Re: [PATCH v2 04/17] xen: Introduce XEN_DOMCTL_CDF_not_hypercall_target

[Julien Grall]

Hi Jason,

On 16/07/2025 22:14, Jason Andryuk wrote:
Add a new create domain flag  to indicate if a domain can be the target
of hypercalls.  By default all domains can be targetted - subject to any
other permission checks.
This property is useful in a safety environment to isolate domains for
freedom from interference.

I see the flag is exposed to the toolstack. However, I don't see how you 
can successfully create a VM if you are not allowed to call hypercalls 
(for instance to add some memory).

I think, at minimum, you would want to allow hypercalls while the domain 
is created. That said, I wonder if this setup would not be better to 
describe with XSM?


[...]

diff --git a/xen/include/public/domctl.h b/xen/include/public/domctl.h
index 88a294c5be..f1f6f96bc2 100644
--- a/xen/include/public/domctl.h
+++ b/xen/include/public/domctl.h
@@ -70,9 +70,11 @@ struct xen_domctl_createdomain {
  #define XEN_DOMCTL_CDF_trap_unmapped_accesses  (1U << 8)
  /* Allow domain to provide device model for multiple other domains */
  #define XEN_DOMCTL_CDF_device_model   (1U << 9)
+/* Domain cannot be the target of hypercalls */
+#define XEN_DOMCTL_CDF_not_hypercall_target   (1U << 10)
  
  /* Max XEN_DOMCTL_CDF_* constant.  Used for ABI checking. */
-#define XEN_DOMCTL_CDF_MAX XEN_DOMCTL_CDF_device_model
+#define XEN_DOMCTL_CDF_MAX XEN_DOMCTL_CDF_not_hypercall_target

I am not sure where to comment. But aren't both flags mutually exclusive?

diff --git a/xen/include/xsm/dummy.h b/xen/include/xsm/dummy.h
index 0b341efd18..f2205575ed 100644
--- a/xen/include/xsm/dummy.h
+++ b/xen/include/xsm/dummy.h
@@ -91,12 +91,16 @@ static always_inline int xsm_default_action(
              return 0;
          fallthrough;
      case XSM_DM_PRIV:
+        if ( target && !is_hypercall_target(target) )
+            return -EPERM;
          if ( is_dm_domain(src) )
              return 0;
          if ( target && evaluate_nospec(src->target == target) )
              return 0;
          fallthrough;
      case XSM_PRIV:
+        if ( target && !is_hypercall_target(target) )
+            return -EPERM;
          if ( is_control_domain(src) )
              return 0;
          return -EPERM;

Cheers,

--
Julien Grall