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docs(threat-model): cover threats inherent to receiving data over UART
TF-A supports reading input data from UART interfaces. This opens up an attack vector for arbitrary data to be injected into TF-A, which is not covered in the threat model right now. Fill this gap by: - Updating the data flow diagrams. Data may flow from the UART into TF-A (and not only the other way around). - Documenting the threats inherent to reading untrusted data from a UART. Change-Id: I508da5d2f7ad5d20717b958d76ab9337c5eca50f Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
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Sandrine Bailleux
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4 changed files with 62 additions and 7 deletions
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@ -25,7 +25,7 @@ digraph tfa_dfd {
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nsec [label="Non-secure\nClients"]
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sec [label="Secure\nClients"]
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dbg [label="Debug & Trace"]
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logs [label="Logs\n(UART)"]
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uart [label="UART"]
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nvm [label="Non-volatile\nMemory"]
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# Trust boundary cluster
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@ -56,7 +56,7 @@ digraph tfa_dfd {
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# Interactions between nodes
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nvm -> bl31 [lhead=cluster_tfa label="DF1"]
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logs -> bl31 [dir="back" lhead=cluster_tfa label="DF2"]
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uart -> bl31 [dir="both" lhead=cluster_tfa label="DF2"]
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dbg -> bl2 [dir="both" lhead=cluster_tfa label="DF3"]
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sec -> bl2 [dir="both" lhead=cluster_tfa label="DF4"]
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nsec -> bl1 [dir="both" lhead=cluster_tfa, label="DF5"]
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@ -25,7 +25,7 @@ digraph tfa_dfd {
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nsec [label="Non-secure\nClients"]
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sec [label="Secure\nClients"]
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dbg [label="Debug & Trace"]
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logs [label="Logs\n(UART)"]
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uart [label="UART"]
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nvm [label="Non-volatile\nMemory"]
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@ -65,7 +65,7 @@ digraph tfa_dfd {
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# Interactions between nodes
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nvm -> bl31 [lhead=cluster_tfa label="DF1"]
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logs -> bl31 [dir="back" lhead=cluster_tfa label="DF2"]
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uart -> bl31 [dir="both" lhead=cluster_tfa label="DF2"]
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dbg -> bl2 [dir="both" lhead=cluster_tfa label="DF3"]
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sec -> bl2 [dir="both" lhead=cluster_tfa label="DF4"]
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nsec -> bl1 [dir="both" lhead=cluster_tfa, label="DF5"]
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@ -63,8 +63,10 @@ are considered untrusted by TF-A.
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| | images include TF-A BL2 and BL31 images, as well as |
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| | other secure and non-secure images. |
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+-----------------+--------------------------------------------------------+
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| DF2 | | TF-A log system framework outputs debug messages |
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| | over a UART interface. |
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| DF2 | | TF-A log system framework outputs debug or |
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| | informative messages over a UART interface. |
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| | |
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| | | Also, characters can be read from a UART interface. |
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+-----------------+--------------------------------------------------------+
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| DF3 | | Debug and trace IP on a platform can allow access |
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| | to registers and memory of TF-A. |
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@ -552,6 +554,57 @@ General Threats for All Firmware Images
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| | soon as they are not needed anymore. |
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+------------------------+-----------------------------------------------------+
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| Mitigations | | Yes / Platform specific |
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| implemented? | |
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+------------------------+-----------------------------------------------------+
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+------------------------+-----------------------------------------------------+
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| ID | 15 |
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+========================+=====================================================+
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| Threat | | **Improper handling of input data received over |
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| | a UART interface may allow an attacker to tamper |
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| | with TF-A execution environment.** |
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| | |
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| | | The consequences of the attack depend on the |
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| | the exact usage of input data received over UART. |
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| | Examples are injection of arbitrary data, |
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| | sensitive data tampering, influencing the |
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| | execution path, denial of service (if using |
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| | blocking I/O). This list may not be exhaustive. |
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+------------------------+-----------------------------------------------------+
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| Diagram Elements | DF2, DF4, DF5 |
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+------------------------+-----------------------------------------------------+
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| Affected TF-A | BL1, BL2, BL31 |
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| Components | |
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+------------------------+-----------------------------------------------------+
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| Assets | Sensitive Data, Code Execution, Availability |
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+------------------------+-----------------------------------------------------+
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| Threat Agent | NSCode, SecCode |
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+------------------------+-----------------------------------------------------+
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| Threat Type | Tampering, Information Disclosure, Denial of |
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| | service, Elevation of privilege. |
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+------------------------+-------------------+----------------+----------------+
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| Application | Server | IoT | Mobile |
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+------------------------+-------------------+----------------+----------------+
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| Impact | Critical (5) | Critical (5) | Critical (5) |
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+------------------------+-------------------+----------------+----------------+
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| Likelihood | Critical (5) | Critical (5) | Critical (5) |
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+------------------------+-------------------+----------------+----------------+
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| Total Risk Rating | Critical (25) | Critical (25) | Critical (25) |
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+------------------------+-------------------+----------------+----------------+
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| Mitigations | | By default, the code to read input data from UART |
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| | interfaces is disabled (see `ENABLE_CONSOLE_GETC` |
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| | build option). It should only be enabled on a |
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| | need basis. |
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| | |
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| | | Data received over UART interfaces should be |
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| | treated as untrusted data. As such, it should be |
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| | properly sanitized and handled with caution. |
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+------------------------+-----------------------------------------------------+
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| Mitigations | | Platform specific. |
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| implemented? | |
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| | | Generic code does not read any input data from |
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| | UART interface(s). |
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+------------------------+-----------------------------------------------------+
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@ -90,8 +90,10 @@ implementation:
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and since the MPU configuration is equivalent with that for the fvp
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platform and others, this is not expected to be a concern.
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- ID 15: Improper handling of input data received over a UART interface may
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allow an attacker to tamper with TF-A execution environment.
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--------------
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*Copyright (c) 2021, Arm Limited. All rights reserved.*
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*Copyright (c) 2021-2023, Arm Limited. All rights reserved.*
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