1 2# eStoraged Design - Encrypted Secondary Storage Management Daemon 3 4Author: John Wedig (johnwedig@google.com) 5 6Other contributors: 7John Broadbent (jebr@google.com) 8Benjamin Fair (benjaminfair@google.com) 9Nancy Yuenn (yuenn@google.com) 10 11Created: September 2, 2021 12 13## Problem Description 14 15This daemon will serve as an abstraction for an encrypted storage device, 16encapsulating the security functionality and providing a D-Bus interface to 17manage the encrypted filesystem on the device. Using the D-Bus interface, other 18software components can interact with eStoraged to do things like create a 19new encrypted filesystem, wipe its contents, lock/unlock the device, or change 20the password. 21 22## Background and References 23 24This design is intended to manage secondary storage devices and cannot be used 25for the root filesystem, i.e. the BMC needs to be able to boot while the device 26is still locked. 27 28This design makes use of the 29[cryptsetup](https://gitlab.com/cryptsetup/cryptsetup) utility, which in turn 30uses the [dm-crypt](https://en.wikipedia.org/wiki/Dm-crypt) kernel subsystem. 31Dm-crypt provides the encryption and device mapping capability, and Cryptsetup 32provides the [LUKS](https://en.wikipedia.org/wiki/Linux_Unified_Key_Setup) 33capability, which facilitates password management so that we can do things like 34change the password without re-encrypting the entire device. 35 36This design is specifically targeted for use with eMMC devices, and we plan to 37make use of the lock/unlock feature (CMD42) at the eMMC hardware level as an 38additional security measure. This feature prohibits all read or write accesses 39to the device while locked. Some documentation on this feature can be found in 40the 41[JEDEC standard JESD84-B51A](https://www.jedec.org/document_search?search_api_views_fulltext=jesd84-b51), or in this document: 42[Enabling SD/uSD Card Lock/Unlock Feature in Linux](https://media-www.micron.com/-/media/client/global/documents/products/technical-note/sd-cards/tnsd01_enable_sd_lock_unlock_in_linux.pdf?rev=03f03a6bc0f8435fafa93a8fc8e88988). 43 44There are several types of keys referenced in this doc: 45 46- Volume key: The main encryption key used to encrypt the data on the block 47 device. 48- Encryption Password: The password needed to load the volume key into RAM and 49 decrypt the filesystem. 50- Device Password: The password to lock or unlock the device hardware. 51 52## Requirements 53 54This design should provide an interface for the following capabilities: 55- Create a new LUKS encrypted filesystem on the device 56- Securely wipe the device and verify that the data was wiped 57- Lock the device 58- Unlock the device 59- Change the password 60 61In addition, eStoraged should: 62- Generate a volume key using a random number generator with enough entropy, 63 making it sufficiently random and secure. 64- Utilize any security features provided by the hardware (as a defense-in-depth 65 measure). 66- Use interfaces that are generic enough, so that they can be extended to 67 support additional types of storage devices, as desired. For example, 68 different devices will have different command sets for device locking, e.g. 69 MMC, SATA, NVMe. Initially, we plan to only use eStoraged with eMMC devices, 70 but we may wish to use this with other types of storage devices in the 71 future. 72 73The users of this design can be any other software component in the BMC. Some 74client daemon on the BMC will interact with eStoraged to set up a new encrypted 75filesystem on the eMMC. In addition, the client daemon could be configured to 76unlock the eMMC device when the BMC boots. It is the responsibility of the 77client daemon to provide a password. For example, this password could come 78from user input, fetched from a secure location, or the client daemon could 79generate the passwords itself. 80 81## Proposed Design 82 83eStoraged will represent each eMMC device as an object on D-Bus that 84implements an interface providing these methods and properties: 85- (method) Format 86- (method) Erase 87- (method) Lock 88- (method) Unlock 89- (method) Change Password 90- (property) Locked 91- (property) Status 92 93Upon startup, eStoraged will create a D-Bus object for each eMMC device in 94the system. Specifically, we will use udev to launch an eStoraged instance for 95each eMMC. The bus name and object name will be as follows: 96 97Bus Name: xyz.openbmc\_project.eStorage.