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https://github.com/brain-hackers/u-boot-brain
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![Heinrich Schuchardt](/assets/img/avatar_default.png)
Some distributions provide UEFI binaries like Shim that have been signed using a Microsoft certificate. Provide the download paths for the public keys. Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
498 lines
17 KiB
ReStructuredText
498 lines
17 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0+
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.. Copyright (c) 2018 Heinrich Schuchardt
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UEFI on U-Boot
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==============
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The Unified Extensible Firmware Interface Specification (UEFI) [1] has become
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the default for booting on AArch64 and x86 systems. It provides a stable API for
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the interaction of drivers and applications with the firmware. The API comprises
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access to block storage, network, and console to name a few. The Linux kernel
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and boot loaders like GRUB or the FreeBSD loader can be executed.
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Development target
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------------------
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The implementation of UEFI in U-Boot strives to reach the requirements described
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in the "Embedded Base Boot Requirements (EBBR) Specification - Release v1.0"
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[2]. The "Server Base Boot Requirements System Software on ARM Platforms" [3]
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describes a superset of the EBBR specification and may be used as further
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reference.
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A full blown UEFI implementation would contradict the U-Boot design principle
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"keep it small".
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Building U-Boot for UEFI
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------------------------
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The UEFI standard supports only little-endian systems. The UEFI support can be
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activated for ARM and x86 by specifying::
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CONFIG_CMD_BOOTEFI=y
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CONFIG_EFI_LOADER=y
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in the .config file.
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Support for attaching virtual block devices, e.g. iSCSI drives connected by the
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loaded UEFI application [4], requires::
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CONFIG_BLK=y
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CONFIG_PARTITIONS=y
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Executing a UEFI binary
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~~~~~~~~~~~~~~~~~~~~~~~
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The bootefi command is used to start UEFI applications or to install UEFI
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drivers. It takes two parameters::
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bootefi <image address> [fdt address]
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* image address - the memory address of the UEFI binary
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* fdt address - the memory address of the flattened device tree
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Below you find the output of an example session starting GRUB::
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=> load mmc 0:2 ${fdt_addr_r} boot/dtb
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29830 bytes read in 14 ms (2 MiB/s)
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=> load mmc 0:1 ${kernel_addr_r} efi/debian/grubaa64.efi
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reading efi/debian/grubaa64.efi
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120832 bytes read in 7 ms (16.5 MiB/s)
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=> bootefi ${kernel_addr_r} ${fdt_addr_r}
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The environment variable 'bootargs' is passed as load options in the UEFI system
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table. The Linux kernel EFI stub uses the load options as command line
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arguments.
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Launching a UEFI binary from a FIT image
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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A signed FIT image can be used to securely boot a UEFI image via the
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bootm command. This feature is available if U-Boot is configured with::
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CONFIG_BOOTM_EFI=y
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A sample configuration is provided as file doc/uImage.FIT/uefi.its.
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Below you find the output of an example session starting GRUB::
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=> load mmc 0:1 ${kernel_addr_r} image.fit
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4620426 bytes read in 83 ms (53.1 MiB/s)
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=> bootm ${kernel_addr_r}#config-grub-nofdt
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## Loading kernel from FIT Image at 40400000 ...
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Using 'config-grub-nofdt' configuration
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Verifying Hash Integrity ... sha256,rsa2048:dev+ OK
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Trying 'efi-grub' kernel subimage
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Description: GRUB EFI Firmware
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Created: 2019-11-20 8:18:16 UTC
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Type: Kernel Image (no loading done)
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Compression: uncompressed
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Data Start: 0x404000d0
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Data Size: 450560 Bytes = 440 KiB
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Hash algo: sha256
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Hash value: 4dbee00021112df618f58b3f7cf5e1595533d543094064b9ce991e8b054a9eec
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Verifying Hash Integrity ... sha256+ OK
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XIP Kernel Image (no loading done)
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## Transferring control to EFI (at address 404000d0) ...
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Welcome to GRUB!
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See doc/uImage.FIT/howto.txt for an introduction to FIT images.
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Configuring UEFI secure boot
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The UEFI specification[1] defines a secure way of executing UEFI images
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by verifying a signature (or message digest) of image with certificates.
