Subject: [v2, 11/11] arm64: Implement branch predictor hardening for affected Cortex-A CPUs

From: [email-masked] (Jayachandran C)
Date: Sun, 7 Jan 2018 22:31:54 -0800

On Fri, Jan 05, 2018 at 01:12:41PM +0000, Will Deacon wrote:
> Cortex-A57, A72, A73 and A75 are susceptible to branch predictor aliasing
> and can theoretically be attacked by malicious code.
> 
> This patch implements a PSCI-based mitigation for these CPUs when available.
> The call into firmware will invalidate the branch predictor state, preventing
> any malicious entries from affecting other victim contexts.
> 
> Signed-off-by: Marc Zyngier <[email-masked]>
> Signed-off-by: Will Deacon <[email-masked]>
> ---
>  arch/arm64/kernel/bpi.S        | 24 ++++++++++++++++++++++++
>  arch/arm64/kernel/cpu_errata.c | 42 ++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 66 insertions(+)
> 
> diff --git a/arch/arm64/kernel/bpi.S b/arch/arm64/kernel/bpi.S
> index 06a931eb2673..2e9146534174 100644
> --- a/arch/arm64/kernel/bpi.S
> +++ b/arch/arm64/kernel/bpi.S
> @@ -53,3 +53,27 @@ ENTRY(__bp_harden_hyp_vecs_start)
>  	vectors __kvm_hyp_vector
>  	.endr
>  ENTRY(__bp_harden_hyp_vecs_end)
> +ENTRY(__psci_hyp_bp_inval_start)
> +	sub	sp, sp, #(8 * 18)
> +	stp	x16, x17, [sp, #(16 * 0)]
> +	stp	x14, x15, [sp, #(16 * 1)]
> +	stp	x12, x13, [sp, #(16 * 2)]
> +	stp	x10, x11, [sp, #(16 * 3)]
> +	stp	x8, x9, [sp, #(16 * 4)]
> +	stp	x6, x7, [sp, #(16 * 5)]
> +	stp	x4, x5, [sp, #(16 * 6)]
> +	stp	x2, x3, [sp, #(16 * 7)]
> +	stp	x0, x1, [sp, #(18 * 8)]
> +	mov	x0, #0x84000000
> +	smc	#0
> +	ldp	x16, x17, [sp, #(16 * 0)]
> +	ldp	x14, x15, [sp, #(16 * 1)]
> +	ldp	x12, x13, [sp, #(16 * 2)]
> +	ldp	x10, x11, [sp, #(16 * 3)]
> +	ldp	x8, x9, [sp, #(16 * 4)]
> +	ldp	x6, x7, [sp, #(16 * 5)]
> +	ldp	x4, x5, [sp, #(16 * 6)]
> +	ldp	x2, x3, [sp, #(16 * 7)]
> +	ldp	x0, x1, [sp, #(18 * 8)]
> +	add	sp, sp, #(8 * 18)
> +ENTRY(__psci_hyp_bp_inval_end)
> diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
> index 16ea5c6f314e..cb0fb3796bb8 100644
> --- a/arch/arm64/kernel/cpu_errata.c
> +++ b/arch/arm64/kernel/cpu_errata.c
> @@ -53,6 +53,8 @@ static int cpu_enable_trap_ctr_access(void *__unused)
>  DEFINE_PER_CPU_READ_MOSTLY(struct bp_hardening_data, bp_hardening_data);
>  
>  #ifdef CONFIG_KVM
> +extern char __psci_hyp_bp_inval_start[], __psci_hyp_bp_inval_end[];
> +
>  static void __copy_hyp_vect_bpi(int slot, const char *hyp_vecs_start,
>  				const char *hyp_vecs_end)
>  {
> @@ -94,6 +96,9 @@ static void __install_bp_hardening_cb(bp_hardening_cb_t fn,
>  	spin_unlock(&bp_lock);
>  }
>  #else
> +#define __psci_hyp_bp_inval_start	NULL
> +#define __psci_hyp_bp_inval_end		NULL
> +
>  static void __install_bp_hardening_cb(bp_hardening_cb_t fn,
>  				      const char *hyp_vecs_start,
>  				      const char *hyp_vecs_end)
> @@ -118,6 +123,21 @@ 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/psci.h>
> +
> +static int enable_psci_bp_hardening(void *data)
> +{
> +	const struct arm64_cpu_capabilities *entry = data;
> +
> +	if (psci_ops.get_version)
> +		install_bp_hardening_cb(entry,
> +				       (bp_hardening_cb_t)psci_ops.get_version,
> +				       __psci_hyp_bp_inval_start,
> +				       __psci_hyp_bp_inval_end);
> +
> +	return 0;
> +}
>  #endif	/* CONFIG_HARDEN_BRANCH_PREDICTOR */
>  
>  #define MIDR_RANGE(model, min, max) \
> @@ -261,6 +281,28 @@ const struct arm64_cpu_capabilities arm64_errata[] = {
>  		MIDR_ALL_VERSIONS(MIDR_CORTEX_A73),
>  	},
>  #endif
> +#ifdef CONFIG_HARDEN_BRANCH_PREDICTOR
> +	{
> +		.capability = ARM64_HARDEN_BRANCH_PREDICTOR,
> +		MIDR_ALL_VERSIONS(MIDR_CORTEX_A57),
> +		.enable = enable_psci_bp_hardening,
> +	},
> +	{
> +		.capability = ARM64_HARDEN_BRANCH_PREDICTOR,
> +		MIDR_ALL_VERSIONS(MIDR_CORTEX_A72),
> +		.enable = enable_psci_bp_hardening,
> +	},
> +	{
> +		.capability = ARM64_HARDEN_BRANCH_PREDICTOR,
> +		MIDR_ALL_VERSIONS(MIDR_CORTEX_A73),
> +		.enable = enable_psci_bp_hardening,
> +	},
> +	{
> +		.capability = ARM64_HARDEN_BRANCH_PREDICTOR,
> +		MIDR_ALL_VERSIONS(MIDR_CORTEX_A75),
> +		.enable = enable_psci_bp_hardening,
> +	},
> +#endif
>  	{
>  	}
>  };

On ThunderX2, we would like to do something similar, but it is important to
find out if the current firmware has support for BTB invalidate in the
PSCI version call. Otherwise, we will end up doing version calls that do
nothing but return version (and waste cycles).

I will follow up with ThunderX2 patches, but it would be good to have a
standard way of figuring out if the firmware has BTB invalidate support.

JC.