Re: [PATCH 2/2] net: Implement SO_PASSCGROUP to enable passing cgroup path

[Andy Lutomirski]

On Wed, Apr 16, 2014 at 11:06 AM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
> On Wed, Apr 16, 2014 at 09:31:25AM -0700, Andy Lutomirski wrote:
> I am not sure how same issue with happen with cgroups. In the case of
> socket example, you are forcing a setuid program to write to standard
> output and that setuid program will run in same cgroup as caller and
> will have same cgroup as caller. So even if somebody was using cgroup
> information for authentication, atleast in this particular case it
> will not be a problem. Both unpriviliged and priviliged programs has
> same cgroups.
>

I'm not sure that there's an actual attackable program.  But I also
see no reason to be convinced that there isn't one, and the problem
can easily be avoided by requiring programs to explicitly ask to send
their cgroup.

>>
>> >
>> > The only one that *may* be reasonable is the "secret" cgroup name one,
>> > however nobody seem to come up with a reason why it is legitimate to
>> > allow to keep cgroup names secret.
>> >
>> > And if you can come up with such a good reason the SO_NOPASSCGROUP
>> > option seem the right solution.
>> >
>> >> This ABI is especially tricky because programs will use it even if
>> >> they don't explicitly try to.  So just adding the ABI may break
>> >> existing assumptions that are relevant to security or correctness.
>> >
>> > It's not clear to me what you mean by this, either you explicitly use
>> > SO_PASSCGROUP or not, it's not like you can involuntarily add a flag ...
>> >
>>
>> The issue here is that the receiver sets SO_(PASS|PEER)CGROUP, forcing
>> the sender to identify or authenticate itself.  The sender might not
>> want to identify itself.  Even if you don't buy any secrecy arguments,
>> the sender might not intend to authenticate.  Certainly no existing
>> callers of connect or write intend to authenticate using their cgroup,
>> since current kernels don't have those semantics.
>
> Ok, so passing cgroup information is not necessarily a problem as long
> as it is not used for authentication. So say somebody is just logging
> all the client request and which cgroup client was in, that should not
> be a problem.

Do you consider correct attribution of logging messages to be
important?  If so, then this is a kind of authentication, albeit one
where the impact of screwing it up is a bit lower.

>
> I agree that before somebody uses cgroup information for authentication
> purposes, may be there needs to be a bigger debate whether this info
> can be used safely for authentication purposes or not and in what
> circumstances it is safe to use for authentication.

I thought that the original intended user of these patches was SSSD.
I have no idea what SSSD wanted them for, but I think it may better.

>
> But that does not mean that API to pass the cgroup information around is
> wrong.
>

It may not be wrong, but it might be extremely difficult or impossible
to use it safely.  I think that's something to avoid.

--Andy
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