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. Link to the corresponding iproute2 changes: https://lore.kernel.org/r/20230919-fdb_limit-v4-1-b4d2dc4df30f@xxxxxx Signed-off-by: Johannes Nixdorf <jnixdorf-oss@xxxxxx> --- Changes in v5: - Set IFLA_BR_FDB_N_LEARNED to NLA_REJECT (from review) - Moved the strict_start_type-commit after the netlink change, used the new attribute. (from review) - Dropped the new build time config option. (from review) - Link to v4: https://lore.kernel.org/r/20230919-fdb_limit-v4-0-39f0293807b8@xxxxxx Changes in v4: - Added the new test to the Makefile. (from review) - Removed _entries from the names. (from iproute2 review, in some places only for consistency) - Wrapped the lines at 80 chars, except when longer lines are consistent with neighbouring code. (from review) - Fixed a race in fdb_delete. (from review) - Link to v3: https://lore.kernel.org/r/20230905-fdb_limit-v3-0-7597cd500a82@xxxxxx Changes in v3: - 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) - Link to v2: https://lore.kernel.org/netdev/20230619071444.14625-1-jnixdorf-oss@xxxxxx/ Changes in v2: - 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. - Link to v1: https://lore.kernel.org/netdev/20230515085046.4457-1-jnixdorf-oss@xxxxxx/ 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. --- Johannes Nixdorf (5): net: bridge: Set BR_FDB_ADDED_BY_USER early in fdb_add_entry net: bridge: Track and limit dynamically learned FDB entries net: bridge: Add netlink knobs for number / max learned FDB entries net: bridge: Set strict_start_type for br_policy selftests: forwarding: bridge_fdb_learning_limit: Add a new selftest include/uapi/linux/if_link.h | 2 + net/bridge/br_fdb.c | 42 ++- net/bridge/br_netlink.c | 17 +- net/bridge/br_private.h | 4 + tools/testing/selftests/net/forwarding/Makefile | 3 +- .../net/forwarding/bridge_fdb_learning_limit.sh | 283 +++++++++++++++++++++ 6 files changed, 344 insertions(+), 7 deletions(-) --- base-commit: 58720809f52779dc0f08e53e54b014209d13eebb change-id: 20230904-fdb_limit-fae5bbf16c88 Best regards, -- Johannes Nixdorf <jnixdorf-oss@xxxxxx>
