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>

4 files changed, 7 insertions, 5 deletions

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 7914fdaf4226..a391147c03d4 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1354,7 +1354,8 @@ static inline __u32 skb_get_hash_flowi6(struct sk_buff *skb, const struct flowi6
 	return skb->hash;
 }
 
-__u32 skb_get_hash_perturb(const struct sk_buff *skb, u32 perturb);
+__u32 skb_get_hash_perturb(const struct sk_buff *skb,
+			   const siphash_key_t *perturb);
 
 static inline __u32 skb_get_hash_raw(const struct sk_buff *skb)
 {
diff --git a/include/net/flow_dissector.h b/include/net/flow_dissector.h
index 90bd210be060..5cd12276ae21 100644
--- a/include/net/flow_dissector.h
+++ b/include/net/flow_dissector.h
@@ -4,6 +4,7 @@
 
 #include <linux/types.h>
 #include <linux/in6.h>
+#include <linux/siphash.h>
 #include <uapi/linux/if_ether.h>
 
 /**
@@ -276,7 +277,7 @@ struct flow_keys_basic {
 struct flow_keys {
 	struct flow_dissector_key_control control;
 #define FLOW_KEYS_HASH_START_FIELD basic
-	struct flow_dissector_key_basic basic;
+	struct flow_dissector_key_basic basic __aligned(SIPHASH_ALIGNMENT);
 	struct flow_dissector_key_tags tags;
 	struct flow_dissector_key_vlan vlan;
 	struct flow_dissector_key_vlan cvlan;
diff --git a/include/net/fq.h b/include/net/fq.h
index d126b5d20261..2ad85e683041 100644
--- a/include/net/fq.h
+++ b/include/net/fq.h
@@ -69,7 +69,7 @@ struct fq {
 	struct list_head backlogs;
 	spinlock_t lock;
 	u32 flows_cnt;
-	u32 perturbation;
+	siphash_key_t	perturbation;
 	u32 limit;
 	u32 memory_limit;
 	u32 memory_usage;
diff --git a/include/net/fq_impl.h b/include/net/fq_impl.h
index be40a4b327e3..107c0d700ed6 100644
--- a/include/net/fq_impl.h
+++ b/include/net/fq_impl.h
@@ -108,7 +108,7 @@ begin:
 
 static u32 fq_flow_idx(struct fq *fq, struct sk_buff *skb)
 {
-	u32 hash = skb_get_hash_perturb(skb, fq->perturbation);
+	u32 hash = skb_get_hash_perturb(skb, &fq->perturbation);
 
 	return reciprocal_scale(hash, fq->flows_cnt);
 }
@@ -308,7 +308,7 @@ static int fq_init(struct fq *fq, int flows_cnt)
 	INIT_LIST_HEAD(&fq->backlogs);
 	spin_lock_init(&fq->lock);
 	fq->flows_cnt = max_t(u32, flows_cnt, 1);
-	fq->perturbation = prandom_u32();
+	get_random_bytes(&fq->perturbation, sizeof(fq->perturbation));
 	fq->quantum = 300;
 	fq->limit = 8192;
 	fq->memory_limit = 16 << 20; /* 16 MBytes */