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, 16 insertions, 22 deletions

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 7c09d87d3269..68eda10d0680 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -1350,30 +1350,21 @@ out_bad:
 }
 EXPORT_SYMBOL(__skb_flow_dissect);
 
-static u32 hashrnd __read_mostly;
+static siphash_key_t hashrnd __read_mostly;
 static __always_inline void __flow_hash_secret_init(void)
 {
 	net_get_random_once(&hashrnd, sizeof(hashrnd));
 }
 
-static __always_inline u32 __flow_hash_words(const u32 *words, u32 length,
-					     u32 keyval)
+static const void *flow_keys_hash_start(const struct flow_keys *flow)
 {
-	return jhash2(words, length, keyval);
-}
-
-static inline const u32 *flow_keys_hash_start(const struct flow_keys *flow)
-{
-	const void *p = flow;
-
-	BUILD_BUG_ON(FLOW_KEYS_HASH_OFFSET % sizeof(u32));
-	return (const u32 *)(p + FLOW_KEYS_HASH_OFFSET);
+	BUILD_BUG_ON(FLOW_KEYS_HASH_OFFSET % SIPHASH_ALIGNMENT);
+	return &flow->FLOW_KEYS_HASH_START_FIELD;
 }
 
 static inline size_t flow_keys_hash_length(const struct flow_keys *flow)
 {
 	size_t diff = FLOW_KEYS_HASH_OFFSET + sizeof(flow->addrs);
-	BUILD_BUG_ON((sizeof(*flow) - FLOW_KEYS_HASH_OFFSET) % sizeof(u32));
 	BUILD_BUG_ON(offsetof(typeof(*flow), addrs) !=
 		     sizeof(*flow) - sizeof(flow->addrs));
 
@@ -1388,7 +1379,7 @@ static inline size_t flow_keys_hash_length(const struct flow_keys *flow)
 		diff -= sizeof(flow->addrs.tipckey);
 		break;
 	}
-	return (sizeof(*flow) - diff) / sizeof(u32);
+	return sizeof(*flow) - diff;
 }
 
 __be32 flow_get_u32_src(const struct flow_keys *flow)
@@ -1454,14 +1445,15 @@ static inline void __flow_hash_consistentify(struct flow_keys *keys)
 	}
 }
 
-static inline u32 __flow_hash_from_keys(struct flow_keys *keys, u32 keyval)
+static inline u32 __flow_hash_from_keys(struct flow_keys *keys,
+					const siphash_key_t *keyval)
 {
 	u32 hash;
 
 	__flow_hash_consistentify(keys);
 
-	hash = __flow_hash_words(flow_keys_hash_start(keys),
-				 flow_keys_hash_length(keys), keyval);
+	hash = siphash(flow_keys_hash_start(keys),
+		       flow_keys_hash_length(keys), keyval);
 	if (!hash)
 		hash = 1;
 
@@ -1471,12 +1463,13 @@ static inline u32 __flow_hash_from_keys(struct flow_keys *keys, u32 keyval)
 u32 flow_hash_from_keys(struct flow_keys *keys)
 {
 	__flow_hash_secret_init();
-	return __flow_hash_from_keys(keys, hashrnd);
+	return __flow_hash_from_keys(keys, &hashrnd);
 }
 EXPORT_SYMBOL(flow_hash_from_keys);
 
 static inline u32 ___skb_get_hash(const struct sk_buff *skb,
-				  struct flow_keys *keys, u32 keyval)
+				  struct flow_keys *keys,
+				  const siphash_key_t *keyval)
 {
 	skb_flow_dissect_flow_keys(skb, keys,
 				   FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL);
@@ -1524,7 +1517,7 @@ u32 __skb_get_hash_symmetric(const struct sk_buff *skb)
 			   &keys, NULL, 0, 0, 0,
 			   FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL);
 
-	return __flow_hash_from_keys(&keys, hashrnd);
+	return __flow_hash_from_keys(&keys, &hashrnd);
 }
 EXPORT_SYMBOL_GPL(__skb_get_hash_symmetric);
 
@@ -1544,13 +1537,14 @@ void __skb_get_hash(struct sk_buff *skb)
 
 	__flow_hash_secret_init();
 
-	hash = ___skb_get_hash(skb, &keys, hashrnd);
+	hash = ___skb_get_hash(skb, &keys, &hashrnd);
 
 	__skb_set_sw_hash(skb, hash, flow_keys_have_l4(&keys));
 }
 EXPORT_SYMBOL(__skb_get_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)
 {
 	struct flow_keys keys;