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@avm.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)) + atomic_dec(&br->fdb_n_learned_entries); 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.

The previous patch added accounting and a limit for the number of dynamically learned FDB entries per bridge. However it did not provide means to actually configure those bounds or read back the count. This patch does that.

Two new netlink attributes are added for the accounting and limit of dynamically learned FDB entries: - IFLA_BR_FDB_N_LEARNED_ENTRIES (RO) for the number of entries accounted for a single bridge. - IFLA_BR_FDB_MAX_LEARNED_ENTRIES (RW) for the configured limit of entries for the bridge.

The new attributes are used like this:

# ip link add name br up type bridge fdb_max_learned_entries 256 # ip link add name v1 up master br type veth peer v2 # ip link set up dev v2 # mausezahn -a rand -c 1024 v2 0.01 seconds (90877 packets per second # bridge fdb | grep -v permanent | wc -l 256 # ip -d link show dev br 13: br: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 [...] [...] fdb_n_learned_entries 256 fdb_max_learned_entries 256

Signed-off-by: Johannes Nixdorf jnixdorf-oss@avm.de --- include/uapi/linux/if_link.h | 2 ++ net/bridge/br_netlink.c | 15 ++++++++++++++- 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/if_link.h b/include/uapi/linux/if_link.h index 0f6a0fe09bdb..bcdf728cfe98 100644 --- a/include/uapi/linux/if_link.h +++ b/include/uapi/linux/if_link.h @@ -510,6 +510,8 @@ enum { IFLA_BR_VLAN_STATS_PER_PORT, IFLA_BR_MULTI_BOOLOPT, IFLA_BR_MCAST_QUERIER_STATE, + IFLA_BR_FDB_N_LEARNED_ENTRIES, + IFLA_BR_FDB_MAX_LEARNED_ENTRIES, __IFLA_BR_MAX, };

diff --git a/net/bridge/br_netlink.c b/net/bridge/br_netlink.c index 1dc4e1bce740..d8595274879d 100644 --- a/net/bridge/br_netlink.c +++ b/net/bridge/br_netlink.c @@ -1254,6 +1254,8 @@ static const struct nla_policy br_policy[IFLA_BR_MAX + 1] = { [IFLA_BR_VLAN_STATS_PER_PORT] = { .type = NLA_U8 }, [IFLA_BR_MULTI_BOOLOPT] = NLA_POLICY_EXACT_LEN(sizeof(struct br_boolopt_multi)), + [IFLA_BR_FDB_N_LEARNED_ENTRIES] = { .type = NLA_U32 }, + [IFLA_BR_FDB_MAX_LEARNED_ENTRIES] = { .type = NLA_U32 }, };

static int br_changelink(struct net_device *brdev, struct nlattr *tb[], @@ -1528,6 +1530,12 @@ static int br_changelink(struct net_device *brdev, struct nlattr *tb[], return err; }

+ if (data[IFLA_BR_FDB_MAX_LEARNED_ENTRIES]) { + u32 val = nla_get_u32(data[IFLA_BR_FDB_MAX_LEARNED_ENTRIES]); + + WRITE_ONCE(br->fdb_max_learned_entries, val); + } + return 0; }

@@ -1582,6 +1590,8 @@ static size_t br_get_size(const struct net_device *brdev) nla_total_size_64bit(sizeof(u64)) + /* IFLA_BR_TOPOLOGY_CHANGE_TIMER */ nla_total_size_64bit(sizeof(u64)) + /* IFLA_BR_GC_TIMER */ nla_total_size(ETH_ALEN) + /* IFLA_BR_GROUP_ADDR */ + nla_total_size(sizeof(u32)) + /* IFLA_BR_FDB_N_LEARNED_ENTRIES */ + nla_total_size(sizeof(u32)) + /* IFLA_BR_FDB_MAX_LEARNED_ENTRIES */ #ifdef CONFIG_BRIDGE_IGMP_SNOOPING nla_total_size(sizeof(u8)) + /* IFLA_BR_MCAST_ROUTER */ nla_total_size(sizeof(u8)) + /* IFLA_BR_MCAST_SNOOPING */ @@ -1657,7 +1667,10 @@ 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_N_LEARNED_ENTRIES, + atomic_read(&br->fdb_n_learned_entries)) || + nla_put_u32(skb, IFLA_BR_FDB_MAX_LEARNED_ENTRIES, br->fdb_max_learned_entries)) return -EMSGSIZE;

#ifdef CONFIG_BRIDGE_VLAN_FILTERING

Add a suite covering the fdb_n_learned_entries and fdb_max_learned_entries bridge features, touching all special cases in accounting at least once.

