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change(bootloader): honor SECURE_BOOT_ALLOW_UNUSED_DIGEST_SLOTS during first boot

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Previously the bootloader unconditionally revoked unused secure boot key
digest slots while permanently enabling secure boot on the first boot,
ignoring CONFIG_SECURE_BOOT_ALLOW_UNUSED_DIGEST_SLOTS. Now the config is
honored on this path too: when set, the unused digest slots are left
un-revoked. This is safe as long as the debug and download interfaces are
disabled.

Update the Kconfig help and the Secure Boot v2 guide (en and zh_CN)
accordingly.
This commit is contained in:
Mahavir Jain
2026-06-16 13:29:49 +05:30
parent 4e09adff8d
commit b08669af99
4 changed files with 8 additions and 13 deletions
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docs/en/security/secure-boot-v2.rst
+1 -3
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@@ -741,9 +741,7 @@ Secure Boot Best Practices
.. note::
Note that enabling the config :ref:`CONFIG_SECURE_BOOT_ALLOW_UNUSED_DIGEST_SLOTS` only makes sure that the **app** does not revoke the unused digest slots.
But if you plan to enable secure boot during the fist boot up, the bootloader will intentionally revoke the unused digest slots while enabling secure boot, even if the above config is enabled. Because keeping the unused key slots unrevoked would be a security hazard.
In case for any development workflow if you need to avoid this revocation, you should :ref:`enable-secure-boot-v2-externally`, rather than enabling it during the boot up, so that the bootloader would not need to enable secure boot, and thus you could avoid its revocation strategy.
Enabling the config :ref:`CONFIG_SECURE_BOOT_ALLOW_UNUSED_DIGEST_SLOTS` keeps the unused digest slots un-revoked in both cases: at runtime in the **app**, and in the **bootloader** when secure boot is enabled during the first boot up. Note that leaving unused key slots un-revoked could pose a security risk, unless the debug and download interfaces are completely disabled and remote interfaces are fully audited for security risks.
Conservative Approach
~~~~~~~~~~~~~~~~~~~~~~