linux-kernel - Re: [PATCH 10/14] arm64: ssbd: Add prctl interface for per-thread mitigation

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Date:   Thu, 24 May 2018 13:10:05 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Marc Zyngier <marc.zyngier@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu, Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Christoffer Dall <christoffer.dall@....com>
Subject: Re: [PATCH 10/14] arm64: ssbd: Add prctl interface for per-thread
 mitigation

On Tue, May 22, 2018 at 04:06:44PM +0100, Marc Zyngier wrote:
> If running on a system that performs dynamic SSBD mitigation, allow
> userspace to request the mitigation for itself. This is implemented
> as a prctl call, allowing the mitigation to be enabled or disabled at
> will for this particular thread.
> 
> Signed-off-by: Marc Zyngier <marc.zyngier@....com>
> ---
>  arch/arm64/kernel/Makefile |   1 +
>  arch/arm64/kernel/ssbd.c   | 107 +++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 108 insertions(+)
>  create mode 100644 arch/arm64/kernel/ssbd.c
> 
> diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
> index bf825f38d206..0025f8691046 100644
> --- a/arch/arm64/kernel/Makefile
> +++ b/arch/arm64/kernel/Makefile
> @@ -54,6 +54,7 @@ arm64-obj-$(CONFIG_ARM64_RELOC_TEST)	+= arm64-reloc-test.o
>  arm64-reloc-test-y := reloc_test_core.o reloc_test_syms.o
>  arm64-obj-$(CONFIG_CRASH_DUMP)		+= crash_dump.o
>  arm64-obj-$(CONFIG_ARM_SDE_INTERFACE)	+= sdei.o
> +arm64-obj-$(CONFIG_ARM64_SSBD)		+= ssbd.o
>  
>  obj-y					+= $(arm64-obj-y) vdso/ probes/
>  obj-m					+= $(arm64-obj-m)
> diff --git a/arch/arm64/kernel/ssbd.c b/arch/arm64/kernel/ssbd.c
> new file mode 100644
> index 000000000000..34e3c430176b
> --- /dev/null
> +++ b/arch/arm64/kernel/ssbd.c
> @@ -0,0 +1,107 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) 2018 ARM Ltd, All Rights Reserved.
> + */
> +

#include <linux/errno.h>

... for the error numbers you return below.

> +#include <linux/sched.h>
> +#include <linux/thread_info.h>
> +
> +#include <asm/cpufeature.h>
> +
> +/*
> + * prctl interface for SSBD
> + * FIXME: Drop the below ifdefery once the common interface has been merged.
> + */
> +#ifdef PR_SPEC_STORE_BYPASS
> +static int ssbd_prctl_set(struct task_struct *task, unsigned long ctrl)
> +{
> +	int state = arm64_get_ssbd_state();
> +
> +	/* Unsupported or already mitigated */
> +	if (state == ARM64_SSBD_UNKNOWN)
> +		return -EINVAL;
> +	if (state == ARM64_SSBD_MITIGATED)
> +		return -EPERM;
> +
> +	/*
> +	 * Things are a bit backward here: the arm64 internal API
> +	 * *enables the mitigation* when the userspace API *disables
> +	 * speculation*. So much fun.
> +	 */
> +	switch (ctrl) {
> +	case PR_SPEC_ENABLE:
> +		/* If speculation is force disabled, enable is not allowed */
> +		if (state == ARM64_SSBD_FORCE_ENABLE ||
> +		    task_spec_ssb_force_disable(task))
> +			return -EPERM;
> +		task_clear_spec_ssb_disable(task);
> +		clear_tsk_thread_flag(task, TIF_SSBD);
> +		break;
> +	case PR_SPEC_DISABLE:
> +		if (state == ARM64_SSBD_FORCE_DISABLE)
> +			return -EPERM;
> +		task_set_spec_ssb_disable(task);
> +		set_tsk_thread_flag(task, TIF_SSBD);
> +		break;
> +	case PR_SPEC_FORCE_DISABLE:
> +		if (state == ARM64_SSBD_FORCE_DISABLE)
> +			return -EPERM;
> +		task_set_spec_ssb_disable(task);
> +		task_set_spec_ssb_force_disable(task);
> +		set_tsk_thread_flag(task, TIF_SSBD);
> +		break;
> +	default:
> +		return -ERANGE;
> +	}
> +
> +	return 0;
> +}

I'll have to take a look at the core implementation to make sense of
the rest.

Mark.