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Message-ID: <ZGM64ODoVwK8J4u2@u-jnixdorf.ads.avm.de>
Date: Tue, 16 May 2023 10:12:16 +0200
From: Johannes Nixdorf <jnixdorf-oss@....de>
To: Nikolay Aleksandrov <razor@...ckwall.org>
Cc: netdev@...r.kernel.org, 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
Hi,
On Mon, May 15, 2023 at 12:35:03PM +0300, Nikolay Aleksandrov wrote:
> 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.
Thanks for the hint. I'll do that in the future, including a potential v2.
> I've had a patch that implements this feature for a while but didn't get to upstreaming it. :)
I'm not too attached to my name on it, so if your patch is further along,
I'd be happy if you submitted your version instead.
> 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.
This will be taken out in a v2.
> > 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.
This will now always be decremented in a v2.
> > 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.
The callers usually map this return code to -ENOMEM, which I deemed an
appropriate return code for violating the new limit, as I understood it
as a memory limit for the FDB table.
Is there a different error return code you had in mind here, or would
you rather only count dynamic entries towards the limit at all?
> > 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.
For a v2 this now also returns the current entry count under
IFLA_BR_FDB_CUR_ENTRIES.
> > 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.
For a v2 I now moved it into (conditional) holes now in front of
CONFIG_BRIDGE_VLAN_FILTERING (only a hole if it is enabled) and
CONFIG_SWITCHDEV (only a hole if it is disabled). I could not find any
other holes, but please tell me if you had any others in mind.
> > struct list_head port_list;
> > #if IS_ENABLED(CONFIG_BRIDGE_NETFILTER)
> > union {
>
Thanks for your detailed feedback.
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