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

[Doug Goldstein <cardoe@xxxxxxxxxx>]

On 3/10/16 11:10 AM, Konrad Rzeszutek Wilk wrote:
> 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);
>  
> 

+1

You also found a spot that I didn't find when doing the Kconfig
conversion of CONFIG_XSM / CONFIG_FLASK. My search didn't catch
XSM\_ENABLE so follow on patch from me for that will be coming.

Reviewed-by: Doug Goldstein <cardoe@xxxxxxxxxx>

-- 
Doug Goldstein

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