Linux Ethernet Bridging - Jun 2023 - [Bridge] [PATCH net-next v2 1/3] bridge: Set BR_FDB_ADDED_BY_USER early in fdb_add_entry

Introduce a limit on the amount of learned FDB entries on a bridge,
configured by netlink with a build time default on bridge creation in
the kernel config.

For backwards compatibility the kernel config default is disabling the
limit (0).

Without any limit a malicious actor may OOM a kernel by spamming packets
with changing MAC addresses on their bridge port, so allow the bridge
creator to limit the number of entries.

Currently the manual entries are identified by the bridge flags
BR_FDB_LOCAL or BR_FDB_ADDED_BY_USER, and changes to those flags are
protected under a lock. This means the limit also applies to entries
created with BR_FDB_ADDED_BY_EXT_LEARN but none of the other two,
e.g. ones added by SWITCHDEV_FDB_ADD_TO_BRIDGE.

v1: https://lore.kernel.org/netdev/20230515085046.4457-1-jnixdorf-oss at avm.de/

Changes since v1:
 - Added BR_FDB_ADDED_BY_USER earlier in fdb_add_entry to ensure the
   limit is not applied.
 - Do not initialize fdb_*_entries to 0. (from review)
 - Do not skip decrementing on 0. (from review)
 - Moved the counters to a conditional hole in struct net_bridge to
   avoid growing the struct. (from review, it still grows the struct as
   there are 2 32-bit values)
 - Add IFLA_BR_FDB_CUR_LEARNED_ENTRIES (from review)
 - Fix br_get_size() with the added attributes.
 - Only limit learned entries, rename to
   *_(CUR|MAX)_LEARNED_ENTRIES. (from review)
 - Added a default limit in Kconfig. (deemed acceptable in review
   comments, helps with embedded use-cases where a special purpose kernel
   is built anyways)
 - Added an iproute2 patch for easier testing.

Obsolete v1 review comments:
 - Return better errors to users: Due to limiting the limit to
   automatically created entries, netlink fdb add requests and changing
   bridge ports are never rejected, so they do not yet need a more
   friendly error returned.

net-next:

Johannes Nixdorf (3):
  bridge: Set BR_FDB_ADDED_BY_USER early in fdb_add_entry
  bridge: Add a limit on learned FDB entries
  net: bridge: Add a configurable default FDB learning limit

 include/uapi/linux/if_link.h |  2 +
 net/bridge/Kconfig           | 13 +++++++
 net/bridge/br_device.c       |  2 +
 net/bridge/br_fdb.c          | 73 ++++++++++++++++++++++++++++++++----
 net/bridge/br_netlink.c      | 13 ++++++-
 net/bridge/br_private.h      |  6 +++
 6 files changed, 101 insertions(+), 8 deletions(-)

iproute2-next:

Johannes Nixdorf (1):
  iplink: bridge: Add support for bridge FDB learning limits

 include/uapi/linux/if_link.h |  2 ++
 ip/iplink_bridge.c           | 21 +++++++++++++++++++++
 man/man8/ip-link.8.in        |  9 +++++++++
 3 files changed, 32 insertions(+)

-- 
2.40.1

This allows the called fdb_create to detect that the entry was added by
the user early in the process. This is in preparation to adding limits
in fdb_create that should not apply to user created fdb entries.

Signed-off-by: Johannes Nixdorf <jnixdorf-oss at avm.de>

---

Changes since v1:
 - Added this change to ensure user added entries are not limited.

 net/bridge/br_fdb.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/bridge/br_fdb.c b/net/bridge/br_fdb.c
index e69a872bfc1d..ac1dc8723b9c 100644
--- a/net/bridge/br_fdb.c
+++ b/net/bridge/br_fdb.c
@@ -1056,7 +1056,7 @@ static int fdb_add_entry(struct net_bridge *br, struct
net_bridge_port *source,
 		if (!(flags & NLM_F_CREATE))
 			return -ENOENT;
 
