Re: [PATCH net-next 0/3] ipmr/ip6mr: add Netlink notifications on cache reports

[Julien Gomes <julien@arista.com>]

On 06/15/2017 06:00 AM, Nikolay Aleksandrov wrote:
> On 15/06/17 14:44, Nikolay Aleksandrov wrote:
>> On 15/06/17 14:33, Nikolay Aleksandrov wrote:
>>> On 15/06/17 00:51, Julien Gomes wrote:
>>>> Hi Nikolay,
>>>>
>>>> On 06/14/2017 05:04 AM, Nikolay Aleksandrov wrote:
>>>>
>>>>> This has been on our todo list and I'm definitely interested in the implementation.
>>>>> A few things that need careful consideration from my POV. First are the security
>>>>> implications - this sends rtnl multicast messages but the rtnl socket has
>>>>> the NL_CFG_F_NONROOT_RECV flag thus allowing any user on the system to listen in.
>>>>> This would allow them to see the full packets and all reports (granted they can see
>>>>> the notifications even now), but the full packet is like giving them the opportunity
>>>>> to tcpdump the PIM traffic.
>>>> I definitely see how this can be an issue.
>>>> From what I see, this means that either the packet should be
>>>> transmitted another way, or another Netlink family should be used.
>>>>
>>>> NETLINK_ROUTE looks to be the logical family to choose though,
>>>> but then I do not see a proper other way to handle this.
>>> Right, currently me neither, unless it provides a bind callback when registering
>>> the kernel socket.
>>>
>>>> However I may just not be looking into the right direction,
>>>> maybe you currently have another approach in mind?
>>> I haven't gotten around to make (or even try) them but I was thinking about 2 options
>>> ending up with a similar result:
>>>
>>> 1) genetlink
>>>  It also has the NONROOT_RECV flag, but it also allows for a callback - mcast_bind()
>>>  which can be used to filter.
>>>
>>> or
>>>
>>> 2) Providing a bind callback to the NETLINK_ROUTE socket.
>>>
>> Ah nevermind, these cannot be used for filtering currently, so it seems
>> the netlink interface would need to be extended too if going down this road.
>>
> Sorry for the multiple emails, just to be thorough - again if going down this
> road all of these would obviously require a different group to bind to in order
> to be able to filter on it, because users must keep receiving their notifications
> for the ipmr one.

Actually, using a bind callback for NETLINK_ROUTE with a new group,
without netlink interface extension, could  work.

I quickly tested something like this:
> static int rtnetlink_bind(struct net *net, int group)
> {
>     switch (group) {
>     case RTNLGRP_IPV4_MROUTE_R:
>     case RTNLGRP_IPV6_MROUTE_R:
>        if (!ns_capable(net->user_ns, CAP_NET_ADMIN))
>            return -EPERM;
>        break;
>     }
>     return 0;
> }

With the addition of one/two groups this does restrict the reports'
potential listeners.
The group names here are just placeholders, I am not especially fixed
on these ones.

It is not perfect as this would introduce groups with specific
requirements in NETLINK_ROUTE, but I think it can be decent.

What do you think about this?

-- 
Julien Gomes