docs(spm): threat model for memory sharing functionality
Update the SPM threat model with information about FF-A v1.1
memory sharing functionality.
Change-Id: I65ea0d53aba8ac2f8432539968ceaab6be109ac8
Signed-off-by: J-Alves <joao.alves@arm.com>
diff --git a/docs/threat_model/threat_model_spm.rst b/docs/threat_model/threat_model_spm.rst
index 98dbf76..9458a9f 100644
--- a/docs/threat_model/threat_model_spm.rst
+++ b/docs/threat_model/threat_model_spm.rst
@@ -35,7 +35,7 @@
- The TF-A implementation for the S-EL2 SPMC based on the Hafnium hypervisor
running in the secure world of TrustZone (at S-EL2 exception level).
The threat model is not related to the normal world Hypervisor or VMs.
- The S-EL1 SPMC solution is not covered.
+ The S-EL1 and EL3 SPMC solutions are not covered.
- The implementation complies with the FF-A v1.0 specification, and a few
features of FF-A v1.1 specification.
- Secure partitions are statically provisioned at boot time.
@@ -235,8 +235,8 @@
+------------------------+------------------+-----------------+---------------+
| ``Total Risk Rating`` | High (16) | High (16) | |
+------------------------+------------------+-----------------+---------------+
-| ``Mitigations`` | In context of FF-A v1.0 this is the case of sharing|
-| | the RX/TX buffer pair and usage in the |
+| ``Mitigations`` | In context of FF-A v1.0 and v1.1 this is the case |
+| | of sharing the RX/TX buffer pair and usage in the |
| | PARTITION_INFO_GET or mem sharing primitives. |
| | The SPMC must copy the contents of the TX buffer |
| | to an internal temporary buffer before processing |
@@ -1151,11 +1151,189 @@
| | interrupted. |
+------------------------+----------------------------------------------------+
++------------------------+----------------------------------------------------+
+| ID | 25 |
++========================+====================================================+
+| ``Threat`` | **A rogue FF-A endpoint can use memory sharing |
+| | calls to exhaust SPMC resources.** |
+| | For each on-going operation that involves an SP, |
+| | the SPMC allocates resources to track its state. |
+| | If the operation is never concluded, the resources |
+| | are never freed. |
+| | In the worst scenario, multiple operations that |
+| | never conclude may exhaust the SPMC resources to a |
+| | point in which renders memory sharing operations |
+| | impossible. This could affect other, non-harmful |
+| | FF-A endpoints, from legitimately using memory |
+| | share functionality. The intent might even be |
+| | to cause the SPMC to consume excessive CPU cycles, |
+| | attempting to make it deny its service to the NWd. |
++------------------------+----------------------------------------------------+
+| ``Diagram Elements`` | DF1, DF2 |
++------------------------+----------------------------------------------------+
+| ``Affected TF-A | SPMC, SPMD |
+| Components`` | |
++------------------------+----------------------------------------------------+
+| ``Assets`` | SPMC state |
++------------------------+----------------------------------------------------+
+| ``Threat Agent`` | NS-Endpoint, S-Endpoint |
++------------------------+----------------------------------------------------+
+| ``Threat Type`` | Denial of Service |
++------------------------+------------------+-----------------+---------------+
+| ``Application`` | ``Server`` | ``Mobile`` | |
++------------------------+------------------+-----------------+---------------+
+| ``Impact`` | High (4) | Medium (3) | |
++------------------------+------------------+-----------------+---------------+
+| ``Likelihood`` | High (4) | Medium (3) | |
++------------------------+------------------+-----------------+---------------+
+| ``Total Risk Rating`` | High (16) | Medium (9) | |
++------------------------+------------------+-----------------+---------------+
+| ``Mitigations`` | The TF-A SPMC uses a statically allocated pool of |
+| | memory to keep track of on-going memory sharing |
+| | operations. After a possible attack, this could |
+| | fail due to insufficient memory, and return an |
+| | error to the caller. At this point, any other |
+| | endpoint that requires use of memory sharing for |
+| | its operation could get itself in an unusable |
+| | state. |
+| | Regarding CPU cycles starving threat, the SPMC |
+| | doesn't provide any mitigation for this, as any |
+| | FF-A endpoint, at the virtual FF-A instance is |
+| | allowed to invoke memory share/lend/donate. |
++------------------------+----------------------------------------------------+
+
++------------------------+----------------------------------------------------+
+| ID | 26 |
++========================+====================================================+
+| ``Threat`` | **A borrower may interfere with lender's |
+| | operation, if it terminates due to a fatal error |
+| | condition without releasing the memory |
+| | shared/lent.** |
+| | Such scenario may render the lender inoperable. |
++------------------------+----------------------------------------------------+
+| ``Diagram Elements`` | DF1, DF2 |
++------------------------+----------------------------------------------------+
+| ``Affected TF-A | SPMC |
+| Components`` | |
++------------------------+----------------------------------------------------+
+| ``Assets`` | SP state |
++------------------------+----------------------------------------------------+
+| ``Threat Agent`` | NS-Endpoint, S-Endpoint |
++------------------------+----------------------------------------------------+
+| ``Threat Type`` | Denial of Service |
++------------------------+------------------+-----------------+---------------+
+| ``Application`` | ``Server`` | ``Mobile`` | |
++------------------------+------------------+-----------------+---------------+
+| ``Impact`` | High (4) | Low (2) | |
++------------------------+------------------+-----------------+---------------+
+| ``Likelihood`` | Medium (3) | Medium (3) | |
++------------------------+------------------+-----------------+---------------+
