
+	siphash_key_t 	perturbation;
 	u8		cur_depth;	/* depth of longest slot */
 	u8		flags;
 	unsigned short  scaled_quantum; /* SFQ_ALLOT_SIZE(quantum) */
@@ -157,7 +157,7 @@ static inline struct sfq_head *sfq_dep_head(struct sfq_sched_data *q, sfq_index
 static unsigned int sfq_hash(const struct sfq_sched_data *q,
 			     const struct sk_buff *skb)
 {
-	return skb_get_hash_perturb(skb, q->perturbation) & (q->divisor - 1);
+	return skb_get_hash_perturb(skb, &q->perturbation) & (q->divisor - 1);
 }
 
 static unsigned int sfq_classify(struct sk_buff *skb, struct Qdisc *sch,
@@ -607,9 +607,11 @@ static void sfq_perturbation(struct timer_list *t)
 	struct sfq_sched_data *q = from_timer(q, t, perturb_timer);
 	struct Qdisc *sch = q->sch;
 	spinlock_t *root_lock = qdisc_lock(qdisc_root_sleeping(sch));
+	siphash_key_t nkey;
 
+	get_random_bytes(&nkey, sizeof(nkey));
 	spin_lock(root_lock);
-	q->perturbation = prandom_u32();
+	q->perturbation = nkey;
 	if (!q->filter_list && q->tail)
 		sfq_rehash(sch);
 	spin_unlock(root_lock);
@@ -688,7 +690,7 @@ static int sfq_change(struct Qdisc *sch, struct nlattr *opt)
 	del_timer(&q->perturb_timer);
 	if (q->perturb_period) {
 		mod_timer(&q->perturb_timer, jiffies + q->perturb_period);
-		q->perturbation = prandom_u32();
+		get_random_bytes(&q->perturbation, sizeof(q->perturbation));
 	}
 	sch_tree_unlock(sch);
 	kfree(p);
@@ -745,7 +747,7 @@ static int sfq_init(struct Qdisc *sch, struct nlattr *opt,
 	q->quantum = psched_mtu(qdisc_dev(sch));
 	q->scaled_quantum = SFQ_ALLOT_SIZE(q->quantum);
 	q->perturb_period = 0;
-	q->perturbation = prandom_u32();
+	get_random_bytes(&q->perturbation, sizeof(q->perturbation));
 
 	if (opt) {
 		int err = sfq_change(sch, opt);