Date:   Fri, 31 Jan 2020 08:12:01 +0000
From:   James Chapman <jchapman@...alix.com>
To:     Guillaume Nault <gnault@...hat.com>
Cc:     Tom Parkin <tparkin@...alix.com>,
        Ridge Kennedy <ridgek@...iedtelesis.co.nz>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] l2tp: Allow duplicate session creation with UDP

On 30/01/2020 22:34, Guillaume Nault wrote:
> On Thu, Jan 30, 2020 at 10:28:23AM +0000, James Chapman wrote:
>> On 29/01/2020 11:44, Guillaume Nault wrote:
>>> Since userspace is in charge of selecting the session ID, I still can't
>>> see how having the kernel accept duplicate IDs goes against the RFC.
>>> The kernel doesn't assign duplicate IDs on its own. Userspace has full
>>> control on the IDs and can implement whatever constraint when assigning
>>> session IDs (even the DOCSIS DEPI way of partioning the session ID
>>> space).
>> Perhaps another example might help.
>>
>> Suppose there's an L2TPv3 app out there today that creates two tunnels
>> to a peer, one of which is used as a hot-standby backup in case the main
>> tunnel fails. This system uses separate network interfaces for the
>> tunnels, e.g. a router using a mobile network as a backup. If the main
>> tunnel fails, it switches traffic of sessions immediately into the
>> second tunnel. Userspace is deliberately using the same session IDs in
>> both tunnels in this case. This would work today for IP-encap, but not
>> for UDP. However, if the kernel treats session IDs as scoped by 3-tuple,
>> the application would break. The app would need to be modified to add
>> each session ID into both tunnels to work again.
>>
> That's an interesting use case. I can imagine how this works on Rx, but
> how can packets be transmitted on the new tunnel? The session will
> still send packets through the original tunnel with the original
> 3-tuple, and there's no way to reassign a session to a new tunnel. We
> could probably rebind/reconnect the tunnel socket, but then why
> creating the second tunnel in the kernel?

It might use some netfilter or BPF code to change the IPs and redirect
outbound packets. TBH, it's a hypothetical use case and probably easier
to implement using scoped session IDs. :-)


>>>>> I would have to read the RFC with scoped session IDs in mind, but, as
>>>>> far as I can see, the only things that global session IDs allow which
>>>>> can't be done with scoped session IDs are:
>>>>>   * Accepting L2TPoIP sessions to receive L2TPoUDP packets and
>>>>>     vice-versa.
>>>>>   * Accepting L2TPv3 packets from peers we're not connected to.
>>>>>
>>>>> I don't find any of these to be desirable. Although Tom convinced me
>>>>> that global session IDs are in the spirit of the RFC, I still don't
>>>>> think that restricting their scope goes against it in any practical
>>>>> way. The L2TPv3 control plane requires a two way communication, which
>>>>> means that the session is bound to a given 3/5-tuple for control
>>>>> messages. Why would the data plane behave differently?
>>>> The Cable Labs / DOCSIS DEPI protocol is a good example. It is based on
>>>> L2TPv3 and uses the L2TPv3 data plane. It treats the session ID as
>>>> unscoped and not associated with a given tunnel.
>>>>
>>> Fair enough. Then we could add a L2TP_ATTR_SCOPE netlink attribute to
>>> sessions. A global scope would reject the session ID if another session
>>> already exists with this ID in the same network namespace. Sessions with
>>> global scope would be looked up solely based on their ID. A non-global
>>> scope would allow a session ID to be duplicated as long as the 3/5-tuple
>>> is different and no session uses this ID with global scope.
>>>
>>>>> I agree that it looks saner (and simpler) for a control plane to never
>>>>> assign the same session ID to sessions running over different tunnels,
>>>>> even if they have different 3/5-tuples. But that's the user space
>>>>> control plane implementation's responsability to select unique session
>>>>> IDs in this case. The fact that the kernel uses scoped or global IDs is
>>>>> irrelevant. For unmanaged tunnels, the administrator has complete
>>>>> control over the local and remote session IDs and is free to assign
>>>>> them globally if it wants to, even if the kernel would have accepted
>>>>> reusing session IDs.
>>>> I disagree. Using scoped session IDs may break applications that assume
>>>> RFC behaviour. I mentioned one example where session IDs are used
>>>> unscoped above.
>>>>
>>> I'm sorry, but I still don't understand how could that break any
>>> existing application.
>> Does my example of the hot-standby backup tunnel help?
>>
> Yes, even though I'm not sure how it precisely translate in terms of
> userspace/kernel interraction. But anyway, with L2TP_ATTR_SCOPE, we'd
> have the possibility to keep session ID unscoped for l2tp_ip by
> default. That should be enough to keep any such scenario working
> without any modification.
>
>>> For L2TPoUDP, session IDs are always looked up in the context of the
>>> UDP socket. So even though the kernel has stopped accepting duplicate
>>> IDs, the session IDs remain scoped in practice. And with the
>>> application being responsible for assigning IDs, I don't see how making
>>> the kernel less restrictive could break any existing implementation.
>>> Again, userspace remains in full control for session ID assignment
>>> policy.
>>>
>>> Then we have L2TPoIP, which does the opposite, always looks up sessions
>>> globally and depends on session IDs being unique in the network
>>> namespace. But Ridge's patch does not change that. Also, by adding the
>>> L2TP_ATTR_SCOPE attribute (as defined above), we could keep this
>>> behaviour (L2TPoIP session could have global scope by default).
>> I'm looking at this with an end goal of having the UDP rx path later
>> modified to work the same way as IP-encap currently does. I know Linux
>> has never worked this way in the L2TPv3 UDP path and no-one has
>> requested that it does yet, but I think it would improve the
>> implementation if UDP and IP encap behaved similarly.
>>
> Yes, unifying UDP and IP encap would be really nice.
>
>> L2TP_ATTR_SCOPE would be a good way for the app to select which
>> behaviour it prefers.
>>
> Yes. But do we agree that it's also a way to keep the existing
> behaviour: unscoped for IP, scoped to the 5-tuple for UDP? That is, IP
> and UDP encap would use a different default value when user space
> doesn't request a specific behaviour.

