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

src/platform/vboot_reference/vboot_firmware/lib/firmware_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 firmware image.
  * (Firmware Portion)
  */
 
 #include "firmware_image_fw.h"
 
@@ skipping 271 matching lines @@
                                                     firmwareB)) {
         WriteStoredVersion(FIRMWARE_KEY_VERSION,
                            (uint16_t) (min_lversion >> 16));
         WriteStoredVersion(FIRMWARE_VERSION,
                            (uint16_t) (min_lversion & 0x00FFFF));
         stored_lversion = min_lversion;  /* Update stored version as it's used
                                           * later. */
       }
     }
   }
-  /* Lock Firmware 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 Firmware TPM rollback indices from further writes.  In this design,
+   * this is done by setting the globalLock bit, which is cleared only by
+   * TPM_Init at reboot.
    */
-  LockStoredVersion(FIRMWARE_KEY_VERSION);
-  LockStoredVersion(FIRMWARE_VERSION);
+  LockFirmwareVersions();
 
   /* Determine which firmware (if any) to jump to.
    *
    * We always attempt to jump to FirmwareA first. If verification of FirmwareA
    * fails, we try FirmwareB. In all cases, if the firmware successfully
    * verified but is a rollback, we jump to recovery.
    *
    * Note: This means that if FirmwareA verified successfully and is a
    * rollback, then no attempt is made to check FirmwareB. We still jump to
    * recovery. FirmwareB is only used as a backup in case FirmwareA gets
@@ skipping 12 matching lines @@
      */
     if (stored_lversion <= firmwareB_lversion &&
         (VERIFY_FIRMWARE_SUCCESS == VerifyFirmware(root_key_blob,
                                                    verification_headerB,
                                                    firmwareB)))
         return BOOT_FIRMWARE_B_CONTINUE;
   }
   /* D'oh: No bootable firmware. */
   return BOOT_FIRMWARE_RECOVERY_CONTINUE;
 }