domctl: handle XEN_DOMCTL_getdomaininfo without acquiring domctl lock

getdomaininfo() is not called under consistently the same lock. Thus,
with caller side locking irrelevant, it can as well be called with the
domctl lock not held. (Callers not pausing the domain they want to
retrieve information for already need to be aware that not all of the
data returned can be relied on as being consistent; most data will also
be stale by the time the caller gets to look at it.)

Move the handling not only ahead of acquiring the lock, but also ahead
of the XSM check, leveraging that the sub-op has its own hook.

While moving, convert an assignment to an assertion: The domain in
question was determined from the field which previously was "updated".

This is part of XSA-492.

Fixes: 5513bd0b46 ("add xenstore domain flag to hypervisor")
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Daniel P. Smith <dpsmith@apertussolutions.com>
(cherry picked from commit 784bd4dc0b)

xen/common/domctl.c

@@ -323,6 +323,26 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xen_domctl_t) u_domctl)
         break;
     }
 
+    /* Handle sub-ops not requiring the domctl lock. */
+    switch ( op->cmd )
+    {
+    case XEN_DOMCTL_getdomaininfo:
+        ret = xsm_getdomaininfo(XSM_XS_PRIV, d);
+        if ( !ret )
+        {
+            getdomaininfo(d, &op->u.getdomaininfo);
+
+            ASSERT(op->domain == op->u.getdomaininfo.domain);
+            copyback = true;
+        }
+
+        goto domctl_out_unlock_domonly;
+
+    default:
+        /* Everything else handled further down. */
+        break;
+    }
+
     ret = xsm_domctl(XSM_OTHER, d, op->cmd,
                      /* SSIDRef only applicable for cmd == createdomain */
                      op->u.createdomain.ssidref);
@@ -539,17 +559,6 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xen_domctl_t) u_domctl)
         copyback = 1;
         break;
 
-    case XEN_DOMCTL_getdomaininfo:
-        ret = xsm_getdomaininfo(XSM_XS_PRIV, d);
-        if ( ret )
-            break;
-
-        getdomaininfo(d, &op->u.getdomaininfo);
-
-        op->domain = op->u.getdomaininfo.domain;
-        copyback = 1;
-        break;
-
     case XEN_DOMCTL_getvcpucontext:
     {
         vcpu_guest_context_u c = { .nat = NULL };

xen/include/xsm/dummy.h

@@ -172,8 +172,11 @@ static XSM_INLINE int cf_check xsm_domctl(
     case XEN_DOMCTL_bind_pt_irq:
     case XEN_DOMCTL_unbind_pt_irq:
         return xsm_default_action(XSM_DM_PRIV, current->domain, d);
+
     case XEN_DOMCTL_getdomaininfo:
-        return xsm_default_action(XSM_XS_PRIV, current->domain, d);
+        ASSERT_UNREACHABLE();
+        return -EILSEQ;
+
     default:
         return xsm_default_action(XSM_PRIV, current->domain, d);
     }

xen/xsm/flask/hooks.c

@@ -680,8 +680,12 @@ static int cf_check flask_domctl(struct domain *d, unsigned int cmd,
          */
         return avc_current_has_perm(ssidref, SECCLASS_DOMAIN, DOMAIN__CREATE, NULL);
 
-    /* These have individual XSM hooks (common/domctl.c) */
+    /* These have individual XSM hooks and don't make it here. */
     case XEN_DOMCTL_getdomaininfo:
+        ASSERT_UNREACHABLE();
+        return -EILSEQ;
+
+    /* These have individual XSM hooks (common/domctl.c) */
     case XEN_DOMCTL_scheduler_op:
     case XEN_DOMCTL_irq_permission:
     case XEN_DOMCTL_iomem_permission: