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Message-ID: <e8d98be6-d540-59c6-79eb-353715625ea5@blackwall.org>
Date: Mon, 15 May 2023 12:35:03 +0300
From: Nikolay Aleksandrov <razor@...ckwall.org>
To: Johannes Nixdorf <jnixdorf-oss@....de>, netdev@...r.kernel.org
Cc: bridge@...ts.linux-foundation.org, "David S. Miller"
<davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
Roopa Prabhu <roopa@...dia.com>
Subject: Re: [PATCH net-next 1/2] bridge: Add a limit on FDB entries
On 15/05/2023 11:50, 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 adding a bridge netlink setting IFLA_BR_FDB_MAX_ENTRIES,
> which, if nonzero, limits the amount of entries to a user specified
> maximum.
>
> For backwards compatibility the default setting of 0 disables the limit.
>
> All changes to fdb_n_entries are under br->hash_lock, which means we do
> not need additional locking. The call paths are (✓ denotes that
> br->hash_lock is taken around the next call):
>
> - fdb_delete <-+- fdb_delete_local <-+- br_fdb_changeaddr ✓
> | +- br_fdb_change_mac_address ✓
> | +- br_fdb_delete_by_port ✓
> +- br_fdb_find_delete_local ✓
> +- fdb_add_local <-+- br_fdb_changeaddr ✓
> | +- br_fdb_change_mac_address ✓
> | +- br_fdb_add_local ✓
> +- br_fdb_cleanup ✓
> +- br_fdb_flush ✓
> +- br_fdb_delete_by_port ✓
> +- fdb_delete_by_addr_and_port <--- __br_fdb_delete ✓
> +- br_fdb_external_learn_del ✓
> - fdb_create <-+- fdb_add_local <-+- br_fdb_changeaddr ✓
> | +- br_fdb_change_mac_address ✓
> | +- br_fdb_add_local ✓
> +- br_fdb_update ✓
> +- fdb_add_entry <--- __br_fdb_add ✓
> +- br_fdb_external_learn_add ✓
>
> Signed-off-by: Johannes Nixdorf <jnixdorf-oss@....de>
> ---
> include/uapi/linux/if_link.h | 1 +
> net/bridge/br_device.c | 2 ++
> net/bridge/br_fdb.c | 6 ++++++
> net/bridge/br_netlink.c | 9 ++++++++-
> net/bridge/br_private.h | 2 ++
> 5 files changed, 19 insertions(+), 1 deletion(-)
>
Hi,
If you're sending a patch series please add a cover letter (--cover-letter) which
explains what the series are trying to do and why.
I've had a patch that implements this feature for a while but didn't get to upstreaming it. :)
Anyway more comments below,
> diff --git a/include/uapi/linux/if_link.h b/include/uapi/linux/if_link.h
> index 4ac1000b0ef2..27cf5f2d8790 100644
> --- a/include/uapi/linux/if_link.h
> +++ b/include/uapi/linux/if_link.h
> @@ -510,6 +510,7 @@ enum {
> IFLA_BR_VLAN_STATS_PER_PORT,
> IFLA_BR_MULTI_BOOLOPT,
> IFLA_BR_MCAST_QUERIER_STATE,
> + IFLA_BR_FDB_MAX_ENTRIES,
> __IFLA_BR_MAX,
> };
>
> diff --git a/net/bridge/br_device.c b/net/bridge/br_device.c
> index 8eca8a5c80c6..d455a28df7c9 100644
> --- a/net/bridge/br_device.c
> +++ b/net/bridge/br_device.c
> @@ -528,6 +528,8 @@ void br_dev_setup(struct net_device *dev)
> br->bridge_hello_time = br->hello_time = 2 * HZ;
> br->bridge_forward_delay = br->forward_delay = 15 * HZ;
> br->bridge_ageing_time = br->ageing_time = BR_DEFAULT_AGEING_TIME;
> + br->fdb_n_entries = 0;
> + br->fdb_max_entries = 0;
Unnecessary, the private area is already cleared.
