1 .. SPDX-License-Identifier: GPL-2.0+
2 .. Copyright (c) 2018 Heinrich Schuchardt
7 The Unified Extensible Firmware Interface Specification (UEFI) [1] has become
8 the default for booting on AArch64 and x86 systems. It provides a stable API for
9 the interaction of drivers and applications with the firmware. The API comprises
10 access to block storage, network, and console to name a few. The Linux kernel
11 and boot loaders like GRUB or the FreeBSD loader can be executed.
16 The implementation of UEFI in U-Boot strives to reach the requirements described
17 in the "Embedded Base Boot Requirements (EBBR) Specification - Release v1.0"
18 [2]. The "Server Base Boot Requirements System Software on ARM Platforms" [3]
19 describes a superset of the EBBR specification and may be used as further
22 A full blown UEFI implementation would contradict the U-Boot design principle
25 Building U-Boot for UEFI
26 ------------------------
28 The UEFI standard supports only little-endian systems. The UEFI support can be
29 activated for ARM and x86 by specifying::
36 Support for attaching virtual block devices, e.g. iSCSI drives connected by the
37 loaded UEFI application [4], requires::
42 Executing a UEFI binary
43 ~~~~~~~~~~~~~~~~~~~~~~~
45 The bootefi command is used to start UEFI applications or to install UEFI
46 drivers. It takes two parameters::
48 bootefi <image address> [fdt address]
50 * image address - the memory address of the UEFI binary
51 * fdt address - the memory address of the flattened device tree
53 Below you find the output of an example session starting GRUB::
55 => load mmc 0:2 ${fdt_addr_r} boot/dtb
56 29830 bytes read in 14 ms (2 MiB/s)
57 => load mmc 0:1 ${kernel_addr_r} efi/debian/grubaa64.efi
58 reading efi/debian/grubaa64.efi
59 120832 bytes read in 7 ms (16.5 MiB/s)
60 => bootefi ${kernel_addr_r} ${fdt_addr_r}
62 The environment variable 'bootargs' is passed as load options in the UEFI system
63 table. The Linux kernel EFI stub uses the load options as command line
66 Launching a UEFI binary from a FIT image
67 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
69 A signed FIT image can be used to securely boot a UEFI image via the
70 bootm command. This feature is available if U-Boot is configured with::
74 A sample configuration is provided as file doc/uImage.FIT/uefi.its.
76 Below you find the output of an example session starting GRUB::
78 => load mmc 0:1 ${kernel_addr_r} image.fit
79 4620426 bytes read in 83 ms (53.1 MiB/s)
80 => bootm ${kernel_addr_r}#config-grub-nofdt
81 ## Loading kernel from FIT Image at 40400000 ...
82 Using 'config-grub-nofdt' configuration
83 Verifying Hash Integrity ... sha256,rsa2048:dev+ OK
84 Trying 'efi-grub' kernel subimage
85 Description: GRUB EFI Firmware
86 Created: 2019-11-20 8:18:16 UTC
87 Type: Kernel Image (no loading done)
88 Compression: uncompressed
89 Data Start: 0x404000d0
90 Data Size: 450560 Bytes = 440 KiB
92 Hash value: 4dbee00021112df618f58b3f7cf5e1595533d543094064b9ce991e8b054a9eec
93 Verifying Hash Integrity ... sha256+ OK
94 XIP Kernel Image (no loading done)
95 ## Transferring control to EFI (at address 404000d0) ...
98 See doc/uImage.FIT/howto.txt for an introduction to FIT images.
100 Configuring UEFI secure boot
101 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
103 The UEFI specification[1] defines a secure way of executing UEFI images
104 by verifying a signature (or message digest) of image with certificates.