-		fdb = fdb_create(br, source, addr, vid, 0);
+		fdb = fdb_create(br, source, addr, vid, BR_FDB_ADDED_BY_USER);
 		if (!fdb)
 			return -ENOMEM;
 
@@ -1069,6 +1069,8 @@ static int fdb_add_entry(struct net_bridge *br, struct
net_bridge_port *source,
 			WRITE_ONCE(fdb->dst, source);
 			modified = true;
 		}
+
+		set_bit(BR_FDB_ADDED_BY_USER, &fdb->flags);
 	}
 
 	if (fdb_to_nud(br, fdb) != state) {
@@ -1100,8 +1102,6 @@ static int fdb_add_entry(struct net_bridge *br, struct
net_bridge_port *source,
 	if (fdb_handle_notify(fdb, notify))
 		modified = true;
 
-	set_bit(BR_FDB_ADDED_BY_USER, &fdb->flags);
-
 	fdb->used = jiffies;
 	if (modified) {
 		if (refresh)
-- 
2.40.1

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_LEARNED_ENTRIES, which, if nonzero, limits the amount
of learned entries to a user specified maximum.

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.

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 ?

The flags that imply an entry does not come from learning
(BR_FDB_NOT_LEARNED_MASK) are now only set or cleared under br->hash_lock
as well, and when the boolean value of (fdb->flags &
BR_FDB_NOT_LEARNED_MASK) changes the accounting is updated.

This introduces one additional locked update in br_fdb_update if
BR_FDB_ADDED_BY_USER was set. This is only the case when creating a new
entry via netlink, and never in the packet handling fast path.

Signed-off-by: Johannes Nixdorf <jnixdorf-oss at avm.de>

---

Changes since v1:
 - Do not initialize fdb_*_entries to 0. (from review)
 - Do not skip decrementing on 0. (from review)
 - Moved the counters to a conditional hole in struct net_bridge to
   avoid growing the struct. (from review, it still grows the struct as
   there are 2 32-bit values)
 - Add IFLA_BR_FDB_CUR_LEARNED_ENTRIES (from review)
 - Fix br_get_size()
 - Only limit learned entries, rename to
   *_(CUR|MAX)_LEARNED_ENTRIES. (from review)

Obsolete v1 review comments:
 - Return better errors to users: Due to limiting the limit to
   automatically created entries, netlink fdb add requests and changing
   bridge ports are never rejected, so they do not yet need a more
   friendly error returned.

 include/uapi/linux/if_link.h |  2 ++
 net/bridge/br_fdb.c          | 67 +++++++++++++++++++++++++++++++++---
 net/bridge/br_netlink.c      | 13 ++++++-
 net/bridge/br_private.h      |  6 ++++
 4 files changed, 83 insertions(+), 5 deletions(-)

diff --git a/include/uapi/linux/if_link.h b/include/uapi/linux/if_link.h
index 4ac1000b0ef2..165b9014379b 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_CUR_LEARNED_ENTRIES,
+	IFLA_BR_FDB_MAX_LEARNED_ENTRIES,
 	__IFLA_BR_MAX,
 };
 
diff --git a/net/bridge/br_fdb.c b/net/bridge/br_fdb.c
index ac1dc8723b9c..bc61d1fd5fcf 100644
--- a/net/bridge/br_fdb.c
+++ b/net/bridge/br_fdb.c
@@ -301,6 +301,38 @@ static void fdb_add_hw_addr(struct net_bridge *br, const
unsigned char *addr)
 	}
 }
 