> dev->max_mtu = ETH_MAX_MTU;
>
> br_netfilter_rtable_init(br);
> diff --git a/net/bridge/br_fdb.c b/net/bridge/br_fdb.c
> index e69a872bfc1d..8a833e6dee92 100644
> --- a/net/bridge/br_fdb.c
> +++ b/net/bridge/br_fdb.c
> @@ -329,6 +329,8 @@ 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 (!WARN_ON(!br->fdb_n_entries))
> + br->fdb_n_entries--;
This is pointless, just put the WARN_ON(!br->fdb_n_entries) above decrementing, if we
hit that we are already in trouble and not decrementing doesn't help us.
> fdb_notify(br, f, RTM_DELNEIGH, swdev_notify);
> call_rcu(&f->rcu, fdb_rcu_free);
> }
> @@ -391,6 +393,9 @@ static struct net_bridge_fdb_entry *fdb_create(struct net_bridge *br,
> struct net_bridge_fdb_entry *fdb;
> int err;
>
> + if (unlikely(br->fdb_max_entries && br->fdb_n_entries >= br->fdb_max_entries))
> + return NULL;
> +
This one needs more work, fdb_create() is also used when user-space is adding new
entries, so it would be nice to return a proper error.
> fdb = kmem_cache_alloc(br_fdb_cache, GFP_ATOMIC);
> if (!fdb)
> return NULL;
> @@ -408,6 +413,7 @@ static struct net_bridge_fdb_entry *fdb_create(struct net_bridge *br,
> }
>
> hlist_add_head_rcu(&fdb->fdb_node, &br->fdb_list);
> + br->fdb_n_entries++;
>
> return fdb;
> }
> diff --git a/net/bridge/br_netlink.c b/net/bridge/br_netlink.c
> index 05c5863d2e20..e5b8d36a3291 100644
> --- a/net/bridge/br_netlink.c
> +++ b/net/bridge/br_netlink.c
> @@ -1527,6 +1527,12 @@ static int br_changelink(struct net_device *brdev, struct nlattr *tb[],
> return err;
> }
>
> + if (data[IFLA_BR_FDB_MAX_ENTRIES]) {
> + u32 val = nla_get_u32(data[IFLA_BR_FDB_MAX_ENTRIES]);
> +
> + br->fdb_max_entries = val;
> + }
> +
> return 0;
> }
>
> @@ -1656,7 +1662,8 @@ static int br_fill_info(struct sk_buff *skb, const struct net_device *brdev)
> nla_put_u8(skb, IFLA_BR_TOPOLOGY_CHANGE_DETECTED,
> br->topology_change_detected) ||
> nla_put(skb, IFLA_BR_GROUP_ADDR, ETH_ALEN, br->group_addr) ||
> - nla_put(skb, IFLA_BR_MULTI_BOOLOPT, sizeof(bm), &bm))
> + nla_put(skb, IFLA_BR_MULTI_BOOLOPT, sizeof(bm), &bm) ||
> + nla_put_u32(skb, IFLA_BR_FDB_MAX_ENTRIES, br->fdb_max_entries))
You are not returning the current entry count, that is also needed.
> return -EMSGSIZE;
>
> #ifdef CONFIG_BRIDGE_VLAN_FILTERING
> diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
> index 2119729ded2b..64fb359c6e3e 100644
> --- a/net/bridge/br_private.h
> +++ b/net/bridge/br_private.h
> @@ -494,6 +494,8 @@ struct net_bridge {
> #endif
>
> struct rhashtable fdb_hash_tbl;
> + u32 fdb_n_entries;
> + u32 fdb_max_entries;
These are not critical, so I'd use 4 byte holes in net_bridge and pack it better
instead of making it larger.
> struct list_head port_list;
> #if IS_ENABLED(CONFIG_BRIDGE_NETFILTER)
> union {
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