Message-ID: <20191022200632.al5ajlaahsvjeuma@madcap2.tricolour.ca>
Date:   Tue, 22 Oct 2019 16:06:32 -0400
From:   Richard Guy Briggs <rgb@...hat.com>
To:     Neil Horman <nhorman@...driver.com>
Cc:     Paul Moore <paul@...l-moore.com>,
        containers@...ts.linux-foundation.org, linux-api@...r.kernel.org,
        Linux-Audit Mailing List <linux-audit@...hat.com>,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        netdev@...r.kernel.org, netfilter-devel@...r.kernel.org,
        sgrubb@...hat.com, omosnace@...hat.com, dhowells@...hat.com,
        simo@...hat.com, Eric Paris <eparis@...isplace.org>,
        Serge Hallyn <serge@...lyn.com>, ebiederm@...ssion.com,
        Dan Walsh <dwalsh@...hat.com>, mpatel@...hat.com
Subject: Re: [PATCH ghak90 V7 20/21] audit: add capcontid to set contid
 outside init_user_ns

On 2019-10-22 08:13, Neil Horman wrote:
> On Mon, Oct 21, 2019 at 08:31:37PM -0400, Paul Moore wrote:
> > On Mon, Oct 21, 2019 at 7:58 PM Richard Guy Briggs <rgb@...hat.com> wrote:
> > > On 2019-10-21 17:43, Paul Moore wrote:
> > > > On Mon, Oct 21, 2019 at 5:38 PM Richard Guy Briggs <rgb@...hat.com> wrote:
> > > > > On 2019-10-21 15:53, Paul Moore wrote:
> > > > > > On Fri, Oct 18, 2019 at 9:39 PM Richard Guy Briggs <rgb@...hat.com> wrote:
> > > > > > > On 2019-09-18 21:22, Richard Guy Briggs wrote:
> > > > > > > > Provide a mechanism similar to CAP_AUDIT_CONTROL to explicitly give a
> > > > > > > > process in a non-init user namespace the capability to set audit
> > > > > > > > container identifiers.
> > > > > > > >
> > > > > > > > Use audit netlink message types AUDIT_GET_CAPCONTID 1027 and
> > > > > > > > AUDIT_SET_CAPCONTID 1028.  The message format includes the data
> > > > > > > > structure:
> > > > > > > > struct audit_capcontid_status {
> > > > > > > >         pid_t   pid;
> > > > > > > >         u32     enable;
> > > > > > > > };
> > > > > > >
> > > > > > > Paul, can I get a review of the general idea here to see if you're ok
> > > > > > > with this way of effectively extending CAP_AUDIT_CONTROL for the sake of
> > > > > > > setting contid from beyond the init user namespace where capable() can't
> > > > > > > reach and ns_capable() is meaningless for these purposes?
> > > > > >
> > > > > > I think my previous comment about having both the procfs and netlink
> > > > > > interfaces apply here.  I don't see why we need two different APIs at
> > > > > > the start; explain to me why procfs isn't sufficient.  If the argument
> > > > > > is simply the desire to avoid mounting procfs in the container, how
> > > > > > many container orchestrators can function today without a valid /proc?
> > > > >
> > > > > Ok, sorry, I meant to address that question from a previous patch
> > > > > comment at the same time.
> > > > >
> > > > > It was raised by Eric Biederman that the proc filesystem interface for
> > > > > audit had its limitations and he had suggested an audit netlink
> > > > > interface made more sense.
> > > >
> > > > I'm sure you've got it handy, so I'm going to be lazy and ask: archive
> > > > pointer to Eric's comments?  Just a heads-up, I'm really *not* a fan
> > > > of using the netlink interface for this, so unless Eric presents a
> > > > super compelling reason for why we shouldn't use procfs I'm inclined
> > > > to stick with /proc.
> > >
> > > It was actually a video call with Eric and Steve where that was
> > > recommended, so I can't provide you with any first-hand communication
> > > about it.  I'll get more details...
> > 
> > Yeah, that sort of information really needs to be on the list.
> > 
> > > So, with that out of the way, could you please comment on the general
> > > idea of what was intended to be the central idea of this mechanism to be
> > > able to nest containers beyond the initial user namespace (knowing that
> > > a /proc interface is available and the audit netlink interface isn't
> > > necessary for it to work and the latter can be easily removed)?
> > 
> > I'm not entirely clear what you are asking about, are you asking why I
> > care about nesting container orchestrators?  Simply put, it is not
> > uncommon for the LXC/LXD folks to see nested container orchestrators,
> > so I felt it was important to support that use case.  When we
> > originally started this effort we probably should have done a better
> > job reaching out to the LXC/LXD folks, we may have caught this
> > earlier.  Regardless, we caught it, and it looks like we are on our
> > way to supporting it (that's good).
> > 
> > Are you asking why I prefer the procfs approach to setting/getting the
> > audit container ID?  For one, it makes it easier for a LSM to enforce
> > the audit container ID operations independent of the other audit
> > control APIs.  It also provides a simpler interface for container
> > orchestrators.  Both seem like desirable traits as far as I'm
> > concerned.
> 
> I agree that one api is probably the best approach here, but I actually
> think that the netlink interface is the more flexible approach.  Its a
> little more work for userspace (you have to marshal your data into a
> netlink message before sending it, and wait for an async response), but
> thats a well known pattern, and it provides significantly more
> flexibility for the kernel.  LSM already has a hook to audit netlink
> messages in sock_sendmsg, so thats not a problem, and if you use
> netlink, you get the advantage of being able to broadcast messages
> within your network namespaces, facilitating any needed orchestrator
> co-ordination.  To do the same thing with a filesystem api, you need to
> use the fanotify api, which IIRC doesn't work on proc.

One api was the intent, deprecating proc for loginuid and sessionid if
netlink was the chosen way to go.

I don't think we had discussed the possibility or need to use netlink
multicast for this purpose and see it as a liability to limiting access
to only those processes that need it.

> Neil
> 
> > > > > The intent was to switch to the audit netlink interface for contid,
> > > > > capcontid and to add the audit netlink interface for loginuid and
> > > > > sessionid while deprecating the proc interface for loginuid and
> > > > > sessionid.  This was alluded to in the cover letter, but not very clear,
> > > > > I'm afraid.  I have patches to remove the contid and loginuid/sessionid
> > > > > interfaces in another tree which is why I had forgotten to outline that
> > > > > plan more explicitly in the cover letter.
> > 
> > paul moore

- RGB

--
Richard Guy Briggs <rgb@...hat.com>
Sr. S/W Engineer, Kernel Security, Base Operating Systems
Remote, Ottawa, Red Hat Canada
IRC: rgb, SunRaycer
Voice: +1.647.777.2635, Internal: (81) 32635

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