[PATCH net] net/ipv6: Reduce chance of collisions in inet6_hashfn()

[Samuel Mendoza-Jonas]

From: Stewart Smith <trawets@xxxxxxxxxx>

For both IPv4 and IPv6 incoming TCP connections are tracked in a hash
table with a hash over the source & destination addresses and ports.
However, the IPv6 hash is insufficient and can lead to a high rate of
collisions.

The IPv6 hash used an XOR to fit everything into the 96 bits for the
fast jenkins hash, meaning it is possible for an external entity to
ensure the hash collides, thus falling back to a linear search in the
bucket, which is slow.

We take the approach of hash half the data; hash the other half; and
then hash them together. We do this with 3x jenkins hashes rather than
2x to calculate the hashing value for the connection covering the full
length of the addresses and ports.

While this may look like it adds overhead, the reality of modern CPUs
means that this is unmeasurable in real world scenarios.

In simulating with llvm-mca, the increase in cycles for the hashing code
was ~5 cycles on Skylake (from a base of ~50), and an extra ~9 on
Nehalem (base of ~62).

In commit dd6d2910c5e0 ("netfilter: conntrack: switch to siphash")
netfilter switched from a jenkins hash to a siphash, but even the faster
hsiphash is a more significant overhead (~20-30%) in some preliminary
testing. So, in this patch, we keep to the more conservative approach to
ensure we don't add much overhead per SYN.

In testing, this results in a consistently even spread across the
connection buckets. In both testing and real-world scenarios, we have
not found any measurable performance impact.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 08dcdbf6a7b9 ("ipv6: use a stronger hash for tcp")
Fixes: b3da2cf37c5c ("[INET]: Use jhash + random secret for ehash.")
Signed-off-by: Stewart Smith <trawets@xxxxxxxxxx>
Signed-off-by: Samuel Mendoza-Jonas <samjonas@xxxxxxxxxx>
---
 include/net/ipv6.h          | 4 +---
 net/ipv6/inet6_hashtables.c | 5 ++++-
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/include/net/ipv6.h b/include/net/ipv6.h
index 7332296eca44..f9bb54869d82 100644
--- a/include/net/ipv6.h
+++ b/include/net/ipv6.h
@@ -752,9 +752,7 @@ static inline u32 ipv6_addr_hash(const struct in6_addr *a)
 /* more secured version of ipv6_addr_hash() */
 static inline u32 __ipv6_addr_jhash(const struct in6_addr *a, const u32 initval)
 {
-	u32 v = (__force u32)a->s6_addr32[0] ^ (__force u32)a->s6_addr32[1];
-
-	return jhash_3words(v,
+	return jhash_3words((__force u32)a->s6_addr32[1],
 			    (__force u32)a->s6_addr32[2],
 			    (__force u32)a->s6_addr32[3],
 			    initval);
diff --git a/net/ipv6/inet6_hashtables.c b/net/ipv6/inet6_hashtables.c
index b64b49012655..bb7198081974 100644
--- a/net/ipv6/inet6_hashtables.c
+++ b/net/ipv6/inet6_hashtables.c
@@ -33,7 +33,10 @@ u32 inet6_ehashfn(const struct net *net,
 	net_get_random_once(&inet6_ehash_secret, sizeof(inet6_ehash_secret));
 	net_get_random_once(&ipv6_hash_secret, sizeof(ipv6_hash_secret));
 
-	lhash = (__force u32)laddr->s6_addr32[3];
+	lhash = jhash_3words((__force u32)laddr->s6_addr32[3],
+			    (((u32)lport) << 16) | (__force u32)fport,
+			    (__force u32)faddr->s6_addr32[0],
+			    ipv6_hash_secret);
 	fhash = __ipv6_addr_jhash(faddr, ipv6_hash_secret);
 
 	return __inet6_ehashfn(lhash, lport, fhash, fport,
-- 
2.25.1