netdev - Re: [PATCH net-next 07/16] net: bridge: Maintain number of MDB entries in net_bridge_mcast

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Date:   Sun, 29 Jan 2023 11:40:49 +0200
From:   Nikolay Aleksandrov <razor@...ckwall.org>
To:     Petr Machata <petrm@...dia.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Roopa Prabhu <roopa@...dia.com>, netdev@...r.kernel.org
Cc:     bridge@...ts.linux-foundation.org, Ido Schimmel <idosch@...dia.com>
Subject: Re: [PATCH net-next 07/16] net: bridge: Maintain number of MDB
 entries in net_bridge_mcast_port

On 26/01/2023 19:01, Petr Machata wrote:
> The MDB maintained by the bridge is limited. When the bridge is configured
> for IGMP / MLD snooping, a buggy or malicious client can easily exhaust its
> capacity. In SW datapath, the capacity is configurable through the
> IFLA_BR_MCAST_HASH_MAX parameter, but ultimately is finite. Obviously a
> similar limit exists in the HW datapath for purposes of offloading.
> 
> In order to prevent the issue of unilateral exhaustion of MDB resources,
> introduce two parameters in each of two contexts:
> 
> - Per-port and per-port-VLAN number of MDB entries that the port
>   is member in.
> 
> - Per-port and (when BROPT_MCAST_VLAN_SNOOPING_ENABLED is enabled)
>   per-port-VLAN maximum permitted number of MDB entries, or 0 for
>   no limit.
> 
> The per-port multicast context is used for tracking of MDB entries for the
> port as a whole. This is available for all bridges.
> 
> The per-port-VLAN multicast context is then only available on
> VLAN-filtering bridges on VLANs that have multicast snooping on.
> 
> With these changes in place, it will be possible to configure MDB limit for
> bridge as a whole, or any one port as a whole, or any single port-VLAN.
> 
> Note that unlike the global limit, exhaustion of the per-port and
> per-port-VLAN maximums does not cause disablement of multicast snooping.
> It is also permitted to configure the local limit larger than hash_max,
> even though that is not useful.
> 
> In this patch, introduce only the accounting for number of entries, and the
> max field itself, but not the means to toggle the max. The next patch
> introduces the netlink APIs to toggle and read the values.
> 
> Note that the per-port-VLAN mcast_max_groups value gets reset when VLAN
> snooping is enabled. The reason for this is that while VLAN snooping is
> disabled, permanent entries can be added above the limit imposed by the
> configured maximum. Under those circumstances, whatever caused the VLAN
> context enablement, would need to be rolled back, adding a fair amount of
> code that would be rarely hit and tricky to maintain. At the same time,
> the feature that this would enable is IMHO not interesting: I posit that
> the usefulness of keeping mcast_max_groups intact across
> mcast_vlan_snooping toggles is marginal at best.
> 
> Signed-off-by: Petr Machata <petrm@...dia.com>
> Reviewed-by: Ido Schimmel <idosch@...dia.com>
> ---
>  net/bridge/br_multicast.c | 131 +++++++++++++++++++++++++++++++++++++-
>  net/bridge/br_private.h   |   2 +
>  2 files changed, 132 insertions(+), 1 deletion(-)
> 

Acked-by: Nikolay Aleksandrov <razor@...ckwall.org>