Subject: Re: [PATCH v3 06/20] arm64: mm: Fix and re-enable ARM64_SW_TTBR0_PAN
To: Will Deacon <will.deacon@arm.com>,
        <linux-arm-kernel@lists.infradead.org>
References: <1512563739-25239-1-git-send-email-will.deacon@arm.com>
 <1512563739-25239-7-git-send-email-will.deacon@arm.com>
CC: <linux-kernel@vger.kernel.org>, <catalin.marinas@arm.com>,
        <mark.rutland@arm.com>, <ard.biesheuvel@linaro.org>,
        <sboyd@codeaurora.org>, <dave.hansen@linux.intel.com>,
        <keescook@chromium.org>, <msalter@redhat.com>,
        <labbott@redhat.com>, <tglx@linutronix.de>,
        Xiexiuqi <xiexiuqi@huawei.com>
From: Yisheng Xie <xieyisheng1@huawei.com>
Message-ID: <cddbd896-2a23-1aa2-bbd8-3bc7cf5367e2@huawei.com>
Date: Wed, 17 Jan 2018 10:58:23 +0800
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101
 Thunderbird/45.1.0
MIME-Version: 1.0
In-Reply-To: <1512563739-25239-7-git-send-email-will.deacon@arm.com>
Content-Type: text/plain; charset="windows-1252"
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org

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