On Thu, Jun 29, 2023 at 3:59 AM Samuel Mendoza-Jonas samjonas@amazon.com wrote:
From: Stewart Smith trawets@amazon.com
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.
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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@vger.kernel.org Fixes: 08dcdbf6a7b9 ("ipv6: use a stronger hash for tcp") Fixes: b3da2cf37c5c ("[INET]: Use jhash + random secret for ehash.") Signed-off-by: Stewart Smith trawets@amazon.com Signed-off-by: Samuel Mendoza-Jonas samjonas@amazon.com
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);
Hmmm... see my following comment.
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);
This seems wrong to me.
Reusing ipv6_hash_secret and other keys twice is not good, I am sure some security researchers would love this...
Please just change __ipv6_addr_jhash(), so that all users can benefit from a more secure version ? It also leaves lhash / fhash names relevant here.
We will probably have to switch to sip (or other stronger hash than jhash) at some point, it is a tradeoff.
We might also add a break in the loop when a bucket exceeds a given safety length, because attackers can eventually exploit hashes after some point.
The following patch looks much saner to me.
diff --git a/include/net/ipv6.h b/include/net/ipv6.h index 7332296eca44b84dca1bbecb545f6824a0e8ed3d..2acc4c808d45d1c1bb1c5076e79842e136203e4c 100644 --- a/include/net/ipv6.h +++ b/include/net/ipv6.h @@ -752,12 +752,8 @@ 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, - (__force u32)a->s6_addr32[2], - (__force u32)a->s6_addr32[3], - initval); + return jhash2((__force const u32 *)a->s6_addr32, + ARRAY_SIZE(a->s6_addr32), initval); }