Re: XSM permissive by default.

From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: xen-devel@lists.xenproject.org, dgdegra@tycho.nsa.gov
Subject: Re: XSM permissive by default.
Date: Wed, 9 Mar 2016 16:17:35 -0500	[thread overview]
Message-ID: <20160309211735.GA28919@char.us.oracle.com> (raw)
In-Reply-To: <56E023FF.1020706@citrix.com>

On Wed, Mar 09, 2016 at 01:24:15PM +0000, Andrew Cooper wrote:
> On 09/03/16 01:51, Konrad Rzeszutek Wilk wrote:
> > Hey,
> >
> > I was wondering if it we should change the default flask_bootparam
> > option from permissive to disabled?
> >
> > The reason being is that I was startled to see that my xSplice
> > code was able to patch the hypervisor from within an PV guest!
> >
> > Further testing showed that I could do 'xl debug-keys R' from
> > within the guests. This being possible with released 4.6 if I have
> > XSM enabled.
> >
> > All of this is due to the fact that I had forgotten to load the policy,
> > but Xen just told me:
> >
> > Flask:  Access controls disabled until policy is loaded.
> >
> > which is an understatement. I somehow had expected that if no
> > policy was loaded it would revert to the dummy one which has the
> > same permission as the non-XSM build. Ha! What a surprise..
> >
> > Now that the XSM is enabled via config it becomes much more
> > easy to enable it..
> >
> > Or perhaps change the code to flask so that if there are any

s/flask/dummy/
> > errors loading the policy it uses the dummy one?
> 
> By the looks of it, "permissive" shouldn't be an available option at all.
> 
> If a misconfiguration occurs, the behaviour should revert back to the
> current "dom0 all powerful, everything else unprivileged" state which
> currently exists without XSM.

Looking deeper in the code I believe it should be possible to swap
from the 'flask_ops' to the 'dummy_ops' (which is what you have without
XSM) if there is a failure during booting to load the policy file
(or the person simply forgot to include it). 

However it was not clear to me whether changing the ops from dummy_ops
to flask_ops during runtime (When the policy being loaded) would work.
It looks like it should be possible as FLASK_DISABLE does it..

Or whether one can FLASK_LOAD if the ops are dummy_ops instead
of flask_ops.

I will try to spin out a patch for this next week.
> 
> ~Andrew

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