Re: Potential issues (security and otherwise) with the current cgroup-bpf API

[David Ahern <dsahern@gmail.com>]

On 12/19/16 6:56 PM, Andy Lutomirski wrote:
> On Mon, Dec 19, 2016 at 5:44 PM, David Ahern <dsahern@gmail.com> wrote:
>> On 12/19/16 5:25 PM, Andy Lutomirski wrote:
>>> net.socket_create_filter = "none": no filter
>>> net.socket_create_filter = "bpf:baadf00d": bpf filter
>>> net.socket_create_filter = "disallow": no sockets created period
>>> net.socket_create_filter = "iptables:foobar": some iptables thingy
>>> net.socket_create_filter = "nft:blahblahblah": some nft thingy
>>> net.socket_create_filter = "address_family_list:1,2,3": allow AF 1, 2, and 3
>>
>> Such a scheme works for the socket create filter b/c it is a very simple use case. It does not work for the ingress and egress which allow generic bpf filters.
> 
> Can you elaborate on what goes wrong?  (Obviously the
> "address_family_list" example makes no sense in that context.)

Being able to dump a filter or see that one exists would be a great add-on, but I don't see how 'net.socket_create_filter = "bpf:baadf00d"' is a viable API for loading generic BPF filters. Simple cases like "disallow" are easy -- just return 0 in the filter, no complicated BPF code needed. The rest are specific cases of the moment which goes against the intent of ebpf and generic programmability. 
 
>>
>> ...
>>
>>>> you're ignoring use cases I described earlier.
>>>> In vrf case there is only one ifindex it needs to bind to.
>>>
>>> I'm totally lost.  Can you explain what this has to do with the cgroup
>>> hierarchy?
>>
>> I think the point is that a group hierarchy makes no sense for the VRF use case. What I put into iproute2 is
>>
>>     cgrp2/vrf/NAME
>>
>> where NAME is the vrf name. The filter added to it binds ipv4 and ipv6 sockets to a specific device index. cgrp2/vrf is the "default" vrf and does not have a filter. A user can certainly add another layer cgrp2/vrf/NAME/NAME2 but it provides no value since VRF in a VRF does not make sense.
> 
> I tend to agree.  I still think that the mechanism as it stands is
> broken in other respects and should be fixed before it goes live.  I
> have no desire to cause problems for the vrf use case.
> 
> But keep in mind that the vrf use case is, in Linus' tree, a bit
> broken right now in its interactions with other users of the same
> mechanism.  Suppose I create a container and want to trace all of its
> created sockets.  I'll set up cgrp2/container and load my tracer as a
> socket creation hook.  Then a container sets up
> cgrp2/container/vrf/NAME (using delgation) and loads your vrf binding
> filter.  Now the tracing stops working -- oops.

There are other ways to achieve socket tracing, but I get your point -- nested cases do not work as users may want.


>>>>> I like this last one, but IT'S NOT A POSSIBLE FUTURE EXTENSION.  You
>>>>> have to do it now (or disable the feature for 4.10).  This is why I'm
>>>>> bringing this whole thing up now.
>>>>
>>>> We don't have to touch user visible api here, so extensions are fine.
>>>
>>> Huh?  My example in the original email attaches a program in a
>>> sub-hierarchy.  Are you saying that 4.11 could make that example stop
>>> working?
>>
>> Are you suggesting sub-cgroups should not be allowed to override the filter of a parent cgroup?
> 
> Yes, exactly.  I think there are two sensible behaviors:
> 
> a) sub-cgroups cannot have a filter at all of the parent has a filter.
> (This is the "punt" approach -- it lets different semantics be
> assigned later without breaking userspace.)
> 
> b) sub-cgroups can have a filter if a parent does, too.  The semantics
> are that the sub-cgroup filter runs first and all side-effects occur.
> If that filter says "reject" then ancestor filters are skipped.  If
> that filter says "accept", then the ancestor filter is run and its
> side-effects happen as well.  (And so on, all the way up to the root.)

That comes with a big performance hit for skb / data path cases.

I'm riding my use case on Daniel's work, and as I understand it the nesting case has been discussed. I'll defer to Daniel and Alexei on this part.