[PATCH 03/11] Documentation: security: require signature verification to be pinned

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

From: Ahmad Fatoum <a.fatoum@barebox.org>

global.bootm.verify defaults to "available", which accepts an image that
carries nothing, and "hash" never looks at the configuration signature.

Document that we expect signature to be set and enforced in verified
boot setups.

Signed-off-by: Ahmad Fatoum <a.fatoum@barebox.org>
---
 Documentation/user/security.rst | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/Documentation/user/security.rst b/Documentation/user/security.rst
index e650c03a1023..7160f8f8e3c2 100644
--- a/Documentation/user/security.rst
+++ b/Documentation/user/security.rst
@@ -91,6 +91,19 @@ be allowed to boot any images that have not been signed by the correct key.
 This can be enforced by setting ``CONFIG_BOOTM_FORCE_SIGNED_IMAGES=y``
 and disabling any ways that could be used to override this.
 
+How thoroughly an image is checked is controlled by
+:ref:`global.bootm.verify <magicvar_global_bootm_verify>`. Only ``signature``
+is suitable for verified boot. ``hash`` checks the image hashes, but not the
+configuration signature, so it detects corruption, not tampering. The default
+``available`` verifies whatever the image carries and accepts an image
+carrying nothing.
+
+As a global variable, the setting may be changeable at runtime unless barebox
+is built with ``CONFIG_BOOTM_FORCE_SIGNED_IMAGES=y`` or the
+:ref:`security policy <use_security-policies>` denies
+``SCONFIG_BOOT_UNSIGNED_IMAGES``. Either pins it to ``signature``. This
+will also result in :ref:`command_bootm` refusing to boot any non-FIT images.
+
 For development convenience ``CONFIG_CRYPTO_BUILTIN_DEVELOPMENT_KEYS``
 can be enabled after enabling ``CONFIG_INSECURE`` to compile well known
 development keys into the barebox binary.
-- 
2.47.3