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Advisory TFV-5 (CVE-2017-15031)
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===============================
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+----------------+-------------------------------------------------------------+
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| Title | Not initializing or saving/restoring ``PMCR_EL0`` can leak |
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| | secure world timing information |
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+================+=============================================================+
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| CVE ID | `CVE-2017-15031`_ |
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+----------------+-------------------------------------------------------------+
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| Date | 02 Oct 2017, updated on 04 Nov 2019 |
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+----------------+-------------------------------------------------------------+
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| Versions | All, up to and including v2.1 |
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| Affected | |
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+----------------+-------------------------------------------------------------+
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| Configurations | All |
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| Affected | |
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+----------------+-------------------------------------------------------------+
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| Impact | Leakage of sensitive secure world timing information |
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+----------------+-------------------------------------------------------------+
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| Fix Version | `Pull Request #1127`_ (merged on 18 October 2017) |
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| | |
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| | `Commit e290a8fcbc`_ (merged on 23 August 2019) |
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| | |
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| | `Commit c3e8b0be9b`_ (merged on 27 September 2019) |
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+----------------+-------------------------------------------------------------+
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| Credit | Arm, Marek Bykowski |
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+----------------+-------------------------------------------------------------+
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The ``PMCR_EL0`` (Performance Monitors Control Register) provides details of the
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Performance Monitors implementation, including the number of counters
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implemented, and configures and controls the counters. If the ``PMCR_EL0.DP``
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bit is set to zero, the cycle counter (when enabled) counts during secure world
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execution, even when prohibited by the debug signals.
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Since TF-A does not save and restore ``PMCR_EL0`` when switching between the
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normal and secure worlds, normal world code can set ``PMCR_EL0.DP`` to zero to
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cause leakage of secure world timing information. This register should be added
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to the list of saved/restored registers both when entering EL3 and also
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transitioning to S-EL1.
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Furthermore, ``PMCR_EL0.DP`` has an architecturally ``UNKNOWN`` reset value.
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Since Arm TF does not initialize this register, it's possible that on at least
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some implementations, ``PMCR_EL0.DP`` is set to zero by default. This and other
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bits with an architecturally UNKNOWN reset value should be initialized to
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sensible default values in the secure context.
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The same issue exists for the equivalent AArch32 register, ``PMCR``, except that
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here ``PMCR_EL0.DP`` architecturally resets to zero.
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NOTE: The original pull request referenced above only fixed the issue for S-EL1
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whereas the EL3 was fixed in the later commits.
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.. _CVE-2017-15031: http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-15031
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.. _Pull Request #1127: https://github.com/ARM-software/arm-trusted-firmware/pull/1127
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.. _Commit e290a8fcbc: https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git/commit/?id=e290a8fcbc
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.. _Commit c3e8b0be9b: https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git/commit/?id=c3e8b0be9b
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