9.6. Advisory TFV-6 (CVE-2017-5753, CVE-2017-5715, CVE-2017-5754)
Trusted Firmware-A exposure to speculative processor vulnerabilities using cache timing side-channels
03 Jan 2018 (Updated 11 Jan, 18 Jan, 26 Jan, 30 Jan and 07 June 2018)
All, up to and including v1.4
Leakage of secure world data to normal world
Google / Arm
This security advisory describes the current understanding of the Trusted Firmware-A exposure to the speculative processor vulnerabilities identified by Google Project Zero. To understand the background and wider impact of these vulnerabilities on Arm systems, please refer to the Arm Processor Security Update.
9.6.1. Variant 1 (CVE-2017-5753)
At the time of writing, no vulnerable patterns have been observed in upstream TF code, therefore no workarounds have been applied or are planned.
9.6.2. Variant 2 (CVE-2017-5715)
Where possible on vulnerable CPUs, Arm recommends invalidating the branch predictor as early as possible on entry into the secure world, before any branch instruction is executed. There are a number of implementation defined ways to achieve this.
For Cortex-A57 and Cortex-A72 CPUs, the Pull Requests (PRs) in this advisory invalidate the branch predictor when entering EL3 by disabling and re-enabling the MMU.
For Cortex-A73 and Cortex-A75 CPUs, the PRs in this advisory invalidate the
branch predictor when entering EL3 by temporarily dropping into AArch32
Secure-EL1 and executing the
BPIALL instruction. This workaround is
significantly more complex than the “MMU disable/enable” workaround. The latter
is not effective at invalidating the branch predictor on Cortex-A73/Cortex-A75.
Note that if other privileged software, for example a Rich OS kernel, implements its own branch predictor invalidation during context switch by issuing an SMC (to execute firmware branch predictor invalidation), then there is a dependency on the PRs in this advisory being deployed in order for those workarounds to work. If that other privileged software is able to workaround the vulnerability locally (for example by implementing “MMU disable/enable” itself), there is no such dependency.
Pull Request #1240 and Pull Request #1405 optimise the earlier fixes by
implementing a specified CVE-2017-5715 workaround SMC
SMCCC_ARCH_WORKAROUND_1) for use by normal world privileged software. This
is more efficient than calling an arbitrary SMC (for example
SMCCC_ARCH_WORKAROUND_1 can be found in the CVE-2017-5715
mitigation specification. The specification and implementation also enable
the normal world to discover the presence of this firmware service.
On Juno R1 we measured the round trip latency for both the
SMCCC_ARCH_WORKAROUND_1 SMCs on Cortex-A57, using both the “MMU
disable/enable” and “BPIALL at AArch32 Secure-EL1” workarounds described above.
This includes the time spent in test code conforming to the SMC Calling
Convention (SMCCC) from AArch64. For the
SMCCC_ARCH_WORKAROUND_1 cases, the
test code uses SMCCC v1.1, which reduces the number of general purpose registers
it needs to save/restore. Although the
BPIALL instruction is not effective
at invalidating the branch predictor on Cortex-A57, the drop into Secure-EL1
with MMU disabled that this workaround entails effectively does invalidate the
branch predictor. Hence this is a reasonable comparison.
The results were as follows:
Due to the high severity and wide applicability of this issue, the above workarounds are enabled by default (on vulnerable CPUs only), despite some performance and code size overhead. Platforms can choose to disable them at compile time if they do not require them. Pull Request #1240 disables the workarounds for unaffected upstream platforms.
For vulnerable AArch32-only CPUs (for example Cortex-A8, Cortex-A9 and
BPIALL instruction should be used as early as possible on
entry into the secure world. For Cortex-A8, also set
ACTLR to 1 during
early processor initialization. Note that the
BPIALL instruction is not
effective at invalidating the branch predictor on Cortex-A15. For that CPU, set
ACTLR to 1 during early processor initialization, and invalidate the
branch predictor by performing an
On AArch32 EL3 systems, the monitor and secure-SVC code is typically tightly
integrated, for example as part of a Trusted OS. Therefore any Variant 2
workaround should be provided by vendors of that software and is outside the
scope of TF. However, an example implementation in the minimal AArch32 Secure
SP_MIN is provided in Pull Request #1228.
Other Arm CPUs are not vulnerable to this or other variants. This includes Cortex-A76, Cortex-A53, Cortex-A55, Cortex-A32, Cortex-A7 and Cortex-A5.
For more information about non-Arm CPUs, please contact the CPU vendor.
9.6.3. Variant 3 (CVE-2017-5754)
This variant is only exploitable between Exception Levels within the same translation regime, for example between EL0 and EL1, therefore this variant cannot be used to access secure memory from the non-secure world, and is not applicable for TF. However, Secure Payloads (for example, Trusted OS) should provide mitigations on vulnerable CPUs to protect themselves from exploited Secure-EL0 applications.
The only Arm CPU vulnerable to this variant is Cortex-A75.