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Date: Thu, 14 Sep 2023 18:15:23 +0300
From: Nikolay Aleksandrov <razor@...ckwall.org>
To: Johannes Nixdorf <jnixdorf-oss@....de>,
"David S. Miller" <davem@...emloft.net>, Andrew Lunn <andrew@...n.ch>,
David Ahern <dsahern@...il.com>, Eric Dumazet <edumazet@...gle.com>,
Florian Fainelli <f.fainelli@...il.com>, Ido Schimmel <idosch@...dia.com>,
Jakub Kicinski <kuba@...nel.org>, Oleksij Rempel <linux@...pel-privat.de>,
Paolo Abeni <pabeni@...hat.com>, Roopa Prabhu <roopa@...dia.com>,
Shuah Khan <shuah@...nel.org>, Vladimir Oltean <vladimir.oltean@....com>
Cc: bridge@...ts.linux-foundation.org, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH net-next v3 3/6] net: bridge: Track and limit dynamically
learned FDB entries
On 9/5/23 14:47, Johannes Nixdorf wrote:
> A malicious actor behind one bridge port may spam the kernel with packets
> with a random source MAC address, each of which will create an FDB entry,
> each of which is a dynamic allocation in the kernel.
>
> There are roughly 2^48 different MAC addresses, further limited by the
> rhashtable they are stored in to 2^31. Each entry is of the type struct
> net_bridge_fdb_entry, which is currently 128 bytes big. This means the
> maximum amount of memory allocated for FDB entries is 2^31 * 128B =
> 256GiB, which is too much for most computers.
>
> Mitigate this by maintaining a per bridge count of those automatically
> generated entries in fdb_n_learned_entries, and a limit in
> fdb_max_learned_entries. If the limit is hit new entries are not learned
> anymore.
>
> For backwards compatibility the default setting of 0 disables the limit.
>
> User-added entries by netlink or from bridge or bridge port addresses
> are never blocked and do not count towards that limit.
>
> Introduce a new fdb entry flag BR_FDB_DYNAMIC_LEARNED to keep track of
> whether an FDB entry is included in the count. The flag is enabled for
> dynamically learned entries, and disabled for all other entries. This
> should be equivalent to BR_FDB_ADDED_BY_USER and BR_FDB_LOCAL being unset,
> but contrary to the two flags it can be toggled atomically.
>
> Atomicity is required here, as there are multiple callers that modify the
> flags, but are not under a common lock (br_fdb_update is the exception
> for br->hash_lock, br_fdb_external_learn_add for RTNL).
>
> Signed-off-by: Johannes Nixdorf <jnixdorf-oss@....de>
> ---
> net/bridge/br_fdb.c | 34 ++++++++++++++++++++++++++++++++--
> net/bridge/br_private.h | 4 ++++
> 2 files changed, 36 insertions(+), 2 deletions(-)
>
> diff --git a/net/bridge/br_fdb.c b/net/bridge/br_fdb.c
> index 06e28ef8d9ff..f8a96ed9a338 100644
> --- a/net/bridge/br_fdb.c
> +++ b/net/bridge/br_fdb.c
> @@ -329,11 +329,18 @@ static void fdb_delete(struct net_bridge *br, struct net_bridge_fdb_entry *f,
> hlist_del_init_rcu(&f->fdb_node);
> rhashtable_remove_fast(&br->fdb_hash_tbl, &f->rhnode,
> br_fdb_rht_params);
> + if (test_bit(BR_FDB_DYNAMIC_LEARNED, &f->flags))
this is racy with br_fdb_update(), the fdb entry is still visible
because a grace period hasn't passed, so in theory br_fdb_update() can
unset the bit after this test and...
> + atomic_dec(&br->fdb_n_learned_entries);
... this will decrease once more wrongly
> fdb_notify(br, f, RTM_DELNEIGH, swdev_notify);
> call_rcu(&f->rcu, fdb_rcu_free);
> }
>
> -/* Delete a local entry if no other port had the same address. */
> +/* Delete a local entry if no other port had the same address.
> + *
> + * This function should only be called on entries with BR_FDB_LOCAL set,
> + * so even with BR_FDB_ADDED_BY_USER cleared we never need to increase
> + * the accounting for dynamically learned entries again.
