linux-kernel - Re: [PATCH v3 06/20] arm64: mm: Fix and re-enable ARM64_SW_TTBR0

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Date:   Wed, 17 Jan 2018 10:58:23 +0800
From:   Yisheng Xie <xieyisheng1@...wei.com>
To:     Will Deacon <will.deacon@....com>,
        <linux-arm-kernel@...ts.infradead.org>
CC:     <linux-kernel@...r.kernel.org>, <catalin.marinas@....com>,
        <mark.rutland@....com>, <ard.biesheuvel@...aro.org>,
        <sboyd@...eaurora.org>, <dave.hansen@...ux.intel.com>,
        <keescook@...omium.org>, <msalter@...hat.com>,
        <labbott@...hat.com>, <tglx@...utronix.de>,
        Xiexiuqi <xiexiuqi@...wei.com>
Subject: Re: [PATCH v3 06/20] arm64: mm: Fix and re-enable ARM64_SW_TTBR0_PAN

Hi Will,

On 2017/12/6 20:35, Will Deacon wrote:
>  config ARM64_SW_TTBR0_PAN
>  	bool "Emulate Privileged Access Never using TTBR0_EL1 switching"
> -	depends on BROKEN       # Temporary while switch_mm is reworked
>  	help
>  	  Enabling this option prevents the kernel from accessing
>  	  user-space memory directly by pointing TTBR0_EL1 to a reserved

I have a question not related to this patch itself, but to ARM64_SW_TTBR0_PAN:

What does ARM64_SW_TTBR0_PAN used for? I means is hardware support PAN, do we
still need SW_TTBR0_PAN?

And if the hardware do not support PAN, is SW_TTBR0_PAN is *must* option? or
there maybe a security risk?

Thanks
Yisheng Xie