New anti-rollback strategy (no TPM NVRAM write cycles for locking).

src/platform/vboot_reference/vboot_firmware/lib/kernel_image_fw.c

 /* Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  *
  * Functions for verifying a verified boot kernel image.
  * (Firmware portion)
  */
 
 #include "kernel_image_fw.h"
 
@@ skipping 429 matching lines @@
           stored_lversion = min_lversion;  /* Update stored version as it's
                                             * used later. */
         }
       }
       kernel_to_boot = try_kernel_which[i];
       break;  /* We found a valid kernel. */
     }
     try_kernel[i]->boot_priority = 0;
     }  /* for loop. */
 
-  /* Lock Kernel TPM rollback indices from further writes.
-   * TODO(gauravsh): Figure out if these can be combined into one
-   * 32-bit location since we seem to always use them together. This can help
-   * us minimize the number of NVRAM writes/locks (which are limited over flash
-   * memory lifetimes.
+  /* Lock Kernel TPM rollback indices from further writes.  In this design,
+   * this is tied to locking physical presence---so (software) physical
+   * presence cannot be asserted after this point.  This is a big side effect,
+   * so we want to make it clear in the function name.
+   * TODO(gauravsh): figure out better abstractions.
    */
-  LockStoredVersion(KERNEL_KEY_VERSION);
-  LockStoredVersion(KERNEL_VERSION);
+  LockKernelVersionsByLockingPP();
   return kernel_to_boot;
 }