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Subject[PATCH v5 net-next 1/3] net/ipv4: multipath routing: configurable seed
Ability for a user to assign seed value to multipath route hashes.
Now kernel uses random seed value to prevent hash-flooding DoS attacks;
however, it disables some use cases, f.e:

+-------+ +------+ +--------+
| |-eth0---| FW0 |---eth0-| |
| | +------+ | |
| GW0 |ECMP ECMP| GW1 |
| | +------+ | |
| |-eth1---| FW1 |---eth1-| |
+-------+ +------+ +--------+

In this use case, two ECMP routers balance traffic between two firewalls.
If some flow transmits a response over a different channel than request,
such flow will be dropped, because keep-state rules are created on
the other firewall.

This patch adds sysctl variable: net.ipv4.fib_multipath_hash_seed.
User can set the same seed value on GW0 and GW1 for traffic to be
mirror-balanced. By default, random value is used.

Signed-off-by: Balaev Pavel <balaevpa@infotecs.ru>
---
Documentation/networking/ip-sysctl.rst | 14 ++++
include/net/flow_dissector.h | 2 +
include/net/netns/ipv4.h | 2 +
net/core/flow_dissector.c | 7 ++
net/ipv4/route.c | 10 ++-
net/ipv4/sysctl_net_ipv4.c | 97 ++++++++++++++++++++++++++
6 files changed, 131 insertions(+), 1 deletion(-)

diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index 9701906f6..d1a67e6fe 100644
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst
@@ -100,6 +100,20 @@ fib_multipath_hash_policy - INTEGER
- 1 - Layer 4
- 2 - Layer 3 or inner Layer 3 if present

+fib_multipath_hash_seed - STRING
+ Controls seed value for multipath route hashes. By default
+ random value is used. Only valid for kernels built with
+ CONFIG_IP_ROUTE_MULTIPATH enabled.
+
+ Valid format: two hex values set off with comma or "random"
+ keyword.
+
+ Example to generate the seed value::
+
+ RAND=$(openssl rand -hex 16) && echo "${RAND:0:16},${RAND:16:16}"
+
+ Default: "random"
+
fib_sync_mem - UNSIGNED INTEGER
Amount of dirty memory from fib entries that can be backlogged before
synchronize_rcu is forced.
diff --git a/include/net/flow_dissector.h b/include/net/flow_dissector.h
index ffd386ea0..d104c013a 100644
--- a/include/net/flow_dissector.h
+++ b/include/net/flow_dissector.h
@@ -348,6 +348,8 @@ static inline bool flow_keys_have_l4(const struct flow_keys *keys)
}

u32 flow_hash_from_keys(struct flow_keys *keys);
+u32 flow_multipath_hash_from_keys(struct flow_keys *keys,
+ const siphash_key_t *seed);
void skb_flow_get_icmp_tci(const struct sk_buff *skb,
struct flow_dissector_key_icmp *key_icmp,
const void *data, int thoff, int hlen);
diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
index 87e161249..cb2830432 100644
--- a/include/net/netns/ipv4.h
+++ b/include/net/netns/ipv4.h
@@ -222,6 +222,8 @@ struct netns_ipv4 {
#ifdef CONFIG_IP_ROUTE_MULTIPATH
u8 sysctl_fib_multipath_use_neigh;
u8 sysctl_fib_multipath_hash_policy;
+ int sysctl_fib_multipath_hash_seed;
+ siphash_key_t __rcu *fib_multipath_hash_seed_ctx;
#endif

struct fib_notifier_ops *notifier_ops;
diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 5985029e4..febd1094c 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -1560,6 +1560,13 @@ u32 flow_hash_from_keys(struct flow_keys *keys)
}
EXPORT_SYMBOL(flow_hash_from_keys);

+u32 flow_multipath_hash_from_keys(struct flow_keys *keys,
+ const siphash_key_t *seed)
+{
+ return __flow_hash_from_keys(keys, seed);
+}
+EXPORT_SYMBOL(flow_multipath_hash_from_keys);
+
static inline u32 ___skb_get_hash(const struct sk_buff *skb,
struct flow_keys *keys,
const siphash_key_t *keyval)
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index f6787c55f..79866b429 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1912,6 +1912,7 @@ int fib_multipath_hash(const struct net *net, const struct flowi4 *fl4,
{
u32 multipath_hash = fl4 ? fl4->flowi4_multipath_hash : 0;
struct flow_keys hash_keys;
+ siphash_key_t *seed_ctx;
u32 mhash;

switch (net->ipv4.sysctl_fib_multipath_hash_policy) {
@@ -1989,7 +1990,14 @@ int fib_multipath_hash(const struct net *net, const struct flowi4 *fl4,
}
break;
}
- mhash = flow_hash_from_keys(&hash_keys);
+
+ rcu_read_lock();
+ seed_ctx = rcu_dereference(net->ipv4.fib_multipath_hash_seed_ctx);
+ if (seed_ctx)
+ mhash = flow_multipath_hash_from_keys(&hash_keys, seed_ctx);
+ else
+ mhash = flow_hash_from_keys(&hash_keys);
+ rcu_read_unlock();

if (multipath_hash)
mhash = jhash_2words(mhash, multipath_hash, 0);
diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index a09e466ce..5dff59733 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -447,6 +447,8 @@ static int proc_tcp_available_ulp(struct ctl_table *ctl,
}

