[PATCH 06/11] Documentation: security: describe shell and environment as trust boundary

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

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

Both sections advise disabling the feature without saying why. The shell
validates nothing by design, so whoever reaches it is as trusted as the
boot chain. The environment sets the global variables that select the
boot source, reach the kernel command line and, unless pinned, decide
whether images are verified at all.

Spell that out and mention SCONFIG_ENVIRONMENT_LOAD next to
CONFIG_ENV_HANDLING, for builds that need environment support but must
not load one from media.

Signed-off-by: Ahmad Fatoum <a.fatoum@barebox.org>
---
 Documentation/user/security.rst | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/Documentation/user/security.rst b/Documentation/user/security.rst
index 9a241b0e2e8d..a618c05b1102 100644
--- a/Documentation/user/security.rst
+++ b/Documentation/user/security.rst
@@ -140,6 +140,12 @@ In addition, there are alternative methods of accessing the shell like
 netconsole, or fastboot. These should preferably be disabled or at least
 not activated by default.
 
+barebox places no restrictions on what the shell does: a command that writes
+a partition, sets a variable or applies a devicetree overlay does exactly
+that. Whoever reaches the shell is as trusted as the boot chain, so any
+remaining way of reaching it is part of the boot chain. A console kept for
+diagnostics should be output-only.
+
 Disabling the non-builtin environment
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -156,6 +162,18 @@ This can be enforced by disabling ``CONFIG_ENV_HANDLING``.
 This does not preclude the use of :ref:`Bootchooser` as the
 :ref:`barebox-state framework <state_framework>` can be used independently.
 
+If environment handling is needed for other purposes, denying
+``SCONFIG_ENVIRONMENT_LOAD`` in the
+:ref:`security policy <use_security-policies>` keeps barebox from loading an
+environment from media.
+
+What matters is not the environment itself, but the global variables it sets.
+They select the boot source, end up on the kernel command line and, unless
+signature checking is pinned, decide whether images are verified at all. Any
+way to arbitrarily set global variables, be it a writable environment,
+a script on media or a shell, defeats verified boot regardless of how well
+the images are signed.
+
 Avoiding use of file systems
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-- 
2.47.3