This commit implements full support for starting the idle domain privileged by
introducing a new flask label xenboot_t which the idle domain is labeled with
at creation.  It then provides the implementation for the XSM hook
xsm_transition_running to relabel the idle domain to the existing xen_t flask
label.

In the reference flask policy a new macro, xen_build_domain(target), is
introduced for creating policies for dom0less/hyperlaunch allowing the
hypervisor to create and assign the necessary resources for domain
construction.

Signed-off-by: Daniel P. Smith <dpsmith@apertussolutions.com>
---
 tools/flask/policy/modules/xen.if      | 6 ++++++
 tools/flask/policy/modules/xen.te      | 1 +
 tools/flask/policy/policy/initial_sids | 1 +
 xen/xsm/flask/hooks.c                  | 7 ++++++-
 xen/xsm/flask/policy/initial_sids      | 1 +
 5 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/tools/flask/policy/modules/xen.if b/tools/flask/policy/modules/xen.if
index 5e2aa472b6..4ec676fff1 100644
--- a/tools/flask/policy/modules/xen.if
+++ b/tools/flask/policy/modules/xen.if
@@ -62,6 +62,12 @@ define(`create_domain_common', `
 			setparam altp2mhvm altp2mhvm_op dm };
 ')
 
+# xen_build_domain(target)
+#   Allow a domain to be created at boot by the hypervisor
+define(`xen_build_domain', `
+	allow xenboot_t $1_channel:event create;
+')
+
 # create_domain(priv, target)
 #   Allow a domain to be created directly
 define(`create_domain', `
diff --git a/tools/flask/policy/modules/xen.te b/tools/flask/policy/modules/xen.te
index 3dbf93d2b8..de98206fdd 100644
--- a/tools/flask/policy/modules/xen.te
+++ b/tools/flask/policy/modules/xen.te
@@ -24,6 +24,7 @@ attribute mls_priv;
 ################################################################################
 
 # The hypervisor itself
+type xenboot_t, xen_type, mls_priv;
 type xen_t, xen_type, mls_priv;
 
 # Domain 0
diff --git a/tools/flask/policy/policy/initial_sids b/tools/flask/policy/policy/initial_sids
index 6b7b7eff21..ec729d3ba3 100644
--- a/tools/flask/policy/policy/initial_sids
+++ b/tools/flask/policy/policy/initial_sids
@@ -2,6 +2,7 @@
 # objects created before the policy is loaded or for objects that do not have a
 # label defined in some other manner.
 
+sid xenboot gen_context(system_u:system_r:xenboot_t,s0)
 sid xen gen_context(system_u:system_r:xen_t,s0)
 sid dom0 gen_context(system_u:system_r:dom0_t,s0)
 sid domxen gen_context(system_u:system_r:domxen_t,s0)
diff --git a/xen/xsm/flask/hooks.c b/xen/xsm/flask/hooks.c
index decebc8231..0643654aba 100644
--- a/xen/xsm/flask/hooks.c
+++ b/xen/xsm/flask/hooks.c
@@ -168,7 +168,7 @@ static int cf_check flask_domain_alloc_security(struct domain *d)
     switch ( d->domain_id )
     {
     case DOMID_IDLE:
-        dsec->sid = SECINITSID_XEN;
+        dsec->sid = SECINITSID_XENBOOT;
         break;
     case DOMID_XEN:
         dsec->sid = SECINITSID_DOMXEN;
@@ -188,6 +188,7 @@ static int cf_check flask_domain_alloc_security(struct domain *d)
 
 static void cf_check flask_transition_running(void)
 {
+    struct domain_security_struct *dsec;
     struct domain *d = current->domain;
 
     if ( d->domain_id != DOMID_IDLE )
@@ -198,6 +199,10 @@ static void cf_check flask_transition_running(void)
      * set to false for the consistency check(s) in the setup code.
      */
     d->is_privileged = false;
+
+    dsec = d->ssid;
+    dsec->sid = SECINITSID_XEN;
+    dsec->self_sid = dsec->sid;
 }
 
 static void cf_check flask_domain_free_security(struct domain *d)
diff --git a/xen/xsm/flask/policy/initial_sids b/xen/xsm/flask/policy/initial_sids
index 7eca70d339..e8b55b8368 100644
--- a/xen/xsm/flask/policy/initial_sids
+++ b/xen/xsm/flask/policy/initial_sids
@@ -3,6 +3,7 @@
 #
 # Define initial security identifiers 
 #
+sid xenboot
 sid xen
 sid dom0
 sid domio
-- 
2.20.1

