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Date:   Tue, 26 Sep 2023 14:22:42 +0300
From:   Ido Schimmel <idosch@idosch.org>
To:     Johannes Nixdorf <jnixdorf-oss@avm.de>
Cc:     "David S. Miller" <davem@davemloft.net>,
        Andrew Lunn <andrew@lunn.ch>, David Ahern <dsahern@gmail.com>,
        Eric Dumazet <edumazet@google.com>,
        Florian Fainelli <f.fainelli@gmail.com>,
        Ido Schimmel <idosch@nvidia.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Nikolay Aleksandrov <razor@blackwall.org>,
        Oleksij Rempel <linux@rempel-privat.de>,
        Paolo Abeni <pabeni@redhat.com>,
        Roopa Prabhu <roopa@nvidia.com>, Shuah Khan <shuah@kernel.org>,
        Vladimir Oltean <vladimir.oltean@nxp.com>,
        bridge@lists.linux-foundation.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org
Subject: Re: [PATCH net-next v4 3/6] net: bridge: Track and limit dynamically
 learned FDB entries
Message-ID: <ZRK/ArWPNHgjVvWA@shredder>
References: <20230919-fdb_limit-v4-0-39f0293807b8@avm.de>
 <20230919-fdb_limit-v4-3-39f0293807b8@avm.de>
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On Tue, Sep 19, 2023 at 10:12:50AM +0200, 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@avm.de>

Reviewed-by: Ido Schimmel <idosch@nvidia.com>