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]>

Backport note: The deletion of the XENMEM_rsrc_acq_caller_owned clause is
correct and intentional.  This was an output-only flag who's use never
survived into the Xen 4.12 release, and was subsequently deleted in Xen 4.13.

diff --git a/xen/common/memory.c b/xen/common/memory.c
index dbc06fb0bf..ff88ebb314 100644
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -1059,6 +1059,14 @@ 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;
 
@@ -1115,16 +1123,6 @@ 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
-         *        non-caller-owned resource pages unless the caller is
-         *        the hardware domain.
-         */
-        if ( !(xmar.flags & XENMEM_rsrc_acq_caller_owned) &&
-             !is_hardware_domain(currd) )
-            return -EACCES;
-
         if ( copy_from_guest(gfn_list, xmar.frame_list, xmar.nr_frames) )
             rc = -EFAULT;