+/* Set a FDB flag that implies the entry was not learned, and account
+ * for changes in the learned status.
+ */
+static void __fdb_set_flag_not_learned(struct net_bridge *br,
+				       struct net_bridge_fdb_entry *fdb,
+				       long nr)
+{
+	WARN_ON_ONCE(!(BIT(nr) & BR_FDB_NOT_LEARNED_MASK));
+
+	/* learned before, but we set a flag that implies it's manually added */
+	if (!(fdb->flags & BR_FDB_NOT_LEARNED_MASK))
+		br->fdb_cur_learned_entries--;
+	set_bit(nr, &fdb->flags);
+}
+
+/* Set a FDB flag that implies the entry was not learned, and account
+ * for changes in the learned status.
+ *
+ * This function takes a lock, so ensure it is not called in the fast
+ * path.
+ */
+static void fdb_set_flag_not_learned(struct net_bridge *br,
+				     struct net_bridge_fdb_entry *fdb,
+				     long nr)
+{
+	spin_lock_bh(&br->hash_lock);
+
+	__fdb_set_flag_not_learned(br, fdb, nr);
+
+	spin_unlock_bh(&br->hash_lock);
+}
+
 /* When a static FDB entry is deleted, the HW address from that entry is
  * also removed from the bridge private HW address list and updates all
  * the ports with needed information.
@@ -321,6 +353,8 @@ static void fdb_del_hw_addr(struct net_bridge *br, const
unsigned char *addr)
 static void fdb_delete(struct net_bridge *br, struct net_bridge_fdb_entry *f,
 		       bool swdev_notify)
 {
+	bool learned = !(f->flags & BR_FDB_NOT_LEARNED_MASK);
+
 	trace_fdb_delete(br, f);
 
 	if (test_bit(BR_FDB_STATIC, &f->flags))
@@ -329,11 +363,16 @@ 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);
+	br->fdb_cur_learned_entries -= learned;
 	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 clear_bit never removes the last bit in BR_FDB_NOT_LEARNED_MASK.
+ */
 static void fdb_delete_local(struct net_bridge *br,
 			     const struct net_bridge_port *p,
 			     struct net_bridge_fdb_entry *f)
@@ -390,6 +429,11 @@ static struct net_bridge_fdb_entry *fdb_create(struct
net_bridge *br,
 {
 	struct net_bridge_fdb_entry *fdb;
 	int err;
+	bool learned = !(flags & BR_FDB_NOT_LEARNED_MASK);
+
+	if (unlikely(learned && br->fdb_max_learned_entries &&
+		     br->fdb_cur_learned_entries >= br->fdb_max_learned_entries))
+		return NULL;
 
 	fdb = kmem_cache_alloc(br_fdb_cache, GFP_ATOMIC);
 	if (!fdb)
@@ -409,6 +453,8 @@ static struct net_bridge_fdb_entry *fdb_create(struct
net_bridge *br,
 
 	hlist_add_head_rcu(&fdb->fdb_node, &br->fdb_list);
 
+	br->fdb_cur_learned_entries += learned;
+
 	return fdb;
 }
 
@@ -894,7 +940,7 @@ void br_fdb_update(struct net_bridge *br, struct
net_bridge_port *source,
 			}
 
 			if (unlikely(test_bit(BR_FDB_ADDED_BY_USER, &flags)))
-				set_bit(BR_FDB_ADDED_BY_USER, &fdb->flags);
+				fdb_set_flag_not_learned(br, fdb, BR_FDB_ADDED_BY_USER);
 			if (unlikely(fdb_modified)) {
 				trace_br_fdb_update(br, source, addr, vid, flags);
 				fdb_notify(br, fdb, RTM_NEWNEIGH, true);
@@ -1070,6 +1116,8 @@ static int fdb_add_entry(struct net_bridge *br, struct
net_bridge_port *source,
 			modified = true;
 		}
 
+		if (!(fdb->flags & BR_FDB_NOT_LEARNED_MASK))
+			br->fdb_cur_learned_entries--;
 		set_bit(BR_FDB_ADDED_BY_USER, &fdb->flags);
 	}
 
