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

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Date:	Tue, 15 Apr 2014 20:47:54 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Tejun Heo <tj@...nel.org>, Daniel Walsh <dwalsh@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	lpoetter@...hat.com, Simo Sorce <ssorce@...hat.com>,
	cgroups@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	kay@...hat.com, Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH 2/2] net: Implement SO_PASSCGROUP to enable passing cgroup path

On Apr 15, 2014 5:20 PM, "Vivek Goyal" <vgoyal@...hat.com> wrote:
>
> On Tue, Apr 15, 2014 at 02:53:13PM -0700, Andy Lutomirski wrote:
> > On Tue, Apr 15, 2014 at 2:15 PM, Vivek Goyal <vgoyal@...hat.com> wrote:
> > > This patch implements socket option SO_PASSCGROUP along the lines of
> > > SO_PASSCRED.
> > >
> > > If SO_PASSCGROUP is set, then recvmsg() will get a control message
> > > SCM_CGROUP which will contain the cgroup path of sender. This cgroup
> > > belongs to first mounted hierarchy in the sytem.
> > >
> > > SCM_CGROUP control message can only be received and sender can not send
> > > a SCM_CGROUP message. Kernel automatically generates one if receiver
> > > chooses to receive one.
> > >
> > > This works both for unix stream and datagram sockets.
> > >
> > > cgroup information is passed only if either the sender or receiver has
> > > SO_PASSCGROUP option set. This means for existing workloads they should
> > > not see any significant performance impact of this change.
> >
> > This is odd.  Shouldn't an SCM_CGROUP cmsg be generated when the
> > receiver has SO_PASSCGROUP set and the sender passes SCM_CGROUP to
> > sendmsg?
>
> How can receiver trust the cgroup info generated by sender. It needs to
> be generated by kernel so that receiver can trust it.
>
> And if receiver needs to know cgroup of sender, receiver can just set
> SO_PASSCGROUP on socket and receiver should get one SCM_CGROUP message
> with each message received.

I think the kernel should validate the data.

Here's an attack against SO_PEERCGROUP: if you create a container with
a super secret name, then every time you connect to any unix socket,
you leak the name.

Here's an attack against SO_PASSCGROUP, as you implemented it: connect
a socket and get someone else to write(2) to it.  This isn't very
hard.  Now you've impersonated.

I advocate for the following semantics: if sendmsg is passed a
SCM_CGROUP cmsg, and that cmsg has the right cgroup, and the receiver
has SO_PASSCGROUP set, then the receiver gets SCM_CGROUP.  If you try
to lie using SCM_CGROUP, you get -EPERM.  If you set SO_PASSCGROUP,
but your peer doesn't sent SCM_CREDS, you get nothing.

This is immune to both attacks.  It should be cheaper, too, since
there's no overhead for people who don't use it.

--Andy
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