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, atomically bundled under the new flag BR_FDB_DYNAMIC_LEARNED. This means the limit also applies to entries created with BR_FDB_ADDED_BY_EXT_LEARN but none of BR_FDB_LOCAL or BR_FDB_ADDED_BY_USER, e.g. ones added by SWITCHDEV_FDB_ADD_TO_BRIDGE. Changes since v2: - Fixed the flags for fdb_create in fdb_add_entry to use BIT(...). Previously we passed garbage. (from review) - Set strict_start_type for br_policy. (from review) - Split out the combined accounting and limit patch, and the netlink patch from the combined patch in v2. (from review) - Count atomically, remove the newly introduced lock. (from review) - Added the new attributes to br_policy. (from review) - Added a selftest for the new feature. (from review) 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. iproute2-next v3: https://lore.kernel.org/netdev/20230905-fdb_limit-v3-1-34bb124556d8@xxxxxx/ v2: https://lore.kernel.org/netdev/20230619071444.14625-1-jnixdorf-oss@xxxxxx/ v1: https://lore.kernel.org/netdev/20230515085046.4457-1-jnixdorf-oss@xxxxxx/ Signed-off-by: Johannes Nixdorf <jnixdorf-oss@xxxxxx> --- Johannes Nixdorf (6): net: bridge: Set BR_FDB_ADDED_BY_USER early in fdb_add_entry net: bridge: Set strict_start_type for br_policy net: bridge: Track and limit dynamically learned FDB entries net: bridge: Add netlink knobs for number / max learned FDB entries net: bridge: Add a configurable default FDB learning limit selftests: forwarding: bridge_fdb_learning_limit: Add a new selftest include/uapi/linux/if_link.h | 2 + net/bridge/Kconfig | 13 + net/bridge/br_device.c | 2 + net/bridge/br_fdb.c | 40 ++- net/bridge/br_netlink.c | 16 +- net/bridge/br_private.h | 4 + .../net/forwarding/bridge_fdb_learning_limit.sh | 283 +++++++++++++++++++++ 7 files changed, 354 insertions(+), 6 deletions(-) --- base-commit: 2dde18cd1d8fac735875f2e4987f11817cc0bc2c change-id: 20230904-fdb_limit-fae5bbf16c88 Best regards, -- Johannes Nixdorf <jnixdorf-oss@xxxxxx>