105 This feature on U-Boot is enabled with::
107 CONFIG_UEFI_SECURE_BOOT=y
109 To make the boot sequence safe, you need to establish a chain of trust;
110 In UEFI secure boot the chain trust is defined by the following UEFI variables
113 * KEK - Key Exchange Keys
114 * db - white list database
115 * dbx - black list database
117 An in depth description of UEFI secure boot is beyond the scope of this
118 document. Please, refer to the UEFI specification and available online
119 documentation. Here is a simple example that you can follow for your initial
120 attempt (Please note that the actual steps will depend on your system and
123 Install the required tools on your host
129 Create signing keys and the key database on your host:
135 openssl req -x509 -sha256 -newkey rsa:2048 -subj /CN=TEST_PK/ \
136 -keyout PK.key -out PK.crt -nodes -days 365
137 cert-to-efi-sig-list -g 11111111-2222-3333-4444-123456789abc \
139 sign-efi-sig-list -c PK.crt -k PK.key PK PK.esl PK.auth
141 The key exchange keys
145 openssl req -x509 -sha256 -newkey rsa:2048 -subj /CN=TEST_KEK/ \
146 -keyout KEK.key -out KEK.crt -nodes -days 365
147 cert-to-efi-sig-list -g 11111111-2222-3333-4444-123456789abc \
149 sign-efi-sig-list -c PK.crt -k PK.key KEK KEK.esl KEK.auth
151 The whitelist database
155 $ openssl req -x509 -sha256 -newkey rsa:2048 -subj /CN=TEST_db/ \
156 -keyout db.key -out db.crt -nodes -days 365
157 $ cert-to-efi-sig-list -g 11111111-2222-3333-4444-123456789abc \
159 $ sign-efi-sig-list -c KEK.crt -k KEK.key db db.esl db.auth
161 Copy the \*.auth files to media, say mmc, that is accessible from U-Boot.
163 Sign an image with one of the keys in "db" on your host
167 sbsign --key db.key --cert db.crt helloworld.efi
169 Now in U-Boot install the keys on your board::
171 fatload mmc 0:1 <tmpaddr> PK.auth
172 setenv -e -nv -bs -rt -at -i <tmpaddr>,$filesize PK
173 fatload mmc 0:1 <tmpaddr> KEK.auth
174 setenv -e -nv -bs -rt -at -i <tmpaddr>,$filesize KEK
175 fatload mmc 0:1 <tmpaddr> db.auth
176 setenv -e -nv -bs -rt -at -i <tmpaddr>,$filesize db
178 Set up boot parameters on your board::
180 efidebug boot add 1 HELLO mmc 0:1 /helloworld.efi.signed ""
182 Now your board can run the signed image via the boot manager (see below).
183 You can also try this sequence by running Pytest, test_efi_secboot,
188 cd <U-Boot source directory>
189 pytest.py test/py/tests/test_efi_secboot/test_signed.py --bd sandbox
191 Using OP-TEE for EFI variables
192 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
194 Instead of implementing UEFI variable services inside U-Boot they can
195 also be provided in the secure world by a module for OP-TEE[1]. The
196 interface between U-Boot and OP-TEE for variable services is enabled by
197 CONFIG_EFI_MM_COMM_TEE=y.
199 Tianocore EDK II's standalone management mode driver for variables can
200 be linked to OP-TEE for this purpose. This module uses the Replay
201 Protected Memory Block (RPMB) of an eMMC device for persisting
202 non-volatile variables. When calling the variable services via the
203 OP-TEE API U-Boot's OP-TEE supplicant relays calls to the RPMB driver
204 which has to be enabled via CONFIG_SUPPORT_EMMC_RPMB=y.
206 [1] https://optee.readthedocs.io/ - OP-TEE documentation
208 Executing the boot manager
209 ~~~~~~~~~~~~~~~~~~~~~~~~~~
211 The UEFI specification foresees to define boot entries and boot sequence via UEFI
212 variables. Booting according to these variables is possible via::
214 bootefi bootmgr [fdt address]
216 As of U-Boot v2018.03 UEFI variables are not persisted and cannot be set at
219 Executing the built in hello world application
220 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
222 A hello world UEFI application can be built with::
224 CONFIG_CMD_BOOTEFI_HELLO_COMPILE=y
226 It can be embedded into the U-Boot binary with::
228 CONFIG_CMD_BOOTEFI_HELLO=y
230 The bootefi command is used to start the embedded hello world application::
232 bootefi hello [fdt address]
234 Below you find the output of an example session::
236 => bootefi hello ${fdtcontroladdr}
237 ## Starting EFI application at 01000000 ...
