domctl: relax getdomaininfo permissions

Qemu needs access to this for the domain it controls, both due to it
being used by xc_domain_memory_mapping() (which qemu calls) and the
explicit use in hw/xenpv/xen_domainbuild.c:xen_domain_poll(). Extend
permissions to that of any "ordinary" domctl: A domain controlling the
targeted domain can invoke this operation for that target domain (which
is being achieved by no longer passing NULL to xsm_domctl()).

This at once avoids a for_each_domain() loop when the ID of an
existing domain gets passed in.

Reported-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v2: Add a comment. Clarify description as to what additional permission
    is being granted.
---
I know there had been an alternative patch suggestion, but that one
doesn't seem have seen a formal submission so far, so here is my
original proposal.

I wonder what good the duplication of the returned domain ID does: I'm
tempted to remove the one in the command-specific structure. Does
anyone have insight into why it was done that way?

I further wonder why we have XSM_OTHER: The respective conversion into
other XSM_* values in xsm/dummy.h could as well move into the callers,
making intentions more obvious when looking at the actual code.

--- a/tools/flask/policy/modules/xen.if
+++ b/tools/flask/policy/modules/xen.if
@@ -149,7 +149,7 @@ define(`device_model', `
 	create_channel($2, $1, $2_channel)
 	allow $1 $2_channel:event create;
 
-	allow $1 $2_target:domain shutdown;
+	allow $1 $2_target:domain { getdomaininfo shutdown };
 	allow $1 $2_target:mmu { map_read map_write adjust physmap target_hack };
 	allow $1 $2_target:hvm { getparam setparam trackdirtyvram hvmctl irqlevel pciroute pcilevel cacheattr send_irq };
 ')
--- a/xen/common/domctl.c
+++ b/xen/common/domctl.c
@@ -396,14 +396,13 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xe
     switch ( op->cmd )
     {
     case XEN_DOMCTL_createdomain:
-    case XEN_DOMCTL_getdomaininfo:
     case XEN_DOMCTL_test_assign_device:
     case XEN_DOMCTL_gdbsx_guestmemio:
         d = NULL;
         break;
     default:
         d = rcu_lock_domain_by_id(op->domain);
-        if ( d == NULL )
+        if ( !d && op->cmd != XEN_DOMCTL_getdomaininfo )
             return -ESRCH;
     }
 
@@ -817,14 +816,22 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xe
 
     case XEN_DOMCTL_getdomaininfo:
     {
-        domid_t dom = op->domain;
-
-        rcu_read_lock(&domlist_read_lock);
+        domid_t dom = DOMID_INVALID;
 
-        for_each_domain ( d )
-            if ( d->domain_id >= dom )
+        if ( !d )
+        {
+            ret = -EINVAL;
+            if ( op->domain >= DOMID_FIRST_RESERVED )
                 break;
 
+            rcu_read_lock(&domlist_read_lock);
+
+            dom = op->domain;
+            for_each_domain ( d )
+                if ( d->domain_id >= dom )
+                    break;
+        }
+
         ret = -ESRCH;
         if ( d == NULL )
             goto getdomaininfo_out;
@@ -839,6 +846,10 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xe
         copyback = 1;
 
     getdomaininfo_out:
+        /* When d was non-NULL upon entry, no cleanup is needed. */
+        if ( dom == DOMID_INVALID )
+            break;
+
         rcu_read_unlock(&domlist_read_lock);
         d = NULL;
         break;
--- a/xen/include/xsm/dummy.h
+++ b/xen/include/xsm/dummy.h
@@ -61,7 +61,12 @@ static always_inline int xsm_default_act
         return 0;
     case XSM_TARGET:
         if ( src == target )
+        {
             return 0;
+    case XSM_XS_PRIV:
+            if ( src->is_xenstore )
+                return 0;
+        }
         /* fall through */
     case XSM_DM_PRIV:
         if ( target && src->target == target )
@@ -71,10 +76,6 @@ static always_inline int xsm_default_act
         if ( src->is_privileged )
             return 0;
         return -EPERM;
-    case XSM_XS_PRIV:
-        if ( src->is_xenstore || src->is_privileged )
-            return 0;
-        return -EPERM;
     default:
         LINKER_BUG_ON(1);
         return -EPERM;