Documentation: Add vboot on Lenovo devices

Describe vboot implementation details for retrofitted Lenovo ThinkPad devices.

Change-Id: Ibabcc939d9d01f00a93fd42adc48057966ad877e
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Signed-off-by: Marcello Sylvester Bauer <sylv@sylv.io>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39151
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

1 files changed, 39 insertions, 0 deletions

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+# Using coreboot's verified boot on Lenovo devices
+
+By default a single instance of coreboot is present in the firmware flash,
+no verification is done and the flash is not write-protected, so as to allow
+firmware updates from the OS.
+The verified boot mechanism also called [VBOOT] allows secure firmware
+updates using an A/B partitioning scheme once enabled.
+
+## Enabling VBOOT
+You can enable [VBOOT] in Kconfig's *Security* section. Besides a verified
+boot you can also enable a measured boot by setting
+`CONFIG_VBOOT_MEASURED_BOOT`. Both options need a working TPM, which is
+present on all recent Lenovo devices.
+
+## Updating and recovery
+As the A/B partition is writeable you can still update them from the OS.
+By using the [VBOOT] mechanism you store a copy of coreboot in the `RO`
+partition that acts as failsafe in case the regular firmware update, that
+goes to the `A` or `B` partition fails.
+
+**Note:** The `RO` partition isn't write-protected by default. There's a patch
+pending on gerrit [CB:32705] that write-protects the `RO` partition.
+
+On *Lenovo* devices you can enable the *Fn* key as recovery mode switch, by
+enabling `CONFIG_H8_FN_KEY_AS_VBOOT_RECOVERY_SW`.
+Holding the *Fn* at boot will then switch to the recovery image, allowing
+to boot and flash a working image to the A/B partition.
+
+## 8 MiB ROM limitation
+*Lenovo* devices with 8 MiB ROM only have a `RO`+`A` partition enabled in the
+default FMAP. They are missing the `B` partition, due to size constaints.
+You can still provide your own FMAP if you need `RO`+`A`+`B` partitions.
+
+## CMOS
+[VBOOT] on *Lenovo* devices uses the CMOS to store configuration data, like
+boot failures and the last successfully booted partition.
+
+[VBOOT]: ../../security/vboot/index.md
+[CB:32705]: https://review.coreboot.org/32705