238 WARNING: using memory device/image path, this may confuse some payloads!
243 Load options: root=/dev/sdb3 init=/sbin/init rootwait ro
244 ## Application terminated, r = 0
246 The environment variable fdtcontroladdr points to U-Boot's internal device tree
249 Executing the built-in self-test
250 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
252 An UEFI self-test suite can be embedded in U-Boot by building with::
254 CONFIG_CMD_BOOTEFI_SELFTEST=y
256 For testing the UEFI implementation the bootefi command can be used to start the
259 bootefi selftest [fdt address]
261 The environment variable 'efi_selftest' can be used to select a single test. If
262 it is not provided all tests are executed except those marked as 'on request'.
263 If the environment variable is set to 'list' a list of all tests is shown.
265 Below you can find the output of an example session::
267 => setenv efi_selftest simple network protocol
269 Testing EFI API implementation
270 Selected test: 'simple network protocol'
271 Setting up 'simple network protocol'
272 Setting up 'simple network protocol' succeeded
273 Executing 'simple network protocol'
275 DHCP reply received from 192.168.76.2 (52:55:c0:a8:4c:02)
276 as broadcast message.
277 Executing 'simple network protocol' succeeded
278 Tearing down 'simple network protocol'
279 Tearing down 'simple network protocol' succeeded
280 Boot services terminated
282 Preparing for reset. Press any key.
287 After the U-Boot platform has been initialized the UEFI API provides two kinds
293 The API can be extended by loading UEFI drivers which come in two variants:
298 UEFI drivers are installed with U-Boot's bootefi command. With the same command
299 UEFI applications can be executed.
301 Loaded images of UEFI drivers stay in memory after returning to U-Boot while
302 loaded images of applications are removed from memory.
304 An UEFI application (e.g. an operating system) that wants to take full control
305 of the system calls ExitBootServices. After a UEFI application calls
308 * boot services are not available anymore
309 * timer events are stopped
310 * the memory used by U-Boot except for runtime services is released
311 * the memory used by boot time drivers is released
313 So this is a point of no return. Afterwards the UEFI application can only return
314 to U-Boot by rebooting.
316 The UEFI object model
317 ---------------------
319 UEFI offers a flexible and expandable object model. The objects in the UEFI API
320 are devices, drivers, and loaded images. These objects are referenced by
323 The interfaces implemented by the objects are referred to as protocols. These
324 are identified by GUIDs. They can be installed and uninstalled by calling the
325 appropriate boot services.
327 Handles are created by the InstallProtocolInterface or the
328 InstallMultipleProtocolinterfaces service if NULL is passed as handle.
330 Handles are deleted when the last protocol has been removed with the
331 UninstallProtocolInterface or the UninstallMultipleProtocolInterfaces service.
333 Devices offer the EFI_DEVICE_PATH_PROTOCOL. A device path is the concatenation
334 of device nodes. By their device paths all devices of a system are arranged in a
337 Drivers offer the EFI_DRIVER_BINDING_PROTOCOL. This protocol is used to connect
338 a driver to devices (which are referenced as controllers in this context).
340 Loaded images offer the EFI_LOADED_IMAGE_PROTOCOL. This protocol provides meta
341 information about the image and a pointer to the unload callback function.
346 In the UEFI terminology an event is a data object referencing a notification
347 function which is queued for calling when the event is signaled. The following
348 types of events exist:
350 * periodic and single shot timer events
351 * exit boot services events, triggered by calling the ExitBootServices() service
352 * virtual address change events
353 * memory map change events
354 * read to boot events
355 * reset system events
356 * system table events
357 * events that are only triggered programmatically
359 Events can be created with the CreateEvent service and deleted with CloseEvent
362 Events can be assigned to an event group. If any of the events in a group is
363 signaled, all other events in the group are also set to the signaled state.
365 The UEFI driver model
366 ---------------------
368 A driver is specific for a single protocol installed on a device. To install a
369 driver on a device the ConnectController service is called. In this context
370 controller refers to the device for which the driver is installed.
