RTC Anti-rollback

Purpose

RTC anti-rollback (RTCAR) is a Open Firmware based security feature intended to prevent "RTC rollback attacks" - subversion of timed leases by setting the real-time clock backward in time.

RTCAR works by recording a series of timestamps in SPI FLASH, thus recording the dates and times of recent boots. When starting the OS, Open Firmware compares the current RTC value with the most recent stored value. If the stored value is later than the current value, the RTC is deemed to have been "rolled back" and boots in "activation mode". This limits the effectiveness of rollback attacks.

The series of recorded timestamps is protected from attack by existing lockout mechanisms that prevent the writing of SPI FLASH by any entity other than Open Firmware.

The timestamp storage format is chosen so that the relatively-dangerous operation of erasing a block of SPI FLASH is done only infrequently, only the order of every 6,500 reboots.

OFW Behavior

• RTCAR is performed only in secure mode.
• The RTCAR recovery test (see #RTCAR Recovery) is performed just after the test for firmware update in Firmware_security#Process.
• The RTCAR boot test is performed only if the "ak" tag is not present in Manufacturing data.
• The RTCAR boot test is performed just prior to the test for a valid lease in Firmware_security#Process. In the presence of rollback or corruption, OFW will boot from actos/actrd. Otherwise it will boot from either runos/runrd or actos/actrd according to the presence or absence of a valid activation lease. In other words, apparent RTC rollback is treated as a missing or invalid lease.
• OFW records the status of the RTCAR check by creating properties in the device tree /chosen node as follows:
  • Property name: rtc-timestamp Property value: Null-terminated ASCII string containing the value of the most recent preexisting (prior to the current boot) timestamp. The timestamp format is http://en.wikipedia.org/wiki/ISO_8601 , using the UTC timezone, e.g. "20110324T093447\0". The date and time come directly from the contents of the real time clock registers, with no offsetting or other adjustment beyond conversion from the internal BCD storage format into decimal ASCII. If there is no previously-recorded timestamp, as would be the case if rtc-status contained "empty", the rtc-timestamp property will not be present.
  • Property name: rtc-count Property value: Null-terminated ASCII string containing a decimal number indicating the number of timestamps that have been recorded up to and including the most recent preexisting (prior to the current boot) one. It is possible, though unlikely, that rtc-timestamp could be missing even though rtc-count is nonzero. It is also possible, and similarly unlikely, that rtc-count could be zero with rtc-timestamp present. The real guarantee is that rtc-count will increment on each boot. In the absence of RTCAR recovery operations, the rtc-count value should accurately reflect the number of timestamps that have been recorded over the lifetime of the system.
  • Property name: rtc-status Property value: Null-terminated ASCII string containing one of the following values:
    • "ok" - the RTC timestamp area is valid and the RTC value is more recent than the last timestamp. In this case, OFW will create a new timestamp recording the current RTC value, for use on the next boot. The timestamp that is exported via the "rtc-timestamp" property is the previously-recorded one, not the new one.
    • "empty" - the RTC timestamp area was initially empty, i.e. no timestamps were recorded in it. In this case, OFW will automatically initialize the timestamp area and create a new timestamp recording the current RTC value, for use on the next boot. This case is not considered as a "rollback", so OFW will not force the use of actos/actrd. The "rtc-timestamp" property will be absent, as no previously-recorded timestamp information is available.
    • "residue" - the RTC timestamp area contained some data, but there was a problem with it, such as an invalid checksum or junk after the last timestamp. This case is considered as a possible attack, so OFW forces the use of actos/actrd. OFW does not record a new timestamp. The "rtc-timestamp" property may be present or absent, according to whether or not there was at least one valid timestamp.
    • "rollback" - the RTC timestamp area is valid format-wise, but the most recent timestamp is later than the current RTC time. This case is considered a possible rollback attempt, so OFW forces the use of actos/actrd. OFW does not record a new timestamp. The "rtc-timestamp" property contains the most recent timestamp, i.e. the one that was more recent than the current RTC value.

RTCAR Recovery

RTCAR recovery permits the repair and recovery of system with an invalid or corrupted RTC timestamp area. OFW will repair the timestamp area if a suitable signed file is present in /security, as detailed below.

Note that this recovery process *does not* set the RTC value; it only sets the timestamp area. The RTC value must be corrected by other means.

• The name of the RTCAR recovery control file is "/security/rtcreset.sig"
• The signing key is the same one that is used to sign activation leases - either the OLPC key or a deployment-specific key.
• The file contains one or more "rtc01:"-format records as defined in Firmware_Key_and_Signature_Formats#RTC_Set
• The "sig01:" or "sig02:" field must have a key signature that matches a public key in the laptop's list of activation keys.
• The signed data must verify according to that key.
• The serial number in the rtc01: record must match the laptop's serial number.
• The currentrtc value in the rtc01: record must match the value that was last reported in the rtc-timestamp property. If the old timestamp is unknown (as might be the case if rtc-status were "residue" and no timestamp were found in the corrupted timestamp area), currentrtc must be set to the value "00000000T000000Z".
• The nonce value in the rtc01: record must be a 10-digit decimal ASCII number in the range 0000000000..2147483647, indicating the timestamp count value to restore. After insertion of the new timestamp, the new count will be "count+1". In general, nonce should be the same value that was reported in the rtc-count property, or 0000000000 if there was no such property. OFW does not check that nonce matches the value of rtc-count that is implied by the previous state of the timestamp area.
• If all the checks succeed, the newrtc value in the rtc01: record will be recorded as a new timestamp after restoring valid contents in the timestamp recording area.
• If any of the security checks fails, the timestamp recording area will remain unchanged. In that case, you could recover the timestamp area with a developer key.