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.

initramfs behaviour

The activation initramfs (from dracut-modules-olpc) implements the user-visible behaviour of this system.

• As described above, OFW automatically boots the activation initramfs (actos/actrd) upon detecting rollback or corruption.
• The initramfs observes the presence of the chosen/rtc-* nodes. If rtc-status is "residue" or "rollback", the following steps are executed. Otherwise, the activation initramfs continues with its "normal" behaviour (where it assumes the laptop lacks activation and looks for an activation lease on USB, SD and wifi).
• Instead of searching for an activation, the initramfs enters an alternative mode where it looks for a RTC timestamp reset signature via the network.
• The initramfs connects one by one to all open wifi networks in range, and on each network, connects to the RTC timestamp reset server on the XS predefined addresses on TCP port 191. The logic used here is identical to that used when looking for an activation server.
• When connected to a server, the initramfs sends the message rtcreset <SERIAL> <RTC-TIMESTAMP> <RTC-COUNT> (e.g. rtcreset SHC01601310 20200101T000113Z 19). This is different from the usual message when an activation is sought, where the message would consist of the laptop serial number alone.
• The server may then respond with a RTC timestamp reset signature sent as plain-text.
• If a response is received, the initramfs will save the contents of the response at /security/rtcreset.sig and cleanly reboot the system.
  • Upon reboot, as noted above, the #RTCAR Recovery procedure is executed, which will cause OFW to reprogram the rtc-timestamp field based on the response received from the server.
• If no response is received, the initramfs prints an error message and powers down the XO after a 60 second delay.

In summary, if OFW detects a rollback or rtc-timestamp problem, the activation initramfs is booted. The initramfs attempts to obtain a RTC reset signature from a local server, but if this fails, the system will refuse to boot and power itself off after a short delay.

Viewing RTCAR property values

From Linux:

 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

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 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.

OFW will repair the timestamp area if a suitable RTC timestamp reset signature is present in /security, as detailed below.

• 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.

See #Generating a RTC timestamp reset signature for instructions on how the rtcreset.sig file can be generated using bios-crypto tools. Once generated, the rtcreset.sig file can be provided on external USB or SD (in the 'security' subdirectory of the root) while booting the system.

To perform this operation in a more automated fashion, a RTC timestamp reset server can be installed locally on an open network. When RTCAR prevents boot, the activation initramfs will attempt to connect to a local server and request a RTC reset signature. The server needs to be in possession of the required rtcreset.sig file, or needs to be in the position to be able to create it (with the relevant keys and tools available). See below for a sample implementation of how this can be automated.

Generating a RTC timestamp reset signature

As the RTC timestamp reset signature falls within the regular Firmware security infrastructure, bios-crypto provides the necessary tools in order to generate a valid and signed rtcreset.sig.

Having followed the build/installation instructions documented at bios-crypto, a RTC reset signature can be generated as follows:

 $ ./make-rtcreset.sh <SERIAL> <UUID> <EXISTING-RTC-TIMESTAMP> <EXISTING-RTC-COUNT> <NEW-RTC-TIMESTAMP> <SIGNING-KEY>

e.g.

 $ ./make-rtcreset.sh SHC005007B7 1273E0EC-AEF1-9FF6-45B2-FB706DC24B8D 20120511T024746Z 141 20110511T024746Z lease

The EXISTING-RTC-TIMESTAMP and EXISTING-RTC-COUNT parameters must be extracted from the XO in question - the rtcreset signature is specific to the target laptop with the current recorded count/timestamp values, and will not work with different values.

The final parameter corresponds to the key used to sign the output, in the above example it refers to the keypair of lease.public and lease.private.

The above tool can either be used by hand, or it can form the basis of a more automated system (such as the sample server implementation documented below).

Sample RTC timestamp reset server implementation

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.

A sample server is provided here which will attempt to generate a RTC reset signature for each and every laptop that connects to it. This acts as skeleton code only; a real implementation may want to consider the following items:

• Further input data validation
• UUID database rather than a hardcoded list
• Does the client's rtc-timestamp look bad to begin with? If not, there is no need to generate a reset signature.
• Reset to a timestamp reflective of the current time (but perhaps somewhat in the past) rather than a hardcoded timestamp

Testing

Here are some recipes for verifying the correct operation of this feature, using Open Firmware interactive commands. These recipes assume that you can get to the ok prompt, either with a non-secure system or with a developer key.