#ifdef CONFIG_IP_ROUTE_MULTIPATH
+#define FIB_MULTIPATH_SEED_KEY_LENGTH sizeof(siphash_key_t)
+#define FIB_MULTIPATH_SEED_RANDOM "random"
static int proc_fib_multipath_hash_policy(struct ctl_table *table, int write,
void *buffer, size_t *lenp,
loff_t *ppos)
@@ -461,6 +463,93 @@ static int proc_fib_multipath_hash_policy(struct ctl_table *table, int write,

return ret;
}
+
+static int proc_fib_multipath_hash_seed(struct ctl_table *table, int write,
+ void *buffer, size_t *lenp,
+ loff_t *ppos)
+{
+ struct net *net = container_of(table->data, struct net,
+ ipv4.sysctl_fib_multipath_hash_seed);
+ /* maxlen to print the keys in hex (*2) and a comma in between keys. */
+ struct ctl_table tbl = {
+ .maxlen = ((FIB_MULTIPATH_SEED_KEY_LENGTH * 2) + 2)
+ };
+ siphash_key_t user_key, *ctx;
+ __le64 key[2];
+ int ret;
+
+ tbl.data = kmalloc(tbl.maxlen, GFP_KERNEL);
+
+ if (!tbl.data)
+ return -ENOMEM;
+
+ rcu_read_lock();
+ ctx = rcu_dereference(net->ipv4.fib_multipath_hash_seed_ctx);
+ if (ctx) {
+ put_unaligned_le64(ctx->key[0], &key[0]);
+ put_unaligned_le64(ctx->key[1], &key[1]);
+ user_key.key[0] = le64_to_cpu(key[0]);
+ user_key.key[1] = le64_to_cpu(key[1]);
+
+ snprintf(tbl.data, tbl.maxlen, "%016llx,%016llx",
+ user_key.key[0], user_key.key[1]);
+ } else {
+ snprintf(tbl.data, tbl.maxlen, "%s", FIB_MULTIPATH_SEED_RANDOM);
+ }
+ rcu_read_unlock();
+
+ ret = proc_dostring(&tbl, write, buffer, lenp, ppos);
+
+ if (write && ret == 0) {
+ siphash_key_t *new_ctx, *old_ctx;
+
+ if (!strcmp(tbl.data, FIB_MULTIPATH_SEED_RANDOM)) {
+ rtnl_lock();
+ old_ctx = rtnl_dereference(net->ipv4.fib_multipath_hash_seed_ctx);
+ RCU_INIT_POINTER(net->ipv4.fib_multipath_hash_seed_ctx, NULL);
+ rtnl_unlock();
+ if (old_ctx) {
+ synchronize_net();
+ kfree_sensitive(old_ctx);
+ }
+
+ pr_debug("multipath hash seed set to random value\n");
+ goto out;
+ }
+
+ if (sscanf(tbl.data, "%llx,%llx", user_key.key, user_key.key + 1) != 2) {
+ ret = -EINVAL;
+ goto out;
+ }
+
+ key[0] = cpu_to_le64(user_key.key[0]);
+ key[1] = cpu_to_le64(user_key.key[1]);
+ pr_debug("multipath hash seed set to 0x%llx,0x%llx\n",
+ user_key.key[0], user_key.key[1]);
+
+ new_ctx = kmalloc(sizeof(*new_ctx), GFP_KERNEL);
+ if (!new_ctx) {
+ ret = -ENOMEM;
+ goto out;
+ }
+
+ new_ctx->key[0] = get_unaligned_le64(&key[0]);
+ new_ctx->key[1] = get_unaligned_le64(&key[1]);
+
+ rtnl_lock();
+ old_ctx = rtnl_dereference(net->ipv4.fib_multipath_hash_seed_ctx);
+ rcu_assign_pointer(net->ipv4.fib_multipath_hash_seed_ctx, new_ctx);
+ rtnl_unlock();
+ if (old_ctx) {
+ synchronize_net();
+ kfree_sensitive(old_ctx);
+ }
+ }
+
+out:
+ kfree(tbl.data);
+ return ret;
+}
#endif

static struct ctl_table ipv4_table[] = {
@@ -1052,6 +1141,14 @@ static struct ctl_table ipv4_net_table[] = {
.extra1 = SYSCTL_ZERO,
.extra2 = &two,
},
+ {
+ .procname = "fib_multipath_hash_seed",
+ .data = &init_net.ipv4.sysctl_fib_multipath_hash_seed,
+ /* maxlen to print the keys in hex (*2) and a comma in between keys. */
+ .maxlen = (FIB_MULTIPATH_SEED_KEY_LENGTH * 2) + 2,
+ .mode = 0600,
+ .proc_handler = proc_fib_multipath_hash_seed,
+ },
#endif
{
.procname = "ip_unprivileged_port_start",
--
2.31.1
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