+| ``Total Risk Rating`` | High (12) | Medium(6) | |
++------------------------+------------------+-----------------+---------------+
+| ``Mitigations`` | The TF-A SPMC does not provide mitigation for such |
+| | scenario. The FF-A endpoints must attempt to |
+| | relinquish memory shared/lent themselves in |
+| | case of failure. The memory used to track the |
+| | operation in the SPMC will also remain usuable. |
++------------------------+----------------------------------------------------+
+
++------------------------+----------------------------------------------------+
+| ID | 27 |
++========================+====================================================+
+| ``Threat`` | **A rogue FF-A endpoint may attempt to tamper with |
+| | the content of the memory shared/lent, whilst |
+| | being accessed by other FF-A endpoints.** |
+| | It might attempt to do so: using one of the clear |
+| | flags, when either retrieving or relinquishing |
+| | access to the memory via the respective FF-A |
+| | calls; or directly accessing memory without |
+| | respecting the synchronization protocol between |
+| | all involved endpoints. |
++------------------------+----------------------------------------------------+
+| ``Diagram Elements`` | DF1, DF2 |
++------------------------+----------------------------------------------------+
+| ``Affected TF-A | SPMC, FF-A endpoint |
+| Components`` | |
++------------------------+----------------------------------------------------+
+| ``Assets`` | SP state |
++------------------------+----------------------------------------------------+
+| ``Threat Agent`` | NS-Endpoint, S-Endpoint |
++------------------------+----------------------------------------------------+
+| ``Threat Type`` | Denial of Service, Tampering |
++------------------------+------------------+-----------------+---------------+
+| ``Application`` | ``Server`` | ``Mobile`` | |
++------------------------+------------------+-----------------+---------------+
+| ``Impact`` | Low (2) | Low (2) | |
++------------------------+------------------+-----------------+---------------+
+| ``Likelihood`` | Medium (3) | Medium (3) | |
++------------------------+------------------+-----------------+---------------+
+| ``Total Risk Rating`` | Medium (6) | Medium(6) | |
++------------------------+------------------+-----------------+---------------+
+| ``Mitigations`` | The first case defined in the threat, the TF-A |
+| | SPMC mitigates it, by ensuring a memory is cleared |
+| | only when all borrowers have relinquished access |
+| | to the memory, in a scenario involving multiple |
+| | borrowers. Also, if the receiver is granted RO, |
+| | permissions, the SPMC will reject any request |
+| | to clear memory on behalf of the borrower, by |
+| | returning an error to the respective FF-A call. |
+| | The second case defined in the threat can't be |
+| | mitigated by the SPMC. It is up to the NS/S FF-A |
+| | endpoints to establish a robust protocol for using |
+| | the shared memory. |
++------------------------+----------------------------------------------------+
+
++------------------------+----------------------------------------------------+
+| ID | 28 |
++========================+====================================================+
+| ``Threat`` | **A rogue FF-A endpoint may attempt to share |
+| | memory that is not in its translation regime, or |
+| | attempt to specify attributes more permissive than |
+| | those it possesses at a given time.** |
+| | Both ways could be an attempt for escalating its |
+| | privileges. |
++------------------------+----------------------------------------------------+
+| ``Diagram Elements`` | DF1, DF2 |
++------------------------+----------------------------------------------------+
+| ``Affected TF-A | SPMC, FF-A endpoint |
+| Components`` | |
++------------------------+----------------------------------------------------+
+| ``Assets`` | SP state |
++------------------------+----------------------------------------------------+
+| ``Threat Agent`` | NS-Endpoint, S-Endpoint |
++------------------------+----------------------------------------------------+
+| ``Threat Type`` | Denial of Service, Tampering |
++------------------------+------------------+-----------------+---------------+
+| ``Application`` | ``Server`` | ``Mobile`` | |
++------------------------+------------------+-----------------+---------------+
+| ``Impact`` | High (4) | Low (2) | |
++------------------------+------------------+-----------------+---------------+
+| ``Likelihood`` | Medium (3) | Low (2) | |
++------------------------+------------------+-----------------+---------------+
+| ``Total Risk Rating`` | High (12) | Low (2) | |
++------------------------+------------------+-----------------+---------------+
+| ``Mitigations`` | The TF-A SPMC mitigates this threat by performing |
+| | sanity checks to the provided memory region |
+| | descriptor. |
+| | For operations at the virtual FF-A instance, and |
+| | once the full memory descriptor is provided, |
+| | the SPMC validates that the memory is part of the |
+| | caller's translation regime. The SPMC also checks |
+| | that the memory attributes provided are within |
+| | those the owner possesses, in terms of |
+| | permissiveness. If more permissive attributes are |
+| | specified, the SPMC returns an error |
+| | FFA_INVALID_PARAMETERS. The permissiveness rules |
+| | are enforced in any call to share/lend or donate |
+| | the memory, and in retrieve requests. |
++------------------------+----------------------------------------------------+
+
--------------
-*Copyright (c) 2021-2022, Arm Limited. All rights reserved.*
+*Copyright (c) 2021-2023, Arm Limited. All rights reserved.*
.. _Arm Firmware Framework for Arm A-profile: https://developer.arm.com/docs/den0077/latest
.. _Secure Partition Manager: ../components/secure-partition-manager.html
.. _Generic TF-A threat model: ./threat_model.html#threat-analysis
.. _FF-A ACS: https://github.com/ARM-software/ff-a-acs/releases
+