Subject: Re: [PATCH v6 net-next 1/3] net/ipv4: multipath routing: configurable seed

From: Ido Schimmel <[email-masked]>
Date: Thu, 29 Apr 2021 18:29:20 +0300

On Wed, Apr 28, 2021 at 03:31:33PM +0300, Pavel Balaev wrote:
> 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: Pavel Balaev <[email-masked]>
> ---
>  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;

Why 'int'?

> +	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();

During netns initialization the per-netns seed can be initialized to a
system global seed. When the sysctl is used this seed will be
overridden. You can then remove this check and always call
flow_multipath_hash_from_keys() with the per-netns seed.

I'm not suggesting to initialize the seed of each netns differently as
some users might be inadvertently relying on the fact that it is
currently the same for all namespaces.

>  
>  	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) {

You can reduce nesting by using early return.

> +		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]);

This leaks the seed... I understand how these prints can be useful
during development, but I believe they should be removed prior to
submission.

> +
> +		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);
> +		}
> +	}

This looks overly complex to me and I believe a lot of users will ask
themselves why they need to specify a seed using two hex numbers
separated by a comma. Looking at other implementations that already
allow specifying the seed, it is specified as a single integer.

32-bit in Cumulus:
https://docs.nvidia.com/networking-ethernet-software/cumulus-linux-43/Layer-3/Routing/Equal-Cost-Multipath-Load-Sharing-Hardware-ECMP/#configure-a-hash-seed-to-avoid-hash-polarization

Up to 16-bit in Arista:
https://eos.arista.com/hashing-for-l2-port-channels-and-l3-ecmp/

I believe you chose this interface because of the structure of the
SipHash key that is used for the multipath hash calculation. This is an
internal implementation detail and should not determine the user
interface.

Looking at the history of the code, the flow dissector was migrated to
SipHash in commit 55667441c84f ("net/flow_dissector: switch to
siphash"). The motivating use case was flow label generation since these
are sent on the wire together with the fields from which they were
computed, not multipath hash calculation that also happens to rely on
the flow dissector.

Given the above, do you see a problem with having the user specify a
32-bit number for the multipath hash seed? Note that SipHash is still
used and that the number can be used to fill the entire 128-bit space.

The special value of "0" can be used to revert back to the random seed
(needs to be documented, obviously).

> +
> +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
>