@@ -1440,10 +1488,10 @@ int br_fdb_external_learn_add(struct net_bridge *br,
struct net_bridge_port *p,
 		}
 
 		if (swdev_notify)
-			set_bit(BR_FDB_ADDED_BY_USER, &fdb->flags);
+			__fdb_set_flag_not_learned(br, fdb, BR_FDB_ADDED_BY_USER);
 
 		if (!p)
-			set_bit(BR_FDB_LOCAL, &fdb->flags);
+			__fdb_set_flag_not_learned(br, fdb, BR_FDB_LOCAL);
 
 		if (modified)
 			fdb_notify(br, fdb, RTM_NEWNEIGH, swdev_notify);
@@ -1508,3 +1556,14 @@ void br_fdb_clear_offload(const struct net_device *dev,
u16 vid)
 	spin_unlock_bh(&p->br->hash_lock);
 }
 EXPORT_SYMBOL_GPL(br_fdb_clear_offload);
+
+u32 br_fdb_get_cur_learned_entries(struct net_bridge *br)
+{
+	u32 ret;
+
+	spin_lock_bh(&br->hash_lock);
+	ret = br->fdb_cur_learned_entries;
+	spin_unlock_bh(&br->hash_lock);
+
+	return ret;
+}
diff --git a/net/bridge/br_netlink.c b/net/bridge/br_netlink.c
index 05c5863d2e20..954c468d52ec 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_LEARNED_ENTRIES]) {
+		u32 val = nla_get_u32(data[IFLA_BR_FDB_MAX_LEARNED_ENTRIES]);
+
+		br->fdb_max_learned_entries = val;
+	}
+
 	return 0;
 }
 
@@ -1581,6 +1587,8 @@ 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_CUR_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 */
@@ -1620,6 +1628,7 @@ static int br_fill_info(struct sk_buff *skb, const struct
net_device *brdev)
 	u32 stp_enabled = br->stp_enabled;
 	u16 priority = (br->bridge_id.prio[0] << 8) |
br->bridge_id.prio[1];
 	u8 vlan_enabled = br_vlan_enabled(br->dev);
+	u32 fdb_cur_learned_entries = br_fdb_get_cur_learned_entries(br);
 	struct br_boolopt_multi bm;
 	u64 clockval;
 
@@ -1656,7 +1665,9 @@ 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_CUR_LEARNED_ENTRIES, fdb_cur_learned_entries)
||
+	    nla_put_u32(skb, IFLA_BR_FDB_MAX_LEARNED_ENTRIES,
br->fdb_max_learned_entries))
 		return -EMSGSIZE;
 
 #ifdef CONFIG_BRIDGE_VLAN_FILTERING
diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
index 2119729ded2b..df079191479e 100644
--- a/net/bridge/br_private.h
+++ b/net/bridge/br_private.h
@@ -275,6 +275,8 @@ enum {
 	BR_FDB_LOCKED,
 };
 
+#define BR_FDB_NOT_LEARNED_MASK (BIT(BR_FDB_LOCAL) | BIT(BR_FDB_ADDED_BY_USER))
+
 struct net_bridge_fdb_key {
 	mac_addr addr;
 	u16 vlan_id;
@@ -553,6 +555,9 @@ struct net_bridge {
 	struct kobject			*ifobj;
 	u32				auto_cnt;
 
+	u32				fdb_max_learned_entries;
+	u32				fdb_cur_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.
@@ -838,6 +843,7 @@ int br_fdb_external_learn_del(struct net_bridge *br, struct
net_bridge_port *p,
 			      bool swdev_notify);
 void br_fdb_offloaded_set(struct net_bridge *br, struct net_bridge_port *p,
 			  const unsigned char *addr, u16 vid, bool offloaded);
+u32 br_fdb_get_cur_learned_entries(struct net_bridge *br);
 
 /* br_forward.c */
 enum br_pkt_type {
-- 
2.40.1

This adds a Kconfig option to configure a default FDB learning limit
system wide, so a distributor building a special purpose kernel can
limit all created bridges by default.

The limit is only a soft default setting and overridable per bridge
using netlink.

