linux-kernel - Re: [PATCH net-next v4 3/6] net: bridge: Track and limit dynamically learned FDB entries

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Date:   Wed, 20 Sep 2023 13:49:01 +0300
From:   Nikolay Aleksandrov <razor@...ckwall.org>
To:     Johannes Nixdorf <jnixdorf-oss@....de>,
        "David S. Miller" <davem@...emloft.net>,
        Andrew Lunn <andrew@...n.ch>, David Ahern <dsahern@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Ido Schimmel <idosch@...dia.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Oleksij Rempel <linux@...pel-privat.de>,
        Paolo Abeni <pabeni@...hat.com>,
        Roopa Prabhu <roopa@...dia.com>, Shuah Khan <shuah@...nel.org>,
        Vladimir Oltean <vladimir.oltean@....com>
Cc:     bridge@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH net-next v4 3/6] net: bridge: Track and limit dynamically
 learned FDB entries

On 9/19/23 11:12, 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 maintaining a per bridge count of those automatically
> generated entries in fdb_n_learned, and a limit in fdb_max_learned. If
> the limit is hit new entries are not learned anymore.
> 
> 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.
> 
> Introduce a new fdb entry flag BR_FDB_DYNAMIC_LEARNED to keep track of
> whether an FDB entry is included in the count. The flag is enabled for
> dynamically learned entries, and disabled for all other entries. This
> should be equivalent to BR_FDB_ADDED_BY_USER and BR_FDB_LOCAL being unset,
> but contrary to the two flags it can be toggled atomically.
> 
> Atomicity is required here, as there are multiple callers that modify the
> flags, but are not under a common lock (br_fdb_update is the exception
> for br->hash_lock, br_fdb_external_learn_add for RTNL).
> 
> Signed-off-by: Johannes Nixdorf <jnixdorf-oss@....de>
> ---
>   net/bridge/br_fdb.c     | 35 +++++++++++++++++++++++++++++++++--
>   net/bridge/br_private.h |  4 ++++
>   2 files changed, 37 insertions(+), 2 deletions(-)
> 

I think this is a good counting start. :) It'd be nice to get
more eyes on this one.
Acked-by: Nikolay Aleksandrov <razor@...ckwall.org>