Testing Rollback Detection

First, enable the use of this feature by creating an "rt" tag in manufacturing data:

 ok " "  " rt" $add-tag

Run the anti-rollback check as follows:

 ok rtc-rollback? .
 0

The first time that you invoke rtc-rollback?, it should return 1, indicating that the timestamp area was empty and has now been initialized. Subsequent invocations should return 0, indicating normal operation.

You can see the relevant properties in /chosen with:

 ok dev /chosen  .properties  dend

To simulate a rollback attack:

 ok clock-node @ iselect  get-time 1- set-time  iunselect

That sets the clock back 1 year (the "1-" decrements the year field). Subsequently:

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

You can put the clock back to normal with:

 ok clock-node @ iselect  get-time 1+ set-time  iunselect

Testing Timestamp Reset

Testing the reset function is complicated because you need to make a signed file, which in turn means that you need access to a private key whose public key is on the test machine. Since I don't have access to the OLPC private keys, I had to inject a test key.

First you need to get a copy of the bios-crypto tools:

 $ git clone git://dev.laptop.org/bios-crypto
 $ cd bios-crypto/build
 $ make

Make a keypair for testing:

 $ ./makekey lease

That creates "lease.public" and "lease.private". Put "lease.public" on a USB FLASH drive so it can be transferred to the test machine:

 $ cp lease.public PATH_TO_USB_FLASH_DRIVE/lease.pub
 $ umount PATH_TO_USB_FLASH_DRIVE

Inject the test key into manufacturing data. The following assumes that there are no deployment-specific keys on the machine, so the "a1" key is available. If "a1" is already in use (use ".mfg-data" to check), you will need to use "a2" instead.

 ok load u:\lease.pub
 ok loaded " a1" $add-key

Now you will need to put the test machine into a state where it thinks a rollback attack has occurred.

 ok clock-node @ iselect  get-time 1+ set-time  iunselect
 ok rtc-rollback? .
 0
 ok clock-node @ iselect  get-time 1+ set-time  iunselect
 ok rtc-rollback? .
 ffffffff
 ok dev /chosen  .properties  dend
 rtc-count                141
 rtc-timestamp            20120511T024746Z
 rtc-status               rollback

What that did was to set the clock ahead by a year, then rtc-rollback? recorded a new timestamp that is a year in the future. Then it set the clock back to the current year, and rtc-rollback? detected an attack because the current (correct) clock is earlier than the year-in-the-future timestamp. The rtc-count and rtc-timestamp properties give us some of the information that we need to construct a signed file to fix the timestamp. The other information that we need is in manufacturing data:

 ok " SN" find-tag drop type
 SHC005007B7
 ok " U#" find-tag drop type
 1273E0EC-AEF1-9FF6-45B2-FB706DC24B8D

Armed with those values (which are machine-specific, so you will have to supply your own values), you can make a signed rtcreset.sig file:

 $ ./make-rtcreset.sh SHC005007B7 1273E0EC-AEF1-9FF6-45B2-FB706DC24B8D 20120511T024746Z 141 20110511T024746Z lease

Note that the next-to-last field - 20110511T024746Z - is similar to the first date field ("2012...") except that the year was manually restored to the current year - 2011 instead of 2012. That final date field is the timestamp that will be restored into the test machine; it can be any date/time that is before the current RTC date/time.

The make-rtcreset.sh script writes the result into the output file rtcreset.sig. Copy it onto the USB FLASH drive for transfer to the test machine:

 $ mkdir -p PATH_TO_USB_FLASH_DRIVE/security
 $ cp rtcreset.sig PATH_TO_USB_FLASH_DRIVE/security
 $ umount PATH_TO_USB_FLASH_DRIVE

Insert the USB FLASH drive into the test machine and:

 ok load-crypto . get-my-sn .
 0 0
 ok " u:" dn-buf place
 ok ?rtc-update
 Trying u:\security\rtcreset.sig
 RTCRESET found -   Signature valid
 Writing
 e0000

Instead of typing the commands above, you could instead boot the test machine in secure mode, for example by holding down the "X" game button while booting. In secure mode, the recovery procedure happens automatically.

The recovery will only work once because the timestamp in the signed file will no longer match the last recorded timestamp. To re-test the recovery procedure, you must repeat the above steps beginning with simulating the rollback attack, and then make a new signed rtcreset.sig file.