Re: [Xen-devel] XSM permissive by default.

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Mar 09, 2016 at 08:40:05PM -0600, Doug Goldstein wrote:
> On 3/9/16 4:09 PM, Daniel De Graaf wrote:
> > On 03/09/2016 04:17 PM, Konrad Rzeszutek Wilk wrote:
> >> 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?
> 
> >>>
> >>> By the looks of it, "permissive" shouldn't be an available option at
> >>> all.
> > 
> > Permissive is meant for developing (or debugging) a disaggregated system,
> > where the restrictions on non-dom0 would also break the system.  However,
> > I agree that it needs to be harder to end up in this mode by accident.
> > 
> > The simplest solution in my opinion is to change the boot parameter to
> > default to "flask=enforcing", which will fail the boot if a policy is
> > not available prior to dom0 creation.  This would require any setup
> > where the policy is loaded from userspace to explicitly specify
> > "flask=late", whereas they can currently get away with no parameter.
> > 
> > Another solution would be to default to "flask=late" and either deny the
> > creation of domains if a policy is not present, or automatically revert
> > to the dummy module on domain creation with no loaded policy.  The latter
> > probably deserves a different name ("flask=auto"?).
> > 
> 
> Honestly I'm in favor of secure by default approach. Since Xen is not
> built with flask by default to me the sane approach would be to default
> the system to "flask=enforcing".
> 
> "flask=late" not allowing the creation of domains sounds good but what
> if you're using a disaggregated dom0 with some domDs and one of them
> needs to be up to fetch your policy? Just a hypothetical.
> 
> XSMs like LSMs just aren't meant to be swapped around at runtime and
> like Daniel points out if go down the road of swapping to the dummy
> module there could be further dragons and whose to say someone won't
> look at that and put something in that allows you to switch to another
> later on (yes I know there's only really 1 but I'm speaking of the
> hypothetical).


I presume this patch would be to folks +1:

From 3373a50f386b41eea6ecede4b430e4fa09b2fe7e Mon Sep 17 00:00:00 2001
From: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Date: Thu, 10 Mar 2016 12:05:29 -0500
Subject: [PATCH] flask: By default be in FLASK_BOOTPARAM_ENFORCING mode.

By default the mode was 'permissive' which is "meant for
developing (or debugging) a disaggregated system,
where the restrictions on non-dom0 would also break the system."

However this default mode made it possible to boot an machine
in this state if a policy file during bootup was not provided.

The end was less secure than with XSM-enabled - any guest
could do any operation (including rebooting the machine).

Alternative solutions such as switching from flask to dummy.
However "The main issue with starting with dummy and then
switching to FLASK is that any domains created while using
the dummy policy won't have flask_domain_alloc_security called
to populate domain->ssid, and the rest of the flask code relies
on this being non-NULL. The same would be true for event channels,
but inlining the field to save space makes that a non-issue."

(both excerpts are from Daniel De Graaf emails).

This is a much easier fix.

Suggested-by: Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
 docs/misc/xen-command-line.markdown | 2 +-
 xen/xsm/flask/flask_op.c            | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/docs/misc/xen-command-line.markdown 
b/docs/misc/xen-command-line.markdown
index ca77e3b..9e77f8a 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -662,7 +662,7 @@ to use the default.
 ### flask
 > `= permissive | enforcing | late | disabled`
 
-> Default: `permissive`
+> Default: `enforcing`
 
 Specify how the FLASK security server should be configured.  This option is 
only
 available if the hypervisor was compiled with XSM support (which can be enabled
diff --git a/xen/xsm/flask/flask_op.c b/xen/xsm/flask/flask_op.c
index f4f5dd1..aaed75d 100644
--- a/xen/xsm/flask/flask_op.c
+++ b/xen/xsm/flask/flask_op.c
@@ -25,7 +25,7 @@
 #define _copy_to_guest copy_to_guest
 #define _copy_from_guest copy_from_guest
 
-enum flask_bootparam_t __read_mostly flask_bootparam = 
FLASK_BOOTPARAM_PERMISSIVE;
+enum flask_bootparam_t __read_mostly flask_bootparam = 
FLASK_BOOTPARAM_ENFORCING;
 static void parse_flask_param(char *s);
 custom_param("flask", parse_flask_param);
 
-- 
2.5.0


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