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This feature on U-Boot is enabled with::
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CONFIG_UEFI_SECURE_BOOT=y
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To make the boot sequence safe, you need to establish a chain of trust;
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In UEFI secure boot the chain trust is defined by the following UEFI variables
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* PK - Platform Key
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* KEK - Key Exchange Keys
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* db - white list database
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* dbx - black list database
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An in depth description of UEFI secure boot is beyond the scope of this
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document. Please, refer to the UEFI specification and available online
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documentation. Here is a simple example that you can follow for your initial
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attempt (Please note that the actual steps will depend on your system and
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environment.):
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Install the required tools on your host
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* openssl
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* efitools
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* sbsigntool
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Create signing keys and the key database on your host:
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The platform key
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.. code-block:: bash
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openssl req -x509 -sha256 -newkey rsa:2048 -subj /CN=TEST_PK/ \
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-keyout PK.key -out PK.crt -nodes -days 365
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cert-to-efi-sig-list -g 11111111-2222-3333-4444-123456789abc \
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PK.crt PK.esl;
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sign-efi-sig-list -c PK.crt -k PK.key PK PK.esl PK.auth
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The key exchange keys
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.. code-block:: bash
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openssl req -x509 -sha256 -newkey rsa:2048 -subj /CN=TEST_KEK/ \
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-keyout KEK.key -out KEK.crt -nodes -days 365
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cert-to-efi-sig-list -g 11111111-2222-3333-4444-123456789abc \
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KEK.crt KEK.esl
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sign-efi-sig-list -c PK.crt -k PK.key KEK KEK.esl KEK.auth
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The whitelist database
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.. code-block:: bash
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$ openssl req -x509 -sha256 -newkey rsa:2048 -subj /CN=TEST_db/ \
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-keyout db.key -out db.crt -nodes -days 365
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$ cert-to-efi-sig-list -g 11111111-2222-3333-4444-123456789abc \
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db.crt db.esl
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$ sign-efi-sig-list -c KEK.crt -k KEK.key db db.esl db.auth
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Copy the \*.auth files to media, say mmc, that is accessible from U-Boot.
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Sign an image with one of the keys in "db" on your host
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.. code-block:: bash
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sbsign --key db.key --cert db.crt helloworld.efi
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Now in U-Boot install the keys on your board::
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fatload mmc 0:1 <tmpaddr> PK.auth
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setenv -e -nv -bs -rt -at -i <tmpaddr>,$filesize PK
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fatload mmc 0:1 <tmpaddr> KEK.auth
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setenv -e -nv -bs -rt -at -i <tmpaddr>,$filesize KEK
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fatload mmc 0:1 <tmpaddr> db.auth
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setenv -e -nv -bs -rt -at -i <tmpaddr>,$filesize db
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Set up boot parameters on your board::
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efidebug boot add 1 HELLO mmc 0:1 /helloworld.efi.signed ""
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Now your board can run the signed image via the boot manager (see below).
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You can also try this sequence by running Pytest, test_efi_secboot,
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on the sandbox
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.. code-block:: bash
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cd <U-Boot source directory>
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pytest.py test/py/tests/test_efi_secboot/test_signed.py --bd sandbox
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UEFI binaries may be signed by Microsoft using the following certificates:
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* KEK: Microsoft Corporation KEK CA 2011
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http://go.microsoft.com/fwlink/?LinkId=321185.
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* db: Microsoft Windows Production PCA 2011
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http://go.microsoft.com/fwlink/p/?linkid=321192.
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* db: Microsoft Corporation UEFI CA 2011
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http://go.microsoft.com/fwlink/p/?linkid=321194.
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Using OP-TEE for EFI variables
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Instead of implementing UEFI variable services inside U-Boot they can
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also be provided in the secure world by a module for OP-TEE[1]. The
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interface between U-Boot and OP-TEE for variable services is enabled by
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CONFIG_EFI_MM_COMM_TEE=y.
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Tianocore EDK II's standalone management mode driver for variables can
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be linked to OP-TEE for this purpose. This module uses the Replay
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Protected Memory Block (RPMB) of an eMMC device for persisting
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non-volatile variables. When calling the variable services via the
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OP-TEE API U-Boot's OP-TEE supplicant relays calls to the RPMB driver
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which has to be enabled via CONFIG_SUPPORT_EMMC_RPMB=y.
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[1] https://optee.readthedocs.io/ - OP-TEE documentation
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Executing the boot manager
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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The UEFI specification foresees to define boot entries and boot sequence via UEFI
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variables. Booting according to these variables is possible via::
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bootefi bootmgr [fdt address]
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As of U-Boot v2018.03 UEFI variables are not persisted and cannot be set at
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runtime.
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Executing the built in hello world application
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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A hello world UEFI application can be built with::
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CONFIG_CMD_BOOTEFI_HELLO_COMPILE=y
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It can be embedded into the U-Boot binary with::
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CONFIG_CMD_BOOTEFI_HELLO=y
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The bootefi command is used to start the embedded hello world application::
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bootefi hello [fdt address]
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Below you find the output of an example session::
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=> bootefi hello ${fdtcontroladdr}
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## Starting EFI application at 01000000 ...
