linux-kernel - Re: [PATCH 08/14] arm64: ssbd: Disable mitigation on CPU resume if required by user

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Date:   Wed, 23 May 2018 11:52:56 +0100
From:   Julien Grall <julien.grall@....com>
To:     Marc Zyngier <marc.zyngier@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu
Cc:     Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Christoffer Dall <christoffer.dall@....com>,
        Andy Lutomirski <luto@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 08/14] arm64: ssbd: Disable mitigation on CPU resume if
 required by user

Hi,

On 05/22/2018 04:06 PM, Marc Zyngier wrote:
> On a system where firmware can dynamically change the state of the
> mitigation, the CPU will always come up with the mitigation enabled,
> including when coming back from suspend.
> 
> If the user has requested "no mitigation" via a command line option,
> let's enforce it by calling into the firmware again to disable it.
> 
> Signed-off-by: Marc Zyngier <marc.zyngier@....com>

Reviewed-by: Julien Grall <julien.grall@....com>

Cheers,

> ---
>   arch/arm64/include/asm/cpufeature.h | 6 ++++++
>   arch/arm64/kernel/cpu_errata.c      | 8 ++++----
>   arch/arm64/kernel/suspend.c         | 8 ++++++++
>   3 files changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 1bacdf57f0af..d9dcb683259e 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -553,6 +553,12 @@ static inline int arm64_get_ssbd_state(void)
>   #endif
>   }
>   
> +#ifdef CONFIG_ARM64_SSBD
> +void arm64_set_ssbd_mitigation(bool state);
> +#else
> +static inline void arm64_set_ssbd_mitigation(bool state) {}
> +#endif
> +
>   #endif /* __ASSEMBLY__ */
>   
>   #endif
> diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
> index 8f686f39b9c1..b4c12e9140f0 100644
> --- a/arch/arm64/kernel/cpu_errata.c
> +++ b/arch/arm64/kernel/cpu_errata.c
> @@ -297,7 +297,7 @@ void __init arm64_enable_wa2_handling(struct alt_instr *alt,
>   		*updptr = cpu_to_le32(aarch64_insn_gen_nop());
>   }
>   
> -static void do_ssbd(bool state)
> +void arm64_set_ssbd_mitigation(bool state)
>   {
>   	switch (psci_ops.conduit) {
>   	case PSCI_CONDUIT_HVC:
> @@ -371,20 +371,20 @@ static bool has_ssbd_mitigation(const struct arm64_cpu_capabilities *entry,
>   	switch (ssbd_state) {
>   	case ARM64_SSBD_FORCE_DISABLE:
>   		pr_info_once("%s disabled from command-line\n", entry->desc);
> -		do_ssbd(false);
> +		arm64_set_ssbd_mitigation(false);
>   		required = false;
>   		break;
>   
>   	case ARM64_SSBD_EL1_ENTRY:
>   		if (required) {
>   			__this_cpu_write(arm64_ssbd_callback_required, 1);
> -			do_ssbd(true);
> +			arm64_set_ssbd_mitigation(true);
>   		}
>   		break;
>   
>   	case ARM64_SSBD_FORCE_ENABLE:
>   		pr_info_once("%s forced from command-line\n", entry->desc);
> -		do_ssbd(true);
> +		arm64_set_ssbd_mitigation(true);
>   		required = true;
>   		break;
>   
> diff --git a/arch/arm64/kernel/suspend.c b/arch/arm64/kernel/suspend.c
> index a307b9e13392..70c283368b64 100644
> --- a/arch/arm64/kernel/suspend.c
> +++ b/arch/arm64/kernel/suspend.c
> @@ -62,6 +62,14 @@ void notrace __cpu_suspend_exit(void)
>   	 */
>   	if (hw_breakpoint_restore)
>   		hw_breakpoint_restore(cpu);
> +
> +	/*
> +	 * On resume, firmware implementing dynamic mitigation will
> +	 * have turned the mitigation on. If the user has forcefully
> +	 * disabled it, make sure their wishes are obeyed.
> +	 */
> +	if (arm64_get_ssbd_state() == ARM64_SSBD_FORCE_DISABLE)
> +		arm64_set_ssbd_mitigation(false);
>   }
>   
>   /*
> 

-- 
Julien Grall