Yes, that would be the safest approach.


>>>> However, there might be an alternative solution to fix this for Ridge's
>>>> use case that doesn't involve adding 3/5-tuple session ID lookups in the
>>>> receive path or adding a control knob...
>>>>
>>>> My understanding is that Ridge's application uses unmanaged tunnels
>>>> (like "ip l2tp" does). These use kernel sockets. The netlink tunnel
>>>> create request does not indicate a valid tunnel socket fd. So we could
>>>> use scoped session IDs for unmanaged UDP tunnels only. If Ridge's patch
>>>> were tweaked to allow scoped IDs only for UDP unmanaged tunnels (adding
>>>> a test for tunnel->fd < 0), managed tunnels would continue to work as
>>>> they do now and any application that uses unmanaged tunnels would get
>>>> scoped session IDs. No control knob or 3/5-tuple session ID lookups
>>>> required.
>>>>
>>> Well, I'd prefer to not introduce another subtle behaviour change. What
>>> does rejecting duplicate IDs bring us if the lookup is still done in
>>> the context of the socket? If the point is to have RFC compliance, then
>>> we'd also need to modify the lookup functions.
>>>
>> I agree, it's not ideal. Rejecting duplicate IDs for UDP will allow the
>> UDP rx path to be modified later to work the same way as IP. So my idea
>> was to allow for that change to be made later but only for managed
>> tunnels (sockets created by userspace). My worry with the original patch
>> is that it suggests that session IDs for UDP are always scoped by the
>> tunnel so tweaking it to apply only for unmanaged tunnels was a way of
>> showing this.
>>
>> However, you've convinced me now that scoping the session ID by
>> 3/5-tuple could work. As long as there's a mechanism that lets
>> applications choose whether the 3/5-tuple is ignored in the rx path, I'm
>> ok with it.
>>
> Do we agree that, with L2TP_ATTR_SCOPE being a long-term solution, we
> shouldn't need to reject duplicate session IDs for UDP tunnels?

Yes.


> To summarise my idea:
>
>   * Short term plan:
>     Integrate a variant of Ridge's patch, as it's simple, can easily be
>     backported to -stable and doesn't prevent the future use of global
>     session IDs (as those will be specified with L2TP_ATTR_SCOPE).
>
>   * Long term plan:
>     Implement L2TP_ATTR_SCOPE, a session attribute defining if the
>     session ID is global or scoped to the X-tuple (3-tuple for IP,
>     5-tuple for UDP).
>     Original behaviour would be respected to avoid breaking existing
>     applications. So, by default, IP encapsulation would use global
>     scope and UDP encapsulation would use 5-tuple scope.
>
> Does that look like a good way forward?

Yes, it sounds good to me.

Your proposed approach of using only the session ID to do the session
lookup but then optionally using the 3/5-tuple to scope it resolves my
concerns.

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