Signed-off-by: Johannes Nixdorf <jnixdorf-oss at avm.de>

---

Changes since v1:
 - Added a default limit in Kconfig. (deemed acceptable in review
   comments)

 net/bridge/Kconfig     | 13 +++++++++++++
 net/bridge/br_device.c |  2 ++
 2 files changed, 15 insertions(+)

diff --git a/net/bridge/Kconfig b/net/bridge/Kconfig
index 3c8ded7d3e84..c0d9c08088c4 100644
--- a/net/bridge/Kconfig
+++ b/net/bridge/Kconfig
@@ -84,3 +84,16 @@ config BRIDGE_CFM
 	  Say N to exclude this support and reduce the binary size.
 
 	  If unsure, say N.
+
+config BRIDGE_DEFAULT_FDB_MAX_LEARNED
+	int "Default FDB learning limit"
+	default 0
+	depends on BRIDGE
+	help
+	  Sets a default limit on the number of learned FDB entries on
+	  new bridges. This limit can be overwritten via netlink on a
+	  per bridge basis.
+
+	  The default of 0 disables the limit.
+
+	  If unsure, say 0.
diff --git a/net/bridge/br_device.c b/net/bridge/br_device.c
index 8eca8a5c80c6..93f081ce8195 100644
--- a/net/bridge/br_device.c
+++ b/net/bridge/br_device.c
@@ -530,6 +530,8 @@ void br_dev_setup(struct net_device *dev)
 	br->bridge_ageing_time = br->ageing_time = BR_DEFAULT_AGEING_TIME;
 	dev->max_mtu = ETH_MAX_MTU;
 
+	br->fdb_max_learned_entries = CONFIG_BRIDGE_DEFAULT_FDB_MAX_LEARNED;
+
 	br_netfilter_rtable_init(br);
 	br_stp_timer_init(br);
 	br_multicast_init(br);
-- 
2.40.1

Support setting the FDB limit through ip link. The arguments is:
 - fdb_max_learned_entries: A 32-bit unsigned integer specifying the
                            maximum number of learned FDB entries, with 0
                            disabling the limit.

Also support reading back the current number of learned FDB entries in
the bridge by this count. The returned value's name is:
 - fdb_cur_learned_entries: A 32-bit unsigned integer specifying the
                             current number of learned FDB entries.

Example:

 # ip -d -j -p link show br0
[ {
...
        "linkinfo": {
            "info_kind": "bridge",
            "info_data": {
...
                "fdb_cur_learned_entries": 2,
                "fdb_max_learned_entries": 0,
...
            }
        },
...
    } ]
 # ip link set br0 type bridge fdb_max_learned_entries 1024
 # ip -d -j -p link show br0
[ {
...
        "linkinfo": {
            "info_kind": "bridge",
            "info_data": {
...
                "fdb_cur_learned_entries": 2,
                "fdb_max_learned_entries": 1024,
...
            }
        },
...
    } ]

Signed-off-by: Johannes Nixdorf <jnixdorf-oss at avm.de>
---
 include/uapi/linux/if_link.h |  2 ++
 ip/iplink_bridge.c           | 21 +++++++++++++++++++++
 man/man8/ip-link.8.in        |  9 +++++++++
 3 files changed, 32 insertions(+)

diff --git a/include/uapi/linux/if_link.h b/include/uapi/linux/if_link.h
index 94fb7ef9e226..5ad1e2727e0d 100644
--- a/include/uapi/linux/if_link.h
+++ b/include/uapi/linux/if_link.h
@@ -508,6 +508,8 @@ enum {
 	IFLA_BR_VLAN_STATS_PER_PORT,
 	IFLA_BR_MULTI_BOOLOPT,
 	IFLA_BR_MCAST_QUERIER_STATE,
+	IFLA_BR_FDB_CUR_LEARNED_ENTRIES,
+	IFLA_BR_FDB_MAX_LEARNED_ENTRIES,
 	__IFLA_BR_MAX,
 };
 