On 21.04.2022 00:28, Daniel P. Smith wrote:
> --- a/xen/xsm/flask/hooks.c
> +++ b/xen/xsm/flask/hooks.c
> @@ -168,7 +168,7 @@ static int cf_check flask_domain_alloc_security(struct domain *d)
>      switch ( d->domain_id )
>      {
>      case DOMID_IDLE:
> -        dsec->sid = SECINITSID_XEN;
> +        dsec->sid = SECINITSID_XENBOOT;
>          break;
>      case DOMID_XEN:
>          dsec->sid = SECINITSID_DOMXEN;
> @@ -188,6 +188,7 @@ static int cf_check flask_domain_alloc_security(struct domain *d)
>  
>  static void cf_check flask_transition_running(void)
>  {
> +    struct domain_security_struct *dsec;
>      struct domain *d = current->domain;
>  
>      if ( d->domain_id != DOMID_IDLE )
> @@ -198,6 +199,10 @@ static void cf_check flask_transition_running(void)
>       * set to false for the consistency check(s) in the setup code.
>       */
>      d->is_privileged = false;
> +
> +    dsec = d->ssid;
> +    dsec->sid = SECINITSID_XEN;
> +    dsec->self_sid = dsec->sid;
>  }

If replacing SIDs is an okay thing to do, perhaps assert that the
values haven't changed from SECINITSID_XENBOOT prior to replacing
them?

Jan

On 4/21/22 05:22, Jan Beulich wrote:
> On 21.04.2022 00:28, Daniel P. Smith wrote:
>> --- a/xen/xsm/flask/hooks.c
>> +++ b/xen/xsm/flask/hooks.c
>> @@ -168,7 +168,7 @@ static int cf_check flask_domain_alloc_security(struct domain *d)
>>      switch ( d->domain_id )
>>      {
>>      case DOMID_IDLE:
>> -        dsec->sid = SECINITSID_XEN;
>> +        dsec->sid = SECINITSID_XENBOOT;
>>          break;
>>      case DOMID_XEN:
>>          dsec->sid = SECINITSID_DOMXEN;
>> @@ -188,6 +188,7 @@ static int cf_check flask_domain_alloc_security(struct domain *d)
>>  
>>  static void cf_check flask_transition_running(void)
>>  {
>> +    struct domain_security_struct *dsec;
>>      struct domain *d = current->domain;
>>  
>>      if ( d->domain_id != DOMID_IDLE )
>> @@ -198,6 +199,10 @@ static void cf_check flask_transition_running(void)
>>       * set to false for the consistency check(s) in the setup code.
>>       */
>>      d->is_privileged = false;
>> +
>> +    dsec = d->ssid;
>> +    dsec->sid = SECINITSID_XEN;
>> +    dsec->self_sid = dsec->sid;
>>  }
> 
> If replacing SIDs is an okay thing to do, perhaps assert that the
> values haven't changed from SECINITSID_XENBOOT prior to replacing
> them?

Yes, changing a domain's SID is a legitimate action that could be done
today via the FLASK_RELABEL_DOMAIN subop of xsm_op hypercall that ends
up calling flask_relabel_domain(), when using flask policy. This is
where Jason was concerned if I was going to be using that call to change
the SID, which would require a policy rule to allow xenboot_t to relabel
itself as xen_t. As flask works today, the system domains use initial
SIDs which are effectively compile-time labels, which means the policy
rule is a static, fixed rule, i.e. it is not possible to use a different
set of labels, that must always be present. This also introduces the
risk for a custom policy writer to inadvertently leave the xenboot_t to
xen_t transitional rule out resulting in a failed access attempt which
would lead to a panic. This is unnecessary pain when we can just handle
the transition internal to the hypervisor as that where it is all really
occurring.

As for the ASSERT, that is a good point since there is a specific state
we are expecting to enter the hook. Pair that with some thinking I have
had to do in answering Jason, Roger, and yourself, I am going to rewire
the hook to return a success/error return value and move the panic
outside the check.

v/r,
dps