x86/ioreq server: correctly handle bogus XEN_DMOP_{,un}map_io_range_to_ioreq_server arguments

Misbehaving device model can pass incorrect XEN_DMOP_map/
unmap_io_range_to_ioreq_server arguments, namely end < start when
specifying address range. When this happens we hit ASSERT(s <= e) in
rangeset_contains_range()/rangeset_overlaps_range() with debug builds.
Production builds will not trap right away but may misbehave later
while handling such bogus ranges.

This is XSA-238.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/hvm/ioreq.c

@@ -820,6 +820,9 @@ int hvm_map_io_range_to_ioreq_server(struct domain *d, ioservid_t id,
     struct hvm_ioreq_server *s;
     int rc;
 
+    if ( start > end )
+        return -EINVAL;
+
     spin_lock_recursive(&d->arch.hvm_domain.ioreq_server.lock);
 
     rc = -ENOENT;
@@ -872,6 +875,9 @@ int hvm_unmap_io_range_from_ioreq_server(struct domain *d, ioservid_t id,
     struct hvm_ioreq_server *s;
     int rc;
 
+    if ( start > end )
+        return -EINVAL;
+
     spin_lock_recursive(&d->arch.hvm_domain.ioreq_server.lock);
 
     rc = -ENOENT;