372 The relevant drivers are identified using the EFI_DRIVER_BINDING_PROTOCOL. This
373 protocol has has three functions:
375 * supported - determines if the driver is compatible with the device
376 * start - installs the driver by opening the relevant protocol with
377 attribute EFI_OPEN_PROTOCOL_BY_DRIVER
378 * stop - uninstalls the driver
380 The driver may create child controllers (child devices). E.g. a driver for block
381 IO devices will create the device handles for the partitions. The child
382 controllers will open the supported protocol with the attribute
383 EFI_OPEN_PROTOCOL_BY_CHILD_CONTROLLER.
385 A driver can be detached from a device using the DisconnectController service.
387 U-Boot devices mapped as UEFI devices
388 -------------------------------------
390 Some of the U-Boot devices are mapped as UEFI devices
397 As of U-Boot 2018.03 the logic for doing this is hard coded.
399 The development target is to integrate the setup of these UEFI devices with the
400 U-Boot driver model [5]. So when a U-Boot device is discovered a handle should
401 be created and the device path protocol and the relevant IO protocol should be
402 installed. The UEFI driver then would be attached by calling ConnectController.
403 When a U-Boot device is removed DisconnectController should be called.
405 UEFI devices mapped as U-Boot devices
406 -------------------------------------
408 UEFI drivers binaries and applications may create new (virtual) devices, install
409 a protocol and call the ConnectController service. Now the matching UEFI driver
410 is determined by iterating over the implementations of the
411 EFI_DRIVER_BINDING_PROTOCOL.
413 It is the task of the UEFI driver to create a corresponding U-Boot device and to
414 proxy calls for this U-Boot device to the controller.
416 In U-Boot 2018.03 this has only been implemented for block IO devices.
421 An UEFI uclass driver (lib/efi_driver/efi_uclass.c) has been created that
422 takes care of initializing the UEFI drivers and providing the
423 EFI_DRIVER_BINDING_PROTOCOL implementation for the UEFI drivers.
425 A linker created list is used to keep track of the UEFI drivers. To create an
426 entry in the list the UEFI driver uses the U_BOOT_DRIVER macro specifying
427 UCLASS_EFI as the ID of its uclass, e.g::
429 /* Identify as UEFI driver */
430 U_BOOT_DRIVER(efi_block) = {
431 .name = "EFI block driver",
436 The available operations are defined via the structure struct efi_driver_ops::
438 struct efi_driver_ops {
439 const efi_guid_t *protocol;
440 const efi_guid_t *child_protocol;
441 int (*bind)(efi_handle_t handle, void *interface);
444 When the supported() function of the EFI_DRIVER_BINDING_PROTOCOL is called the
445 uclass checks if the protocol GUID matches the protocol GUID of the UEFI driver.
446 In the start() function the bind() function of the UEFI driver is called after
448 The stop() function of the EFI_DRIVER_BINDING_PROTOCOL disconnects the child
449 controllers created by the UEFI driver and the UEFI driver. (In U-Boot v2013.03
450 this is not yet completely implemented.)
455 The UEFI block IO driver supports devices exposing the EFI_BLOCK_IO_PROTOCOL.
457 When connected it creates a new U-Boot block IO device with interface type
458 IF_TYPE_EFI, adds child controllers mapping the partitions, and installs the
459 EFI_SIMPLE_FILE_SYSTEM_PROTOCOL on these. This can be used together with the
460 software iPXE to boot from iSCSI network drives [4].
462 This driver is only available if U-Boot is configured with::
473 The load file 2 protocol can be used by the Linux kernel to load the initial
474 RAM disk. U-Boot can be configured to provide an implementation with::
476 EFI_LOAD_FILE2_INITRD=y
477 EFI_INITRD_FILESPEC=interface dev:part path_to_initrd
482 * [1] http://uefi.org/specifications - UEFI specifications
483 * [2] https://github.com/ARM-software/ebbr/releases/download/v1.0/ebbr-v1.0.pdf -
484 Embedded Base Boot Requirements (EBBR) Specification - Release v1.0
485 * [3] https://developer.arm.com/docs/den0044/latest/server-base-boot-requirements-system-software-on-arm-platforms-version-11 -
486 Server Base Boot Requirements System Software on ARM Platforms - Version 1.1
488 * [5] :doc:`../driver-model/index`