> + */
> static void fdb_delete_local(struct net_bridge *br,
> const struct net_bridge_port *p,
> struct net_bridge_fdb_entry *f)
> @@ -388,9 +395,20 @@ static struct net_bridge_fdb_entry *fdb_create(struct net_bridge *br,
> __u16 vid,
> unsigned long flags)
> {
> + bool learned = !test_bit(BR_FDB_ADDED_BY_USER, &flags) &&
> + !test_bit(BR_FDB_LOCAL, &flags);
> + u32 max_learned = READ_ONCE(br->fdb_max_learned_entries);
> struct net_bridge_fdb_entry *fdb;
> int err;
>
> + if (likely(learned)) {
> + int n_learned = atomic_read(&br->fdb_n_learned_entries);
> +
> + if (unlikely(max_learned && n_learned >= max_learned))
> + return NULL;
> + __set_bit(BR_FDB_DYNAMIC_LEARNED, &flags);
> + }
> +
> fdb = kmem_cache_alloc(br_fdb_cache, GFP_ATOMIC);
> if (!fdb)
> return NULL;
> @@ -407,6 +425,9 @@ static struct net_bridge_fdb_entry *fdb_create(struct net_bridge *br,
> return NULL;
> }
>
> + if (likely(learned))
> + atomic_inc(&br->fdb_n_learned_entries);
> +
> hlist_add_head_rcu(&fdb->fdb_node, &br->fdb_list);
>
> return fdb;
> @@ -893,8 +914,11 @@ void br_fdb_update(struct net_bridge *br, struct net_bridge_port *source,
> clear_bit(BR_FDB_LOCKED, &fdb->flags);
> }
>
> - if (unlikely(test_bit(BR_FDB_ADDED_BY_USER, &flags)))
> + if (unlikely(test_bit(BR_FDB_ADDED_BY_USER, &flags))) {
> set_bit(BR_FDB_ADDED_BY_USER, &fdb->flags);
> + if (test_and_clear_bit(BR_FDB_DYNAMIC_LEARNED, &fdb->flags))
> + atomic_dec(&br->fdb_n_learned_entries);
> + }
> if (unlikely(fdb_modified)) {
> trace_br_fdb_update(br, source, addr, vid, flags);
> fdb_notify(br, fdb, RTM_NEWNEIGH, true);
> @@ -1071,6 +1095,8 @@ static int fdb_add_entry(struct net_bridge *br, struct net_bridge_port *source,
> }
>
> set_bit(BR_FDB_ADDED_BY_USER, &fdb->flags);
> + if (test_and_clear_bit(BR_FDB_DYNAMIC_LEARNED, &fdb->flags))
> + atomic_dec(&br->fdb_n_learned_entries);
> }
>
> if (fdb_to_nud(br, fdb) != state) {
> @@ -1445,6 +1471,10 @@ int br_fdb_external_learn_add(struct net_bridge *br, struct net_bridge_port *p,
> if (!p)
> set_bit(BR_FDB_LOCAL, &fdb->flags);
>
> + if ((swdev_notify || !p) &&
> + test_and_clear_bit(BR_FDB_DYNAMIC_LEARNED, &fdb->flags))
> + atomic_dec(&br->fdb_n_learned_entries);
> +
> if (modified)
> fdb_notify(br, fdb, RTM_NEWNEIGH, swdev_notify);
> }
> diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
> index a63b32c1638e..675cc40ae1dc 100644
> --- a/net/bridge/br_private.h
> +++ b/net/bridge/br_private.h
> @@ -274,6 +274,7 @@ enum {
> BR_FDB_NOTIFY,
> BR_FDB_NOTIFY_INACTIVE,
> BR_FDB_LOCKED,
> + BR_FDB_DYNAMIC_LEARNED,
> };
>
> struct net_bridge_fdb_key {
> @@ -554,6 +555,9 @@ struct net_bridge {
> struct kobject *ifobj;
> u32 auto_cnt;
>
> + atomic_t fdb_n_learned_entries;
> + u32 fdb_max_learned_entries;
> +
> #ifdef CONFIG_NET_SWITCHDEV
> /* Counter used to make sure that hardware domains get unique
> * identifiers in case a bridge spans multiple switchdev instances.
>
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