libxc: Protect xc_domain_resume from clobbering domain registers

xc_domain_resume() expects the guest to be in state SHUTDOWN_suspend.
However, nothing verifies the state before modify_returncode() modifies
the domain's registers.  This will crash guest processes or the kernel
itself.

This can be demonstrated with `LIBXL_SAVE_HELPER=/bin/false xl migrate`.

Signed-off-by: Jason Andryuk <andryuk@aero.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

tools/libxc/xc_resume.c

@@ -33,12 +33,21 @@ static int modify_returncode(xc_interface *xch, uint32_t domid)
     struct domain_info_context *dinfo = &_dinfo;
     int rc;
 
-    if ( xc_domain_getinfo(xch, domid, 1, &info) != 1 )
+    if ( xc_domain_getinfo(xch, domid, 1, &info) != 1 ||
+         info.domid != domid )
     {
         PERROR("Could not get domain info");
         return -1;
     }
 
+    if ( !info.shutdown || (info.shutdown_reason != SHUTDOWN_suspend) )
+    {
+        ERROR("Dom %d not suspended: (shutdown %d, reason %d)", domid,
+              info.shutdown, info.shutdown_reason);
+        errno = EINVAL;
+        return -1;
+    }
+
     if ( info.hvm )
     {
         /* HVM guests without PV drivers have no return code to modify. */