Update docs with PMU security information

This patch adds information on the PMU configuration registers
and security considerations related to the PMU.

Signed-off-by: Petre-Ionut Tudor <petre-ionut.tudor@arm.com>
Change-Id: I36b15060b9830a77d3f47f293c0a6dafa3c581fb

1 files changed, 99 insertions, 0 deletions

diff --git a/docs/process/security-hardening.rst b/docs/process/security-hardening.rst
index 43a572125..507046f2e 100644
--- a/docs/process/security-hardening.rst
+++ b/docs/process/security-hardening.rst
@@ -25,6 +25,99 @@ The secure world **should never** crash or become unusable due to receiving too
 many normal world requests (a *Denial of Service* or *DoS* attack). It should
 have a mechanism for throttling or ignoring normal world requests.
 
+Preventing Secure-world timing information leakage via PMU counters
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The Secure world needs to implement some defenses to prevent the Non-secure
+world from making it leak timing information. In general, higher privilege
+levels must defend from those below when the PMU is treated as an attack
+vector.
+
+Refer to the :ref:`Performance Monitoring Unit` guide for detailed information
+on the PMU registers.
+
+Timing leakage attacks from the Non-secure world
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Since the Non-secure world has access to the ``PMCR`` register, it can
+configure the PMU to increment counters at any exception level and in both
+Secure and Non-secure state. Thus, it attempts to leak timing information from
+the Secure world.
+
+Shown below is an example of such a configuration:
+
+-  ``PMEVTYPER0_EL0`` and ``PMCCFILTR_EL0``:
+
+   -  Set ``P`` to ``0``.
+   -  Set ``NSK`` to ``1``.
+   -  Set ``M`` to ``0``.
+   -  Set ``NSH`` to ``0``.
+   -  Set ``SH`` to ``1``.
+
+-  ``PMCNTENSET_EL0``:
+
+   -  Set ``P[0]`` to ``1``.
+   -  Set ``C`` to ``1``.
+
+-  ``PMCR_EL0``:
+
+   -  Set ``DP`` to ``0``.
+   -  Set ``E`` to ``1``.
+
+This configuration instructs ``PMEVCNTR0_EL0`` and ``PMCCNTR_EL0`` to increment
+at Secure EL1, Secure EL2 (if implemented) and EL3.
+
+Since the Non-secure world has fine-grained control over where (at which
+exception levels) it instructs counters to increment, obtaining event counts
+would allow it to carry out side-channel timing attacks against the Secure
+world. Examples include Spectre, Meltdown, as well as extracting secrets from
+cryptographic algorithms with data-dependent variations in their execution
+time.
+
+Secure world mitigation strategies
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The ``MDCR_EL3`` register allows EL3 to configure the PMU (among other things).
+The `Arm ARM`_ details all of the bit fields in this register, but for the PMU
+there are two bits which determine the permissions of the counters:
+
+-  ``SPME`` for the programmable counters.
+-  ``SCCD`` for the cycle counter.
+
+Depending on the implemented features, the Secure world can prohibit counting
+in AArch64 state via the following:
+
+-  ARMv8.2-Debug not implemented:
+
+   -  Prohibit general event counters and the cycle counter:
+      ``MDCR_EL3.SPME == 0 && PMCR_EL0.DP == 1 && !ExternalSecureNoninvasiveDebugEnabled()``.
+
+      -  ``MDCR_EL3.SPME`` resets to ``0``, so by default general events should
+         not be counted in the Secure world.
+      -  The ``PMCR_EL0.DP`` bit therefore needs to be set to ``1`` when EL3 is
+         entered and ``PMCR_EL0`` needs to be saved and restored in EL3.
+      -  ``ExternalSecureNoninvasiveDebugEnabled()`` is an authentication
+         interface which is implementation-defined unless ARMv8.4-Debug is
+         implemented. The `Arm ARM`_ has detailed information on this topic.
+
+   -  The only other way is to disable the ``PMCR_EL0.E`` bit upon entering
+      EL3, which disables counting altogether.
+
+-  ARMv8.2-Debug implemented:
+
+   -  Prohibit general event counters: ``MDCR_EL3.SPME == 0``.
+   -  Prohibit cycle counter: ``MDCR_EL3.SPME == 0 && PMCR_EL0.DP == 1``.
+      ``PMCR_EL0`` therefore needs to be saved and restored in EL3.
+
+-  ARMv8.5-PMU implemented:
+
+   -  Prohibit general event counters: as in ARMv8.2-Debug.
+   -  Prohibit cycle counter: ``MDCR_EL3.SCCD == 1``
+
+In Aarch32 execution state the ``MDCR_EL3`` alias is the ``SDCR`` register,
+which has some of the bit fields of ``MDCR_EL3``, most importantly the ``SPME``
+and ``SCCD`` bits.
+
 Build options
 -------------
 
@@ -71,6 +164,12 @@ Several build options can be used to check for security issues. Refer to the
   NB: The ``Werror`` flag is enabled by default in TF-A and can be disabled by
   setting the ``E`` build flag to 0.
 
+.. rubric:: References
+
+-  `Arm ARM`_
+
 --------------
 
 *Copyright (c) 2019-2020, Arm Limited. All rights reserved.*
+
+.. _Arm ARM: https://developer.arm.com/docs/ddi0487/latest