feat(optee): add loading OP-TEE image via an SMC
This adds the ability to load the OP-TEE image via an SMC called from
non-secure userspace rather than loading it during boot. This should
only be utilized on platforms that can ensure security is maintained up
until the point the SMC is invoked as it breaks the normal barrier
between the secure and non-secure world.
Signed-off-by: Jeffrey Kardatzke <jkardatzke@google.com>
Change-Id: I21cfa9699617c493fa4190f01d1cbb714e7449cc
diff --git a/docs/components/spd/optee-dispatcher.rst b/docs/components/spd/optee-dispatcher.rst
index 63baccc..81476f1 100644
--- a/docs/components/spd/optee-dispatcher.rst
+++ b/docs/components/spd/optee-dispatcher.rst
@@ -6,9 +6,26 @@
To build and execute OP-TEE follow the instructions at
`OP-TEE build.git`_
+There are two different modes for loading the OP-TEE OS. The default mode will
+load it as the BL32 payload during boot, and is the recommended technique for
+platforms to use. There is also another technique that will load OP-TEE OS after
+boot via an SMC call by enabling the option for OPTEE_ALLOW_SMC_LOAD that was
+specifically added for ChromeOS. Loading OP-TEE via an SMC call may be insecure
+depending upon the platform configuration. If using that option, be sure to
+understand the risks involved with allowing the Trusted OS to be loaded this
+way. ChromeOS uses a boot flow where it verifies the signature of the firmware
+before executing it, and then only if the signature is valid will the 'secrets'
+used by the TEE become accessible. The firmware then verifies the signature of
+the kernel using depthcharge, and the kernel verifies the rootfs using
+dm-verity. The SMC call to load OP-TEE is then invoked immediately after the
+kernel finishes loading and before any attack vectors can be opened up by
+mounting writable filesystems or opening network/device connections. this
+ensures the platform is 'closed' and running signed code through the point where
+OP-TEE is loaded.
+
--------------
-*Copyright (c) 2014-2018, Arm Limited and Contributors. All rights reserved.*
+*Copyright (c) 2014-2023, Arm Limited and Contributors. All rights reserved.*
.. _OP-TEE OS: https://github.com/OP-TEE/build
.. _OP-TEE build.git: https://github.com/OP-TEE/build
diff --git a/docs/threat_model/threat_model.rst b/docs/threat_model/threat_model.rst
index 99bbb3a..0e967ba 100644
--- a/docs/threat_model/threat_model.rst
+++ b/docs/threat_model/threat_model.rst
@@ -918,9 +918,54 @@
| Mitigations | | Yes / Platform specific |
+------------------------+-----------------------------------------------------+
++------------------------+-----------------------------------------------------+
+| ID | 14 |
++========================+=====================================================+
+| Threat | | **Security vulnerabilities in the Non-secure OS |
+| | can lead to secure world compromise if the option |
+| | OPTEE_ALLOW_SMC_LOAD is enabled.** |
+| | |
+| | | This option trusts the non-secure world up until |
+| | the point it issues the SMC call to load the |
+| | Secure BL32 payload. If a compromise occurs |
+| | before the SMC call is invoked, then arbitrary |
+| | code execution in S-EL1 can occur or arbitrary |
+| | memory in EL3 can be overwritten. |
++------------------------+-----------------------------------------------------+
+| Diagram Elements | DF5 |
++------------------------+-----------------------------------------------------+
+| Affected TF-A | BL31, BL32 |
+| Components | |
++------------------------+-----------------------------------------------------+
+| Assets | Code Execution, Sensitive Data |
++------------------------+-----------------------------------------------------+
+| Threat Agent | NSCode |
++------------------------+-----------------------------------------------------+
+| Threat Type | Tampering, Information Disclosure, |
+| | Elevation of privilege |
++------------------------+-----------------+-----------------+-----------------+
+| Application | Server | IoT | Mobile |
++------------------------+-----------------+-----------------+-----------------+
+| Impact | Critical (5) | Critical (5) | Critical (5) |
++------------------------+-----------------+-----------------+-----------------+
+| Likelihood | Low (2) | Low (2) | Low (2) |
++------------------------+-----------------+-----------------+-----------------+
+| Total Risk Rating | Medium (10) | Medium (10) | Medium (10) |
++------------------------+-----------------+-----------------+-----------------+
+| Mitigations | When enabling the option OPTEE_ALLOW_SMC_LOAD, |
+| | the non-secure OS must be considered a closed |
+| | platform up until the point the SMC can be invoked |
+| | to load OP-TEE. |
++------------------------+-----------------------------------------------------+
+| Mitigations | | None in TF-A itself. This option is only used by |
+| implemented? | ChromeOS currently which has other mechanisms to |
+| | to mitigate this threat which are described in |
+| | `OP-TEE Dispatcher`_. |
++------------------------+-----------------------------------------------------+
+
--------------
-*Copyright (c) 2021-2022, Arm Limited. All rights reserved.*
+*Copyright (c) 2021-2023, Arm Limited. All rights reserved.*
.. _STRIDE threat analysis technique: https://docs.microsoft.com/en-us/azure/security/develop/threat-modeling-tool-threats#stride-model
@@ -932,3 +977,4 @@
.. _TF-A error handling policy: https://trustedfirmware-a.readthedocs.io/en/latest/process/coding-guidelines.html#error-handling-and-robustness
.. _Secure Development Guidelines: https://trustedfirmware-a.readthedocs.io/en/latest/process/security-hardening.html#secure-development-guidelines
.. _Trusted Firmware-A Tests: https://git.trustedfirmware.org/TF-A/tf-a-tests.git/about/
+.. _OP-TEE Dispatcher: https://github.com/ARM-software/arm-trusted-firmware/blob/master/docs/components/spd/optee-dispatcher.rst