RTC Anti-rollback

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Contents

Purpose and overview

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", effectively preventing the system from booting normally. 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.

The activation mode initramfs attempts to contact a server that can automatically recover the system upon event of corruption of the last recorded timestamp value.

This page details the high-level behaviour of the system, and the items in which deployment technicians may need to interact with. Lower level implementation details can be found on the /Implementation page.

RTCAR properties

Three properties are defined to diagnose and control the state of this system.

  1. rtc-timestamp: Thie property contains the value of the most recent preexisting (prior to the current boot) timestamp. If there is no previously recorded timestamp, this property is not present.
  2. rtc-count: This property indicates how many timestamps have been recorded. In other words, this property counts how many times the system has been booted.
  3. rtc-status: This property indicates the current state of the system, and contains one of the following values:
    • "ok" - the RTC timestamp area is valid and the RTC value is more recent than the last timestamp. The system is running normally.
    • "empty" - the RTC timestamp area was initially empty, i.e. no timestamps were recorded in it. A new timestamp will be recorded and presented on next boot. This case is not considered as a "rollback".
    • "residue" - the RTC timestamp area contained some data, but there was a problem with it (e.g. corruption). This case is considered as a possible attack, so the system will hereon refuse to boot normally.
    • "rollback" - the RTC timestamp is later than the current RTC time. This indicates that the clock was rolled back. This case is considered a possible rollback attempt, so the system will hereon refuse to boot normally.

These RTCAR properties can be viewed from Linux with the following terminal commands:

 echo $(</proc/device-tree/chosen/rtc-timestamp)
 echo $(</proc/device-tree/chosen/rtc-count)
 echo $(</proc/device-tree/chosen/rtc-status)
If /proc/device-tree/ does not exist (i.e. on old builds), use /ofw/ instead.

Note that these properties will only be available when the system is booted in secure mode with RTCAR enabled.

For systems with security disabled (perhaps only temporarily, via a developer key), the same properties can be read from OFW's ok prompt after manually enabling the RTCAR system:

 ok rtc-rollback? .
 ok dev /chosen  .properties  dend

initramfs behaviour

When OFW detects a RTCAR-related problem (e.g. attack or corruption), it will first look for a rtc-timestamp reset signature on all available internal/external media (more details on this are presented below). If no such signature is found, rather than booting the system as normal, the activation initramfs is booted.

Instead of running the normal activation code (which looks for an activation on USB/SD and then looks on the network), the initramfs will enter a "RTC timestamp recovery mode". The assumption here is that the system has failed to boot because of a problem with the rtc-timestamp property (while the RTC correctly contains the current date/time): rtc-timestamp may be corrupt, or may incorrectly contain a future date.

In this mode, the initramfs scans for open wireless networks and connects to them one by one. Once connected, the initramfs attempts to connect to a RTC timestamp reset server on a set of predefined addresses. If a connection is established, the initramfs asks the server for a rtc-timestamp reset signature. This signature includes a new value to be written into the rtc-timestamp property.

If the server provides a response, the RTC reset signature is saved to disk and the system is immediately rebooted. Upon rebooting, the firmware detects the RTC reset signature and reprograms rtc-timestamp based on the server's response.

If the newly programmed rtc-timestamp is less than the current date/time as reported by the RTC, the system now boots normally. However, if the rtc-timestamp value is newer than the RTC's date/time, the system will boot again into the activation initramfs and the above cycle will repeat. Such a situation possibly indicates that the problem preventing boot was not with the rtc-timestamp value (as assumed above), but instead with the RTC value (indicating a possible rollback attack).

If a rollback attack has happened, the above rtc-timestamp recovery procedure will not produce fruitful results (either not finding a server, or not recieving a rtc-timestamp which satisfies the system) and the system will refuse to boot after a short time, reporting the error message "Problem with system clock". The XO will then automatically power off after 60 seconds.

In summary, if a rollback or rtc-timestamp problem is detected, the activation initramfs is booted. The initramfs attempts to obtain a RTC reset signature from a local server which will restore the system to usable state in the face of rtc-timestamp corruption problems, but if this fails (e.g. because a rollback attack was attempted), the system will refuse to boot and power itself off after a short delay.

Recovery

Two possible failure cases exist:

  1. The RTC may have been rolled back beyond the last recorded rtc-timestamp value, which the system will interpret as a rollback attack and hence fail to boot. In this case, the RTC time must be corrected to restore operation of the XO.
  2. The RTC may have the correct time but the rtc-timestamp value may have become invalid or corrupt. This is similarly interpreted as a rollback attack, and the system will fail to boot until the rtc-timestamp value is corrected or reset.

RTC rolled back

If the RTC was erroneously or intentionally rolled back, the system described on this page will detect a rollback attack and the XO will fail to boot. The recovery procedure for such a situation is to correct the RTC time.

First, obtain a developer key for the system. Boot the system into Linux, and run from a root terminal:

 date --utc -s "<current UTC date/time>"

e.g.

 date --utc -s "2011/12/28 12:04:59"

Now reboot the system (without the developer key present) and normal functionality should be restored.

Alternatively, the date can be reprogrammed from OFW's ok prompt. See Fix clock for instructions.

Invalid or corrupted RTC timestamp

The other possible failure case is where the RTC has the correct time value, but the rtc-timestamp value recorded is either invalid or corrupt. One simple cause of such a situation is that the RTC held (for whatever reason) a future time/date, and the system was booted in this condition causing rtc-timestamp to similarly record a future time/date. Upon (innocently) correcting the RTC time to the current date/time, the system will detect a rollback, and the system will fail to boot.

The recovery process involves resetting the firmware's rtc-timestamp value to one that is behind the current RTC date/time. A signed RTC timestamp reset signature must be provided, containing a new value that will be automatically programmed into the rtc-timestamp property.

The RTC timestamp reset signature can be obtained from various sources:

  • OFW will search internal and external media for a valid signed file at /security/rtcreset.sig. If found, the rtc-timestamp will be reprogrammed to the value specified by the file.
  • Otherwise, as detailed elsewhere on this page, the activation initramfs will attempt to obtain this signature from a local server.

See /Implementation for technical details on how to generate such a signature.

RTC timestamp reset server implementations

The RTC timestamp reset server contacted by the initramfs has the responsibility of providing a RTC reset signature for the laptop. Note that this functionality is only useful for the case where the rtc-timestamp value on the XO has become invalid (e.g. in the future) or corrupt -- this server is not used for recovering from an RTC rollback where the rtc-timestamp value is correct but the RTC itself is in the past.

xs-activation

Patches have been submitted to extend xs-activation (which already runs a server on the addresses contacted by the activationinitramfs) to generate rtcreset signatures on-demand.

When such a request is made, xs-activation uses the lease delegation (which it must already possess) in order to generate a delegated RTC timestamp reset signature. This is then sent to the XO, which will use it to reprogram the rtc-timestamp field.

The server will generate a signature for every client that requests it (where a delegation is available), and will always specify that the RTC timestamp should be programmed to the server's current date/time (which is assumed to be correct).

Sample/simple server

A sample server is provided here which will attempt to generate a RTC reset signature for each and every laptop that connects to it. Instead of using delegations (like xs-activation as above), this server uses the master key directly to produce non-delegated RTC timestamp reset signatures.

This acts as skeleton/example code only; be sure to read the comments in the code for some further considerations/ideas.

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