Subject: xen/memory: Don't skip the RCU unlock path in acquire_resource()
From: Andrew Cooper <[email-masked]>

In the case that an HVM Stubdomain makes an XENMEM_acquire_resource hypercall,
the FIXME path will bypass rcu_unlock_domain() on the way out of the function.

Move the check to the start of the function.  This does change the behaviour
of the get-size path for HVM Stubdomains, but that functionality is currently
broken and unused anyway, as well as being quite useless to entities which
can't actually map the resource anyway.

This is XSA-334.

Fixes: 83fa6552ce ("common: add a new mappable resource type: XENMEM_resource_grant_table")
Signed-off-by: Andrew Cooper <[email-masked]>
Reviewed-by: Jan Beulich <[email-masked]>

diff --git a/xen/common/memory.c b/xen/common/memory.c
index 1a3c9ffb30..29741d8904 100644
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -1058,6 +1058,14 @@ static int acquire_resource(
     xen_pfn_t mfn_list[32];
     int rc;
 
+    /*
+     * FIXME: Until foreign pages inserted into the P2M are properly
+     *        reference counted, it is unsafe to allow mapping of
+     *        resource pages unless the caller is the hardware domain.
+     */
+    if ( paging_mode_translate(currd) && !is_hardware_domain(currd) )
+        return -EACCES;
+
     if ( copy_from_guest(&xmar, arg, 1) )
         return -EFAULT;
 
@@ -1114,14 +1122,6 @@ static int acquire_resource(
         xen_pfn_t gfn_list[ARRAY_SIZE(mfn_list)];
         unsigned int i;
 
-        /*
-         * FIXME: Until foreign pages inserted into the P2M are properly
-         *        reference counted, it is unsafe to allow mapping of
-         *        resource pages unless the caller is the hardware domain.
-         */
-        if ( !is_hardware_domain(currd) )
-            return -EACCES;
-
         if ( copy_from_guest(gfn_list, xmar.frame_list, xmar.nr_frames) )
             rc = -EFAULT;