diff --git a/ip/iplink_bridge.c b/ip/iplink_bridge.c
index 7e4e62c81c0c..68ed3c251945 100644
--- a/ip/iplink_bridge.c
+++ b/ip/iplink_bridge.c
@@ -34,6 +34,7 @@ static void print_explain(FILE *f)
 		"		  [ group_fwd_mask MASK ]\n"
 		"		  [ group_address ADDRESS ]\n"
 		"		  [ no_linklocal_learn NO_LINKLOCAL_LEARN ]\n"
+		"		  [ fdb_max_learned_entries FDB_MAX_LEARNED_ENTRIES ]\n"
 		"		  [ vlan_filtering VLAN_FILTERING ]\n"
 		"		  [ vlan_protocol VLAN_PROTOCOL ]\n"
 		"		  [ vlan_default_pvid VLAN_DEFAULT_PVID ]\n"
@@ -168,6 +169,14 @@ static int bridge_parse_opt(struct link_util *lu, int argc,
char **argv,
 				bm.optval |= no_ll_learn_bit;
 			else
 				bm.optval &= ~no_ll_learn_bit;
+		} else if (matches(*argv, "fdb_max_learned_entries") == 0) {
+			__u32 fdb_max_learned_entries;
+
+			NEXT_ARG();
+			if (get_u32(&fdb_max_learned_entries, *argv, 0))
+				invarg("invalid fdb_max_learned_entries", *argv);
+
+			addattr32(n, 1024, IFLA_BR_FDB_MAX_LEARNED_ENTRIES,
fdb_max_learned_entries);
 		} else if (matches(*argv, "fdb_flush") == 0) {
 			addattr(n, 1024, IFLA_BR_FDB_FLUSH);
 		} else if (matches(*argv, "vlan_default_pvid") == 0) {
@@ -544,6 +553,18 @@ static void bridge_print_opt(struct link_util *lu, FILE *f,
struct rtattr *tb[])
 	if (tb[IFLA_BR_GC_TIMER])
 		_bridge_print_timer(f, "gc_timer", tb[IFLA_BR_GC_TIMER]);
 
+	if (tb[IFLA_BR_FDB_CUR_LEARNED_ENTRIES])
+		print_uint(PRINT_ANY,
+			   "fdb_cur_learned_entries",
+			   "fdb_cur_learned_entries %u ",
+			   rta_getattr_u32(tb[IFLA_BR_FDB_CUR_LEARNED_ENTRIES]));
+
+	if (tb[IFLA_BR_FDB_MAX_LEARNED_ENTRIES])
+		print_uint(PRINT_ANY,
+			   "fdb_max_learned_entries",
+			   "fdb_max_learned_entries %u ",
+			   rta_getattr_u32(tb[IFLA_BR_FDB_MAX_LEARNED_ENTRIES]));
+
 	if (tb[IFLA_BR_VLAN_DEFAULT_PVID])
 		print_uint(PRINT_ANY,
 			   "vlan_default_pvid",
diff --git a/man/man8/ip-link.8.in b/man/man8/ip-link.8.in
index bf3605a9fa2e..a29595858a51 100644
--- a/man/man8/ip-link.8.in
+++ b/man/man8/ip-link.8.in
@@ -1620,6 +1620,8 @@ the following additional arguments are supported:
 ] [
 .BI no_linklocal_learn " NO_LINKLOCAL_LEARN "
 ] [
+.BI fdb_max_entries " FDB_MAX_ENTRIES "
+] [
 .BI vlan_filtering " VLAN_FILTERING "
 ] [
 .BI vlan_protocol " VLAN_PROTOCOL "
@@ -1731,6 +1733,13 @@ or off
 When disabled, the bridge will not learn from link-local frames (default:
 enabled).
 
+.BI fdb_max_learned_entries " FDB_MAX_LEARNED_ENTRIES "
+- set the maximum number of learned FDB entries linux may create. If
+.RI ( FDB_MAX_LEARNED_ENTRIES " == 0) "
+the feature is disabled.
+.I FDB_MAX_LEARNED_ENTRIES
+is a 32bit unsigned integer.
+
 .BI vlan_filtering " VLAN_FILTERING "
 - turn VLAN filtering on
 .RI ( VLAN_FILTERING " > 0) "
-- 
2.40.1