Re: [Xen-devel] [PATCH v3] xsm: add a default policy to .init.data

[Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>]

On 06/30/2016 09:45 AM, Konrad Rzeszutek Wilk wrote:
> On Wed, Jun 29, 2016 at 11:09:01AM -0400, Daniel De Graaf wrote:
> > This adds a Kconfig option and support for including the XSM policy from
> > tools/flask/policy in the hypervisor so that the bootloader does not
> > need to provide a policy to get sane behavior from an XSM-enabled
> > hypervisor.  The policy provided by the bootloader, if present, will
> > override the built-in policy.
> >
> > The XSM policy is not moved out of tools because that remains the
> > primary location for installing and configuring the policy.
> >
> > Signed-off-by: Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>
> > ---
> >
> > Changes from v2 (dropped acks and reviewed-by):
> >  - Drop linker script changes, use python binary-to-C file script
> >  - Make the config option always include the policy if selected
> >  - Note the new conditional dependency on checkpolicy in INSTALL
> I liked the previous patch of putting in it in __init section.
> Is that something this patch could do? Ah, n/m. I see that
> the python script generates the binary with __init!
>
> Secondly I was wondering why the suggestion I had - which was to check
> of the 'checkpolicy' availability - and if not found - then
> hide the Kconfig option was not mentioned?
At the moment, since FLASK is the only XSM implementation and it is not
enabled by default, if someone enables XSM they will need to generate a
security policy at some point: either during the hypervisor build or in
the tools build.  Otherwise, they will need to disable FLASK at runtime
or (worst case) run without a policy at all, which means all domains can
act as dom0.  If you plan to disable FLASK at runtime, it is better to
compile without XSM enabled so you don't pay the cost of the function
calls to the XSM hooks on (almost) every hypercall.

Otherwise, I was planning to just change the default from always "y" to
"y" if checkpolicy was found and "n" if not.  This would end up running
"checkpolicy -h" on every (recursive) Make invocation if placed in the
top-level Config.mk - I'm unsure if that would be a problem or not.

> .. snip...
> > +sys.stdout.write("\n};\nconst int __init xsm_init_policy_size = %d;\n" % 
> > policy_size)
> > diff --git a/xen/xsm/xsm_core.c b/xen/xsm/xsm_core.c
> > index 8df1a3c..93c7d43 100644
> > --- a/xen/xsm/xsm_core.c
> > +++ b/xen/xsm/xsm_core.c
> > @@ -36,6 +36,24 @@ static inline int verify(struct xsm_operations *ops)
> >      return 0;
> >  }
> >
> > +#ifdef CONFIG_XSM_POLICY
> > +extern char xsm_init_policy[];
> > +extern int xsm_init_policy_size;
> > +#else
> > +#define xsm_init_policy 0
> > +#endif
> > +
> > +static void __init xsm_policy_init(void)
> > +{
> > +#ifdef CONFIG_XSM_POLICY
> > +    if ( policy_size == 0 )
> > +    {
> > +        policy_buffer = xsm_init_policy;
> > +        policy_size = xsm_init_policy_size;
> > +    }
> > +#endif
> > +}
> > +
> This all looks like it could go in a header file?
Sure, I could move it to xsm.h.  I thought it might be clearer to have the
assignment and the xfree() check in the same file.

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