Signed-off-by: Johannes Nixdorf jnixdorf-oss@avm.de --- .../net/forwarding/bridge_fdb_learning_limit.sh | 283 +++++++++++++++++++++ 1 file changed, 283 insertions(+)

diff --git a/tools/testing/selftests/net/forwarding/bridge_fdb_learning_limit.sh b/tools/testing/selftests/net/forwarding/bridge_fdb_learning_limit.sh new file mode 100755 index 000000000000..4da17ac65357 --- /dev/null +++ b/tools/testing/selftests/net/forwarding/bridge_fdb_learning_limit.sh @@ -0,0 +1,283 @@ +#!/bin/bash +# SPDX-License-Identifier: GPL-2.0 + +# ShellCheck incorrectly believes that most of the code here is unreachable +# because it's invoked by variable name following ALL_TESTS. +# +# shellcheck disable=SC2317 + +ALL_TESTS="check_accounting check_limit" +NUM_NETIFS=6 +source lib.sh + +TEST_MAC_BASE=de:ad:be:ef:42: + +NUM_PKTS=16 +FDB_LIMIT=8 + +FDB_TYPES=( + # name is counted? overrides learned? + 'learned 1 0' + 'static 0 1' + 'user 0 1' + 'extern_learn 0 1' + 'local 0 1' +) + +mac() +{ + printf "${TEST_MAC_BASE}%02x" "$1" +} + +H1_DEFAULT_MAC=$(mac 42) + +switch_create() +{ + ip link add dev br0 type bridge + + ip link set dev "$swp1" master br0 + ip link set dev "$swp2" master br0 + # swp3 is used to add local MACs, so do not add it to the bridge yet. + + # swp2 is only used for replying when learning on swp1, its MAC should not be learned. + ip link set dev "$swp2" type bridge_slave learning off + + ip link set dev br0 up + + ip link set dev "$swp1" up + ip link set dev "$swp2" up + ip link set dev "$swp3" up +} + +switch_destroy() +{ + ip link set dev "$swp3" down + ip link set dev "$swp2" down + ip link set dev "$swp1" down + + ip link del dev br0 +} + +h_create() +{ + ip link set "$h1" addr "$H1_DEFAULT_MAC" + + simple_if_init "$h1" 192.0.2.1/24 + simple_if_init "$h2" 192.0.2.2/24 +} + +h_destroy() +{ + simple_if_fini "$h1" 192.0.2.1/24 + simple_if_fini "$h2" 192.0.2.2/24 +} + +setup_prepare() +{ + h1=${NETIFS[p1]} + swp1=${NETIFS[p2]} + + h2=${NETIFS[p3]} + swp2=${NETIFS[p4]} + + swp3=${NETIFS[p6]} + + vrf_prepare + + h_create + + switch_create +} + +cleanup() +{ + pre_cleanup + + switch_destroy + + h_destroy + + vrf_cleanup +} + +fdb_get_n_learned() +{ + ip -d -j link show dev br0 type bridge | \ + jq '.[]["linkinfo"]["info_data"]["fdb_n_learned_entries"]' +} + +fdb_get_n_mac() +{ + local mac=${1} + + bridge -j fdb show br br0 | \ + jq "map(select(.mac == "${mac}" and (has("vlan") | not))) | length" +} + +fdb_fill_learned() +{ + local i + + for i in $(seq 1 "$NUM_PKTS"); do + fdb_add learned "$(mac "$i")" + done +} + +fdb_reset() +{ + bridge fdb flush dev br0 + + # Keep the default MAC address of h1 in the table. We set it to a different one when + # testing dynamic learning. + bridge fdb add "$H1_DEFAULT_MAC" dev "$swp1" master static use +} + +fdb_add() +{ + local type=$1 mac=$2 + + case "$type" in + learned) + ip link set "$h1" addr "$mac" + # Wait for a reply so we implicitly wait until after the forwarding + # code finished and the FDB entry was created. + PING_COUNT=1 ping_do "$h1" 192.0.2.2 + check_err $? "Failed to ping another bridge port" + ip