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WARNING: using memory device/image path, this may confuse some payloads!
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Hello, world!
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Running on UEFI 2.7
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Have SMBIOS table
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Have device tree
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Load options: root=/dev/sdb3 init=/sbin/init rootwait ro
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## Application terminated, r = 0
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The environment variable fdtcontroladdr points to U-Boot's internal device tree
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(if available).
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Executing the built-in self-test
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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An UEFI self-test suite can be embedded in U-Boot by building with::
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CONFIG_CMD_BOOTEFI_SELFTEST=y
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For testing the UEFI implementation the bootefi command can be used to start the
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self-test::
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bootefi selftest [fdt address]
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The environment variable 'efi_selftest' can be used to select a single test. If
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it is not provided all tests are executed except those marked as 'on request'.
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If the environment variable is set to 'list' a list of all tests is shown.
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Below you can find the output of an example session::
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=> setenv efi_selftest simple network protocol
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=> bootefi selftest
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Testing EFI API implementation
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Selected test: 'simple network protocol'
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Setting up 'simple network protocol'
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Setting up 'simple network protocol' succeeded
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Executing 'simple network protocol'
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DHCP Discover
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DHCP reply received from 192.168.76.2 (52:55:c0:a8:4c:02)
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as broadcast message.
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Executing 'simple network protocol' succeeded
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Tearing down 'simple network protocol'
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Tearing down 'simple network protocol' succeeded
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Boot services terminated
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Summary: 0 failures
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Preparing for reset. Press any key.
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The UEFI life cycle
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-------------------
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After the U-Boot platform has been initialized the UEFI API provides two kinds
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of services:
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* boot services
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* runtime services
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The API can be extended by loading UEFI drivers which come in two variants:
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* boot drivers
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* runtime drivers
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UEFI drivers are installed with U-Boot's bootefi command. With the same command
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UEFI applications can be executed.
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Loaded images of UEFI drivers stay in memory after returning to U-Boot while
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loaded images of applications are removed from memory.
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An UEFI application (e.g. an operating system) that wants to take full control
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of the system calls ExitBootServices. After a UEFI application calls
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ExitBootServices
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* boot services are not available anymore
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* timer events are stopped
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* the memory used by U-Boot except for runtime services is released
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* the memory used by boot time drivers is released
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So this is a point of no return. Afterwards the UEFI application can only return
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to U-Boot by rebooting.
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The UEFI object model
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---------------------
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UEFI offers a flexible and expandable object model. The objects in the UEFI API
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are devices, drivers, and loaded images. These objects are referenced by
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handles.
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The interfaces implemented by the objects are referred to as protocols. These
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are identified by GUIDs. They can be installed and uninstalled by calling the
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appropriate boot services.
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Handles are created by the InstallProtocolInterface or the
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InstallMultipleProtocolinterfaces service if NULL is passed as handle.
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Handles are deleted when the last protocol has been removed with the
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UninstallProtocolInterface or the UninstallMultipleProtocolInterfaces service.
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Devices offer the EFI_DEVICE_PATH_PROTOCOL. A device path is the concatenation
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of device nodes. By their device paths all devices of a system are arranged in a
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tree.
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Drivers offer the EFI_DRIVER_BINDING_PROTOCOL. This protocol is used to connect
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a driver to devices (which are referenced as controllers in this context).
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Loaded images offer the EFI_LOADED_IMAGE_PROTOCOL. This protocol provides meta
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information about the image and a pointer to the unload callback function.
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The UEFI events
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---------------
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In the UEFI terminology an event is a data object referencing a notification
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function which is queued for calling when the event is signaled. The following
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types of events exist:
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* periodic and single shot timer events
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* exit boot services events, triggered by calling the ExitBootServices() service
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* virtual address change events
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* memory map change events
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* read to boot events
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* reset system events
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* system table events
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* events that are only triggered programmatically
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Events can be created with the CreateEvent service and deleted with CloseEvent
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service.
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Events can be assigned to an event group. If any of the events in a group is
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signaled, all other events in the group are also set to the signaled state.
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The UEFI driver model
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---------------------
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A driver is specific for a single protocol installed on a device. To install a
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driver on a device the ConnectController service is called. In this context
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controller refers to the device for which the driver is installed.
