netdev - Re: [PATCH V3 net-next 1/4] net: bridge: add fdb flag to extent locked port feature

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <86v8tu7za3.fsf@gmail.com>
Date:   Wed, 25 May 2022 11:11:00 +0200
From:   Hans Schultz <schultz.hans@...il.com>
To:     Nikolay Aleksandrov <razor@...ckwall.org>,
        Hans Schultz <schultz.hans@...il.com>, davem@...emloft.net,
        kuba@...nel.org
Cc:     netdev@...r.kernel.org, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>, Jiri Pirko <jiri@...nulli.us>,
        Ivan Vecera <ivecera@...hat.com>,
        Roopa Prabhu <roopa@...dia.com>, Shuah Khan <shuah@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Ido Schimmel <idosch@...dia.com>, linux-kernel@...r.kernel.org,
        bridge@...ts.linux-foundation.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH V3 net-next 1/4] net: bridge: add fdb flag to extent
 locked port feature

On ons, maj 25, 2022 at 11:38, Nikolay Aleksandrov <razor@...ckwall.org> wrote:
> On 25/05/2022 11:34, Hans Schultz wrote:
>> On ons, maj 25, 2022 at 11:06, Nikolay Aleksandrov <razor@...ckwall.org> wrote:
>>> On 24/05/2022 19:21, Hans Schultz wrote:
>>>>>
>>>>> Hi Hans,
>>>>> So this approach has a fundamental problem, f->dst is changed without any synchronization
>>>>> you cannot rely on it and thus you cannot account for these entries properly. We must be very
>>>>> careful if we try to add any new synchronization not to affect performance as well.
>>>>> More below...
>>>>>
>>>>>> @@ -319,6 +326,9 @@ static void fdb_delete(struct net_bridge *br, struct net_bridge_fdb_entry *f,
>>>>>>  	if (test_bit(BR_FDB_STATIC, &f->flags))
>>>>>>  		fdb_del_hw_addr(br, f->key.addr.addr);
>>>>>>  
>>>>>> +	if (test_bit(BR_FDB_ENTRY_LOCKED, &f->flags) && !test_bit(BR_FDB_OFFLOADED, &f->flags))
>>>>>> +		atomic_dec(&f->dst->locked_entry_cnt);
>>>>>
>>>>> Sorry but you cannot do this for multiple reasons:
>>>>>  - f->dst can be NULL
>>>>>  - f->dst changes without any synchronization
>>>>>  - there is no synchronization between fdb's flags and its ->dst
>>>>>
>>>>> Cheers,
>>>>>  Nik
>>>>
>>>> Hi Nik,
>>>>
>>>> if a port is decoupled from the bridge, the locked entries would of
>>>> course be invalid, so maybe if adding and removing a port is accounted
>>>> for wrt locked entries and the count of locked entries, would that not
>>>> work?
>>>>
>>>> Best,
>>>> Hans
>>>
>>> Hi Hans,
>>> Unfortunately you need the correct amount of locked entries per-port if you want
>>> to limit their number per-port, instead of globally. So you need a
>>> consistent
>> 
>> Hi Nik,
>> the used dst is a port structure, so it is per-port and not globally.
>> 
>> Best,
>> Hans
>> 
>
> Yeah, I know. :) That's why I wrote it, if the limit is not a feature requirement I'd suggest
> dropping it altogether, it can be enforced externally (e.g. from user-space) if needed.
>
> By the way just fyi net-next is closed right now due to merge window. And one more
> thing please include a short log of changes between versions when you send a new one.
> I had to go look for v2 to find out what changed.
>

Okay, I will drop the limit in the bridge module, which is an easy thing
to do. :) (It is mostly there to ensure against DOS attacks if someone
bombards a locked port with random mac addresses.)
I have a similar limitation in the driver, which should then probably be
dropped too?

The mayor difference between v2 and v3 is in the mv88e6xxx driver, where
I now keep an inventory of locked ATU entries and remove them based on a
timer (mv88e6xxx_switchcore.c).

I guess the mentioned log should be in the cover letter part?


>>> fdb view with all its attributes when changing its dst in this case, which would
>>> require new locking because you have multiple dependent struct fields and it will
>>> kill roaming/learning scalability. I don't think this use case is worth the complexity it
>>> will bring, so I'd suggest an alternative - you can monitor the number of locked entries
>>> per-port from a user-space agent and disable port learning or some similar solution that
>>> doesn't require any complex kernel changes. Is the limit a requirement to add the feature?
>>>
>>> I have an idea how to do it and to minimize the performance hit if it really is needed
>>> but it'll add a lot of complexity which I'd like to avoid if possible.
>>>
>>> Cheers,
>>>  Nik