linux-kernel - Re: [PATCH] ARM: DRA7/OMAP5: Enable ACTLR[0] (Enable invalidates of BTB) for secondary cores

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Date:   Wed, 13 Jun 2018 11:11:54 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Nishanth Menon <nm@...com>
Cc:     Tony Lindgren <tony@...mide.com>, linux-kernel@...r.kernel.org,
        linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] ARM: DRA7/OMAP5: Enable ACTLR[0] (Enable invalidates of
 BTB) for secondary cores

On Tue, Jun 12, 2018 at 04:36:11PM -0500, Nishanth Menon wrote:
> Call secure services to enable ACTLR[0] (Enable invalidates of BTB with
> ICIALLU) when branch hardening is enabled for kernel.

As mentioned elsewhere, I don't think this is a good idea - if the secure
world is not implementing the Spectre workarounds, then the _system_ is
exploitable.

If the secure world is implementing the spectre workarounds, it will
already have enabled the IBE bit (which is r/w from secure, read only
from non-secure.)

So, basically, lack of the IBE bit being set is basically telling the
kernel that it's running on a vulnerable platform _even if the kernel
were to set it through some means_.

> 
> Signed-off-by: Nishanth Menon <nm@...com>
> ---
> 
> Based on: next-20180612 +
> Uboot series posted:  https://marc.info/?l=u-boot&m=152883522011042&w=2
> 
> With Just u-boot changes alone: OMAP5-uevm: https://pastebin.ubuntu.com/p/9yDM22bJ6n/
> with kernel changes added on: https://pastebin.ubuntu.com/p/gXPBGGYRPX/
> 
>  arch/arm/mach-omap2/omap-smp.c | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
> 
> diff --git a/arch/arm/mach-omap2/omap-smp.c b/arch/arm/mach-omap2/omap-smp.c
> index 69df3620eca5..28fc80ea675b 100644
> --- a/arch/arm/mach-omap2/omap-smp.c
> +++ b/arch/arm/mach-omap2/omap-smp.c
> @@ -109,6 +109,32 @@ void omap5_erratum_workaround_801819(void)
>  static inline void omap5_erratum_workaround_801819(void) { }
>  #endif
>  
> +#ifdef CONFIG_HARDEN_BRANCH_PREDICTOR
> +static void omap5_harden_predictor(void)
> +{
> +	u32 acr, acr_mask;
> +
> +	asm volatile ("mrc p15, 0, %0, c1, c0, 1" : "=r" (acr));
> +
> +	/*
> +	 * BIT(0) - Disables streaming. All write-allocate lines allocate in
> +	 */
> +	acr_mask = BIT(0);
> +
> +	/* do we already have it done.. if yes, skip expensive smc */
> +	if ((acr & acr_mask) == acr_mask)
> +		return;
> +
> +	acr |= acr_mask;
> +	omap_smc1(OMAP5_DRA7_MON_SET_ACR_INDEX, acr);
> +
> +	pr_debug("%s: ARM ACR setup for CVE_2017_5715 applied on CPU%d\n",
> +		 __func__, smp_processor_id());
> +}
> +#else
> +static inline void omap5_harden_predictor(void) { }
> +#endif
> +
>  static void omap4_secondary_init(unsigned int cpu)
>  {
>  	/*
> @@ -131,6 +157,8 @@ static void omap4_secondary_init(unsigned int cpu)
>  		set_cntfreq();
>  		/* Configure ACR to disable streaming WA for 801819 */
>  		omap5_erratum_workaround_801819();
> +		/* Enable ACR to allow for ICUALLU workaround */
> +		omap5_harden_predictor();
>  	}
>  
>  	/*
> -- 
> 2.15.1
> 

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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