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The relevant drivers are identified using the EFI_DRIVER_BINDING_PROTOCOL. This
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protocol has has three functions:
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* supported - determines if the driver is compatible with the device
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* start - installs the driver by opening the relevant protocol with
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attribute EFI_OPEN_PROTOCOL_BY_DRIVER
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* stop - uninstalls the driver
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The driver may create child controllers (child devices). E.g. a driver for block
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IO devices will create the device handles for the partitions. The child
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controllers will open the supported protocol with the attribute
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EFI_OPEN_PROTOCOL_BY_CHILD_CONTROLLER.
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A driver can be detached from a device using the DisconnectController service.
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U-Boot devices mapped as UEFI devices
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-------------------------------------
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Some of the U-Boot devices are mapped as UEFI devices
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* block IO devices
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* console
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* graphical output
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* network adapter
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As of U-Boot 2018.03 the logic for doing this is hard coded.
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The development target is to integrate the setup of these UEFI devices with the
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U-Boot driver model [5]. So when a U-Boot device is discovered a handle should
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be created and the device path protocol and the relevant IO protocol should be
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installed. The UEFI driver then would be attached by calling ConnectController.
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When a U-Boot device is removed DisconnectController should be called.
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UEFI devices mapped as U-Boot devices
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-------------------------------------
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UEFI drivers binaries and applications may create new (virtual) devices, install
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a protocol and call the ConnectController service. Now the matching UEFI driver
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is determined by iterating over the implementations of the
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EFI_DRIVER_BINDING_PROTOCOL.
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It is the task of the UEFI driver to create a corresponding U-Boot device and to
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proxy calls for this U-Boot device to the controller.
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In U-Boot 2018.03 this has only been implemented for block IO devices.
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UEFI uclass
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~~~~~~~~~~~
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An UEFI uclass driver (lib/efi_driver/efi_uclass.c) has been created that
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takes care of initializing the UEFI drivers and providing the
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EFI_DRIVER_BINDING_PROTOCOL implementation for the UEFI drivers.
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A linker created list is used to keep track of the UEFI drivers. To create an
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entry in the list the UEFI driver uses the U_BOOT_DRIVER macro specifying
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UCLASS_EFI as the ID of its uclass, e.g::
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/* Identify as UEFI driver */
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U_BOOT_DRIVER(efi_block) = {
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.name = "EFI block driver",
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.id = UCLASS_EFI,
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.ops = &driver_ops,
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};
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The available operations are defined via the structure struct efi_driver_ops::
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struct efi_driver_ops {
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const efi_guid_t *protocol;
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const efi_guid_t *child_protocol;
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int (*bind)(efi_handle_t handle, void *interface);
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};
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When the supported() function of the EFI_DRIVER_BINDING_PROTOCOL is called the
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uclass checks if the protocol GUID matches the protocol GUID of the UEFI driver.
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In the start() function the bind() function of the UEFI driver is called after
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checking the GUID.
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The stop() function of the EFI_DRIVER_BINDING_PROTOCOL disconnects the child
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controllers created by the UEFI driver and the UEFI driver. (In U-Boot v2013.03
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this is not yet completely implemented.)
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UEFI block IO driver
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~~~~~~~~~~~~~~~~~~~~
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The UEFI block IO driver supports devices exposing the EFI_BLOCK_IO_PROTOCOL.
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When connected it creates a new U-Boot block IO device with interface type
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IF_TYPE_EFI, adds child controllers mapping the partitions, and installs the
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EFI_SIMPLE_FILE_SYSTEM_PROTOCOL on these. This can be used together with the
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software iPXE to boot from iSCSI network drives [4].
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This driver is only available if U-Boot is configured with::
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CONFIG_BLK=y
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CONFIG_PARTITIONS=y
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Miscellaneous
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-------------
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Load file 2 protocol
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~~~~~~~~~~~~~~~~~~~~
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The load file 2 protocol can be used by the Linux kernel to load the initial
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RAM disk. U-Boot can be configured to provide an implementation with::
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EFI_LOAD_FILE2_INITRD=y
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EFI_INITRD_FILESPEC=interface dev:part path_to_initrd
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Links
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-----
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* [1] http://uefi.org/specifications - UEFI specifications
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* [2] https://github.com/ARM-software/ebbr/releases/download/v1.0/ebbr-v1.0.pdf -
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Embedded Base Boot Requirements (EBBR) Specification - Release v1.0
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* [3] https://developer.arm.com/docs/den0044/latest/server-base-boot-requirements-system-software-on-arm-platforms-version-11 -
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Server Base Boot Requirements System Software on ARM Platforms - Version 1.1
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* [4] :doc:`iscsi`
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* [5] :doc:`../driver-model/index`
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