Author: John Wedig (johnwedig@google.com)
Other contributors: John Broadbent (jebr@google.com) Benjamin Fair (benjaminfair@google.com) Nancy Yuenn (yuenn@google.com)
Created: September 2, 2021
This daemon will serve as an abstraction for an encrypted storage device, encapsulating the security functionality and providing a D-Bus interface to manage the encrypted filesystem on the device. Using the D-Bus interface, other software components can interact with eStoraged to do things like create a new encrypted filesystem, wipe its contents, lock/unlock the device, or change the password.
This design is intended to manage secondary storage devices and cannot be used for the root filesystem, i.e. the BMC needs to be able to boot while the device is still locked.
This design makes use of the cryptsetup utility, which in turn uses the dm-crypt kernel subsystem. Dm-crypt provides the encryption and device mapping capability, and Cryptsetup provides the LUKS capability, which facilitates password management so that we can do things like change the password without re-encrypting the entire device.
This design is specifically targeted for use with eMMC devices, and we plan to make use of the lock/unlock feature (CMD42) at the eMMC hardware level as an additional security measure. This feature prohibits all read or write accesses to the device while locked. Some documentation on this feature can be found in the JEDEC standard JESD84-B51A, or in this document: Enabling SD/uSD Card Lock/Unlock Feature in Linux.
There are several types of keys referenced in this doc:
This design should provide an interface for the following capabilities:
In addition, eStoraged should:
The users of this design can be any other software component in the BMC. Some client daemon on the BMC will interact with eStoraged to set up a new encrypted filesystem on the eMMC. In addition, the client daemon could be configured to unlock the eMMC device when the BMC boots. It is the responsibility of the client daemon to provide a password. For example, this password could come from user input, fetched from a secure location, or the client daemon could generate the passwords itself.
eStoraged will represent each eMMC device as an object on D-Bus that implements an interface providing these methods and properties:
Upon startup, eStoraged will create a D-Bus object for each eMMC device in the system. Specifically, we will use udev to launch an eStoraged instance for each eMMC. The bus name and object name will be as follows:
Bus Name: xyz.openbmc_project.eStorage.<device name> Object Path: /xyz/openbmc_project/storage/<device name>
The object path is intended to be generic enough, so that we could ultimately have multiple daemons managing the same storage device, while using the same object path. For example, this daemon would handle the encryption, whereas another daemon could provide stats for the same device.
To manage the encrypted filesystem, we will make use of the cryptsetup API. This library provides functions to create a new LUKS header, set the password, etc.
For eMMC devices, we plan to enable the password locking feature (CMD42), to prevent all read or write accesses to the device while locked. So, the "Locked" property will mean both locked at the hardware level and locked at the encryption level. We will likely use the ioctl interface to send the relevant commands to the eMMC, similar to what mmc utils does.
Support for hardware locking on other types of devices can be added as needed, but at the very least, encryption-only locking will be available, even if hardware locking isn't supported for a particular device.
As mentioned earlier, the client will provide a password. This password will be used by eStoraged to generate two different passwords: the encryption password and the device password (if hardware locking is available). The passwords will be different, so that in the event that one password is compromised, we still have some protection from the other password.
The Erase method should provide a way to specify the type of erase, e.g. write all zeros, or do something else. Different organizations may have different opinions of what a secure erase entails.
Since some of the D-Bus methods may take a while (e.g. installing a new encrypted filesystem), the D-Bus interface will be asynchronous, with the "Status" property that can be queried to indicate one of the following: success, error, or in-progress.
An alternative would be to use systemd targets to manage the eMMC. For example, the systemd-cryptsetup@.service is often used to unlock an encrypted block device, where it takes the password from a key file or from user input. However, the OpenBMC architecture calls for using D-Bus as the primary form of inter-process communication. In addition, using a daemon with a well-defined D-Bus interface keeps the security functionality more isolated, maintainable, and testable.
Another related piece of software is UDisks2, which also exports a D-Bus object for each storage device in a system. It is capable of setting up an encrypted block device with the Format method: org.freedesktop.UDisks2.Format. And it provides several additional methods related to encryption: Lock, Unlock, and ChangePassphrase. See the D-Bus interface org.freedesktop.UDisks2.Encrypted. The main problem preventing us from leveraging this tool is that it increases our image size too much. We found that the compressed image size increased by 22 MB due to the transitive dependencies being pulled in, e.g. mozjs and python.
To make use of eStoraged, it may be necessary to provide another client daemon that manages the password and invokes the D-Bus API for eStoraged. Since the password management scheme can be unique for different platforms and organizations, it is outside the scope of this design.