Re: [PATCH 2/2] arm64: Branch predictor hardening for Cavium ThunderX2

[Will Deacon <will.deacon@arm.com>]

On Sun, Jan 07, 2018 at 10:53:36PM -0800, Jayachandran C wrote:
> Use PSCI based mitigation for speculative execution attacks targeting
> the branch predictor. The approach is similar to the one used for
> Cortex-A CPUs, but in case of ThunderX2 we add another SMC call to
> test if the firmware supports the capability.
> 
> If the secure firmware has been updated with the mitigation code to
> invalidate the branch target buffer, we use the PSCI version call to
> invoke it.
> 
> Signed-off-by: Jayachandran C <jnair@caviumnetworks.com>
> ---
>  arch/arm64/kernel/cpu_errata.c | 38 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 38 insertions(+)
> 
> diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
> index cb0fb37..abceb5d 100644
> --- a/arch/arm64/kernel/cpu_errata.c
> +++ b/arch/arm64/kernel/cpu_errata.c
> @@ -124,6 +124,7 @@ static void  install_bp_hardening_cb(const struct arm64_cpu_capabilities *entry,
>  	__install_bp_hardening_cb(fn, hyp_vecs_start, hyp_vecs_end);
>  }
>  
> +#include <linux/arm-smccc.h>
>  #include <linux/psci.h>
>  
>  static int enable_psci_bp_hardening(void *data)
> @@ -138,6 +139,33 @@ static int enable_psci_bp_hardening(void *data)
>  
>  	return 0;
>  }
> +
> +#define CAVIUM_TX2_SIP_SMC_CALL		0xC200FF00
> +#define CAVIUM_TX2_BTB_HARDEN_CAP	0xB0A0
> +
> +static int enable_tx2_psci_bp_hardening(void *data)
> +{
> +	const struct arm64_cpu_capabilities *entry = data;
> +	struct arm_smccc_res res;
> +
> +	if (!entry->matches(entry, SCOPE_LOCAL_CPU))
> +		return;
> +
> +	arm_smccc_smc(CAVIUM_TX2_SIP_SMC_CALL, CAVIUM_TX2_BTB_HARDEN_CAP, 0, 0, 0, 0, 0, 0, &res);

One thing to be aware of here is that if somebody configures qemu to emulate
a TX2, this may actually disappear into EL3 and not return. You're better
off sticking with PSCI GET_VERSION in terms of portability, but it's your
call -- I'd expect you to deal with any breakage reports on the list due
to the SMC above. Fair?

> +	if (res.a0 != 0) {
> +		pr_warn("Error: CONFIG_HARDEN_BRANCH_PREDICTOR enabled, but firmware does not support it\n");
> +		return 0;
> +	}

Please don't print this here; see below.

> +	if (res.a1 == 1 && psci_ops.get_version) {
> +		pr_info("CPU%d: Branch predictor hardening enabled\n", smp_processor_id());

If you want to print a message, please put it in the capability structure
.desc field.

Will