Advisory TFV-7 (CVE-2018-3639)
|Title||Trusted Firmware-A exposure to cache speculation vulnerability Variant 4|
|Date||21 May 2018 (Updated 7 June 2018)|
|Versions Affected||All, up to and including v1.5|
|Impact||Leakage of secure world data to normal world|
|Fix Version||Pull Request #1392, Pull Request #1397|
This security advisory describes the current understanding of the Trusted Firmware-A (TF-A) exposure to Variant 4 of the cache speculation 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.
At the time of writing, the TF-A project is not aware of a Variant 4 exploit that could be used against TF-A. It is likely to be very difficult to achieve an exploit against current standard configurations of TF-A, due to the limited interfaces into the secure world with attacker-controlled inputs. However, this is becoming increasingly difficult to guarantee with the introduction of complex new firmware interfaces, for example the Software Delegated Exception Interface (SDEI). Also, the TF-A project does not have visibility of all vendor-supplied interfaces. Therefore, the TF-A project takes a conservative approach by mitigating Variant 4 in hardware wherever possible during secure world execution. The mitigation is enabled by setting an implementation defined control bit to prevent the re-ordering of stores and loads.
For each affected CPU type, TF-A implements one of the two following mitigation approaches in Pull Request #1392 and Pull Request #1397. Both approaches have a system performance impact, which varies for each CPU type and use-case. The mitigation code is enabled by default, but can be disabled at compile time for platforms that are unaffected or where the risk is deemed low enough.
Arm CPUs not mentioned below are unaffected.
For affected CPUs, this approach enables the mitigation during EL3 initialization, following every PE reset. No mechanism is provided to disable the mitigation at runtime.
This approach permanently mitigates the entire software stack and no additional mitigation code is required in other software components.
TF-A implements this approach for the following affected CPUs:
- Cortex-A57 and Cortex-A72, by setting bit 55 (Disable load pass store) of CPUACTLR_EL1 (S3_1_C15_C2_0).
- Cortex-A73, by setting bit 3 of S3_0_C15_C0_0 (not documented in the Technical Reference Manual (TRM)).
- Cortex-A75, by setting bit 35 (reserved in TRM) of CPUACTLR_EL1 (S3_0_C15_C1_0).
For affected CPUs, this approach also enables the mitigation during EL3 initialization, following every PE reset. In addition, this approach implements SMCCC_ARCH_WORKAROUND_2 in the Arm architectural range to allow callers at lower exception levels to temporarily disable the mitigation in their execution context, where the risk is deemed low enough. This approach enables mitigation on entry to EL3, and restores the mitigation state of the lower exception level on exit from EL3. For more information on this approach, see Firmware interfaces for mitigating cache speculation vulnerabilities.
This approach may be complemented by additional mitigation code in other software components, for example code that calls SMCCC_ARCH_WORKAROUND_2. However, even without any mitigation code in other software components, this approach will effectively permanently mitigate the entire software stack, since the default mitigation state for firmware-managed execution contexts is enabled.
Since the expectation in this approach is that more software executes with the mitigation disabled, this may result in better system performance than the static approach for some systems or use-cases. However, for other systems or use-cases, this performance saving may be outweighed by the additional overhead of SMCCC_ARCH_WORKAROUND_2 calls and TF-A exception handling.
TF-A implements this approach for the following affected CPU:
- Cortex-A76, by setting and clearing bit 16 (reserved in TRM) of CPUACTLR2_EL1 (S3_0_C15_C1_1).