# 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_TPM_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, therefore you have to enable the protection in the security Kconfig menu by yourself.

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 constraints. 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.