ip: load balance tcp connections to single dst addr and port

Load balance new TCP connections across nexthops also when they
connect to the same service at a single remote address and port.

This affects only port-based multipath hashing:
fib_multipath_hash_policy 1 or 3.

Local connections must choose both a source address and port when
connecting to a remote service, in ip_route_connect. This
"chicken-and-egg problem" (commit 2d7192d6cbab ("ipv4: Sanitize and
simplify ip_route_{connect,newports}()")) is resolved by first
selecting a source address, by looking up a route using the zero
wildcard source port and address.

As a result multiple connections to the same destination address and
port have no entropy in fib_multipath_hash.

This is not a problem when forwarding, as skb-based hashing has a
4-tuple. Nor when establishing UDP connections, as autobind there
selects a port before reaching ip_route_connect.

Load balance also TCP, by using a random port in fib_multipath_hash.
Port assignment in inet_hash_connect is not atomic with
ip_route_connect. Thus ports are unpredictable, effectively random.

Implementation details:

Do not actually pass a random fl4_sport, as that affects not only
hashing, but routing more broadly, and can match a source port based
policy route, which existing wildcard port 0 will not. Instead,
define a new wildcard flowi flag that is used only for hashing.

Selecting a random source is equivalent to just selecting a random
hash entirely. But for code clarity, follow the normal 4-tuple hash
process and only update this field.

fib_multipath_hash can be reached with zero sport from other code
paths, so explicitly pass this flowi flag, rather than trying to infer
this case in the function itself.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20250424143549.669426-3-willemdebruijn.kernel@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

diff --git a/include/net/flow.h b/include/net/flow.h
index 2a3f0c42f09250d3800d86cca22b22e048bd00a0..a1839c278d8738229f932dbdca7a38c7044f1db7 100644 (file)
--- a/include/net/flow.h
+++ b/include/net/flow.h
@@ -39,6 +39,7 @@ struct flowi_common {
 #define FLOWI_FLAG_ANYSRC              0x01
 #define FLOWI_FLAG_KNOWN_NH            0x02
 #define FLOWI_FLAG_L3MDEV_OIF          0x04
+#define FLOWI_FLAG_ANY_SPORT           0x08
        __u32   flowic_secid;
        kuid_t  flowic_uid;
        __u32           flowic_multipath_hash;

diff --git a/include/net/route.h b/include/net/route.h
index c605fd5ec0c08cc7658c3cf6aa6223790d463ede..8e39aa822cf98601fbf98a0837e2718c07abca9a 100644 (file)
--- a/include/net/route.h
+++ b/include/net/route.h
@@ -326,6 +326,9 @@ static inline void ip_route_connect_init(struct flowi4 *fl4, __be32 dst,
        if (inet_test_bit(TRANSPARENT, sk))
                flow_flags |= FLOWI_FLAG_ANYSRC;
 
+       if (IS_ENABLED(CONFIG_IP_ROUTE_MULTIPATH) && !sport)
+               flow_flags |= FLOWI_FLAG_ANY_SPORT;
+
        flowi4_init_output(fl4, oif, READ_ONCE(sk->sk_mark), ip_sock_rt_tos(sk),
                           ip_sock_rt_scope(sk), protocol, flow_flags, dst,
                           src, dport, sport, sk->sk_uid);

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index e5e4c71be3afbc64d2051ef2256022e301e3efa7..507b2e5dec50f7dbf9d35d8d8bbbb15de61834b2 100644 (file)
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -2037,8 +2037,12 @@ static u32 fib_multipath_custom_hash_fl4(const struct net *net,
                hash_keys.addrs.v4addrs.dst = fl4->daddr;
        if (hash_fields & FIB_MULTIPATH_HASH_FIELD_IP_PROTO)
                hash_keys.basic.ip_proto = fl4->flowi4_proto;
-       if (hash_fields & FIB_MULTIPATH_HASH_FIELD_SRC_PORT)
-               hash_keys.ports.src = fl4->fl4_sport;
+       if (hash_fields & FIB_MULTIPATH_HASH_FIELD_SRC_PORT) {
+               if (fl4->flowi4_flags & FLOWI_FLAG_ANY_SPORT)
+                       hash_keys.ports.src = (__force __be16)get_random_u16();
+               else
+                       hash_keys.ports.src = fl4->fl4_sport;
+       }
        if (hash_fields & FIB_MULTIPATH_HASH_FIELD_DST_PORT)
                hash_keys.ports.dst = fl4->fl4_dport;
 
