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@xxxxxx/ 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
