On 16/05/2023 11:53, Johannes Nixdorf wrote: > On Tue, May 16, 2023 at 11:38:11AM +0300, Nikolay Aleksandrov wrote: >> On 15/05/2023 11:50, Johannes Nixdorf wrote: >>> 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_ENTRIES, >>> which, if nonzero, limits the amount of entries to a user specified >>> maximum. >>> >>> For backwards compatibility the default setting of 0 disables the 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 ✓ >>> >>> Signed-off-by: Johannes Nixdorf <jnixdorf-oss@xxxxxx> >>> --- >>> include/uapi/linux/if_link.h | 1 + >>> net/bridge/br_device.c | 2 ++ >>> net/bridge/br_fdb.c | 6 ++++++ >>> net/bridge/br_netlink.c | 9 ++++++++- >>> net/bridge/br_private.h | 2 ++ >>> 5 files changed, 19 insertions(+), 1 deletion(-) >>> >> >> I completely missed the fact that you don't deal with the situation where you already have fdbs created >> and a limit is set later, then it would be useless because it will start counting from 0 even though >> there are already entries. > > This should not be an issue. The accounting starts with the bridge > creation and is never suspended, so if the user sets a limit later we > do not restart counting at 0. > > The only corner case I can see there is if the user sets a new limit > lower than the current number of FDB entries. In that case the code > currently leaves the bridge in a state where the limit is violated, > but refuses new FDB entries until the total is back below the limit. The > alternative of cleaning out old FDB entries until their number is under > the limit again seems to be more error prone to me as well, so I'd rather > leave it that way. > Ah, good. That's ok then. >> Also another issue that came to mind is that you don't deal with fdb_create() >> for "special" entries, i.e. when adding a port. Currently it will print an error, but you should revisit >> all callers and see where it might be a problem. > > I'll have a look again, also to see whether only counting dynamic > entries created as a reaction to observed packets might be a viable > alternative. If the user creates the entries by adding a port or manually > via netlink I see no reason to restrict them to the same limit. Hmm.. perhaps we can add a flag mask of entries to count. Initially it can be only dynamic entries. We should include more people in this discussion (+CC Ido and Vladimir). Switchdev folks might have more specific requirements and restrictions, so it'd be nice to get their input as well.