@@ -2093,7 +2097,10 @@ int fib_multipath_hash(const struct net *net, const struct flowi4 *fl4,
                        hash_keys.control.addr_type = FLOW_DISSECTOR_KEY_IPV4_ADDRS;
                        hash_keys.addrs.v4addrs.src = fl4->saddr;
                        hash_keys.addrs.v4addrs.dst = fl4->daddr;
-                       hash_keys.ports.src = fl4->fl4_sport;
+                       if (fl4->flowi4_flags & FLOWI_FLAG_ANY_SPORT)
+                               hash_keys.ports.src = (__force __be16)get_random_u16();
+                       else
+                               hash_keys.ports.src = fl4->fl4_sport;
                        hash_keys.ports.dst = fl4->fl4_dport;
                        hash_keys.basic.ip_proto = fl4->flowi4_proto;
                }

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index d0351e95d91618ad9d955605d3cbeda43280bba9..aa6b45bd35157a25cc71e6e3cff36d609f945d57 100644 (file)
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -2492,8 +2492,12 @@ static u32 rt6_multipath_custom_hash_fl6(const struct net *net,
                hash_keys.basic.ip_proto = fl6->flowi6_proto;
        if (hash_fields & FIB_MULTIPATH_HASH_FIELD_FLOWLABEL)
                hash_keys.tags.flow_label = (__force u32)flowi6_get_flowlabel(fl6);
-       if (hash_fields & FIB_MULTIPATH_HASH_FIELD_SRC_PORT)
-               hash_keys.ports.src = fl6->fl6_sport;
+       if (hash_fields & FIB_MULTIPATH_HASH_FIELD_SRC_PORT) {
+               if (fl6->flowi6_flags & FLOWI_FLAG_ANY_SPORT)
+                       hash_keys.ports.src = (__force __be16)get_random_u16();
+               else
+                       hash_keys.ports.src = fl6->fl6_sport;
+       }
        if (hash_fields & FIB_MULTIPATH_HASH_FIELD_DST_PORT)
                hash_keys.ports.dst = fl6->fl6_dport;
 
@@ -2547,7 +2551,10 @@ u32 rt6_multipath_hash(const struct net *net, const struct flowi6 *fl6,
                        hash_keys.control.addr_type = FLOW_DISSECTOR_KEY_IPV6_ADDRS;
                        hash_keys.addrs.v6addrs.src = fl6->saddr;
                        hash_keys.addrs.v6addrs.dst = fl6->daddr;
-                       hash_keys.ports.src = fl6->fl6_sport;
+                       if (fl6->flowi6_flags & FLOWI_FLAG_ANY_SPORT)
+                               hash_keys.ports.src = (__force __be16)get_random_u16();
+                       else
+                               hash_keys.ports.src = fl6->fl6_sport;
                        hash_keys.ports.dst = fl6->fl6_dport;
                        hash_keys.basic.ip_proto = fl6->flowi6_proto;
                }

diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 7dcb33f879ee17b2377f1f0c41f68e4d67798c76..e8e68a14264991132656ddaa8dd9bb84bb586c97 100644 (file)
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -267,6 +267,8 @@ static int tcp_v6_connect(struct sock *sk, struct sockaddr *uaddr,
        fl6.flowi6_mark = sk->sk_mark;
        fl6.fl6_dport = usin->sin6_port;
        fl6.fl6_sport = inet->inet_sport;
+       if (IS_ENABLED(CONFIG_IP_ROUTE_MULTIPATH) && !fl6.fl6_sport)
+               fl6.flowi6_flags = FLOWI_FLAG_ANY_SPORT;
        fl6.flowi6_uid = sk->sk_uid;
 
        opt = rcu_dereference_protected(np->opt, lockdep_sock_is_held(sk));