\<device name\> 98Object Path: /xyz/openbmc\_project/storage/\<device name\> 99 100The object path is intended to be generic enough, so that we could ultimately 101have multiple daemons managing the same storage device, while using the same 102object path. For example, this daemon would handle the encryption, whereas 103another daemon could provide stats for the same device. 104 105To manage the encrypted filesystem, we will make use of the 106[cryptsetup API](https://mbroz.fedorapeople.org/libcryptsetup_API/). This 107library provides functions to create a new LUKS header, set the password, etc. 108 109For eMMC devices, we plan to enable the password locking feature (CMD42), 110to prevent all read or write accesses to the device while locked. So, the 111"Locked" property will mean both locked at the hardware level and locked at the 112encryption level. We will likely use the ioctl interface to send the relevant 113commands to the eMMC, similar to what 114[mmc utils](https://git.kernel.org/pub/scm/utils/mmc/mmc-utils.git/) does. 115 116Support for hardware locking on other types of devices can be added as needed, 117but at the very least, encryption-only locking will be available, even if 118hardware locking isn't supported for a particular device. 119 120As mentioned earlier, the client will provide a password. This password will 121be used by eStoraged to generate two different passwords: the encryption 122password and the device password (if hardware locking is available). The 123passwords will be different, so that in the event that one password is 124compromised, we still have some protection from the other password. 125 126The Erase method should provide a way to specify the type of erase, e.g. write 127all zeros, or do something else. Different organizations may have different 128opinions of what a secure erase entails. 129 130Since some of the D-Bus methods may take a while (e.g. installing a new 131encrypted filesystem), the D-Bus interface will be asynchronous, with the 132"Status" property that can be queried to indicate one of the following: 133success, error, or in-progress. 134 135## Alternatives Considered 136An alternative would be to use systemd targets to manage the eMMC. For example, 137the 138[systemd-cryptsetup@.service](https://www.freedesktop.org/software/systemd/man/systemd-cryptsetup@.service.html) 139is often used to unlock an encrypted block device, where it takes the password 140from a key file or from user input. However, the OpenBMC architecture calls for 141using D-Bus as the primary form of inter-process communication. In addition, 142using a daemon with a well-defined D-Bus interface keeps the security 143functionality more isolated, maintainable, and testable. 144 145Another related piece of software is UDisks2, which also exports a D-Bus object 146for each storage device in a system. It is capable of setting up an encrypted 147block device with the Format method: 148[org.freedesktop.UDisks2.Format](http://storaged.org/doc/udisks2-api/latest/gdbus-org.freedesktop.UDisks2.Block.html#gdbus-method-org-freedesktop-UDisks2-Block.Format). 149And it provides several additional methods related to encryption: Lock, Unlock, 150and ChangePassphrase. See the D-Bus interface 151[org.freedesktop.UDisks2.Encrypted](http://storaged.org/doc/udisks2-api/2.7.5/gdbus-org.freedesktop.UDisks2.Encrypted.html). 152The main problem preventing us from leveraging this tool is that it increases 153our image size too much. We found that the compressed image size increased by 15422 MB due to the transitive dependencies being pulled in, e.g. mozjs and 155python. 156 157## Impacts 158To make use of eStoraged, it may be necessary to provide another client daemon 159that manages the password and invokes the D-Bus API for eStoraged. Since the 160password management scheme can be unique for different platforms and 161organizations, it is outside the scope of this design. 162 163## Testing 164- Unit tests to validate the various code paths in eStoraged. 165- Regression tests will exercise the various D-Bus methods: encrypt, erase, 166 lock, unlock, and change password. 167