link set "$h1" addr "$H1_DEFAULT_MAC" + ;; + local) + ip link set dev "$swp3" addr "$mac" && ip link set "$swp3" master br0 + ;; + static) + bridge fdb replace "$mac" dev "$swp1" master static + ;; + user) + bridge fdb replace "$mac" dev "$swp1" master static use + ;; + extern_learn) + bridge fdb replace "$mac" dev "$swp1" master extern_learn + ;; + esac + + check_err $? "Failed to add a FDB entry of type ${type}" +} + +fdb_del() +{ + local type=$1 mac=$2 + + case "$type" in + local) + ip link set "$swp3" nomaster + ;; + *) + bridge fdb del "$mac" dev "$swp1" master + ;; + esac + + check_err $? "Failed to remove a FDB entry of type ${type}" +} + +check_accounting_one_type() +{ + local type=$1 is_counted=$2 overrides_learned=$3 + shift 3 + RET=0 + + fdb_reset + fdb_add "$type" "$(mac 0)" + learned=$(fdb_get_n_learned) + [ "$learned" -ne "$is_counted" ] + check_fail $? "Inserted FDB type ${type}: Expected the count ${is_counted}, but got ${learned}" + + fdb_del "$type" "$(mac 0)" + learned=$(fdb_get_n_learned) + [ "$learned" -ne 0 ] + check_fail $? "Removed FDB type ${type}: Expected the count 0, but got ${learned}" + + if [ "$overrides_learned" -eq 1 ]; then + fdb_reset + fdb_add learned "$(mac 0)" + fdb_add "$type" "$(mac 0)" + learned=$(fdb_get_n_learned) + [ "$learned" -ne "$is_counted" ] + check_fail $? "Set a learned entry to FDB type ${type}: Expected the count ${is_counted}, but got ${learned}" + fdb_del "$type" "$(mac 0)" + fi + + log_test "FDB accounting interacting with FDB type ${type}" +} + +check_accounting() +{ + local type_args learned + RET=0 + + fdb_reset + learned=$(fdb_get_n_learned) + [ "$learned" -ne 0 ] + check_fail $? "Flushed the FDB table: Expected the count 0, but got ${learned}" + + fdb_fill_learned + sleep 1 + + learned=$(fdb_get_n_learned) + [ "$learned" -ne "$NUM_PKTS" ] + check_fail $? "Filled the FDB table: Expected the count ${NUM_PKTS}, but got ${learned}" + + log_test "FDB accounting" + + for type_args in "${FDB_TYPES[@]}"; do + # This is intentional use of word splitting. + # shellcheck disable=SC2086 + check_accounting_one_type $type_args + done +} + +check_limit_one_type() +{ + local type=$1 is_counted=$2 + local n_mac expected=$((1 - is_counted)) + RET=0 + + fdb_reset + fdb_fill_learned + + fdb_add "$type" "$(mac 0)" + n_mac=$(fdb_get_n_mac "$(mac 0)") + [ "$n_mac" -ne "$expected" ] + check_fail $? "Inserted FDB type ${type} at limit: Expected the count ${expected}, but got ${n_mac}" + + log_test "FDB limits interacting with FDB type ${type}" +} + +check_limit() +{ + local learned + RET=0 + + ip link set br0 type bridge fdb_max_learned_entries "$FDB_LIMIT" + + fdb_reset + fdb_fill_learned + + learned=$(fdb_get_n_learned) + [ "$learned" -ne "$FDB_LIMIT" ] + check_fail $? "Filled the limited FDB table: Expected the count ${FDB_LIMIT}, but got ${learned}" + + log_test "FDB limits" + + for type_args in "${FDB_TYPES[@]}"; do + # This is intentional use of word splitting. + # shellcheck disable=SC2086 + check_limit_one_type $type_args + done +} + +trap cleanup EXIT + +setup_prepare + +tests_run + +exit $EXIT_STATUS