ArmPkg/ArmSvcLib: prevent speculative execution beyond svc

ArmPkg/ArmSvcLib: prevent speculative execution beyond svc

Supervisor Call instruction (SVC) is used by the Arm Standalone MM
environment to request services from the privileged software (such as
ARM Trusted Firmware running in EL3) and also return back to the
non-secure caller via EL3. Some Arm CPUs speculatively executes the
instructions after the SVC instruction without crossing the privilege
level (S-EL0). Although the results of this execution are
architecturally discarded, adversary running on the non-secure side can
manipulate the contents of the general purpose registers to leak the
secure work memory through spectre like micro-architectural side channel
attacks. This behavior is demonstrated by the SafeSide project [1] and
[2]. Add barrier instructions after SVC to prevent speculative execution
to mitigate such attacks.

[1]: https://github.com/google/safeside/blob/master/demos/eret_hvc_smc_wrapper.cc
[2]: https://github.com/google/safeside/blob/master/kernel_modules/kmod_eret_hvc_smc/eret_hvc_smc_module.c

Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>