linux-kernel - Re: [PATCH v3 02/13] arm64: Kconfig: Reword UNMAP_KERNEL_AT

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Message-ID: <20180110192610.GB9723@arm.com>
Date:   Wed, 10 Jan 2018 19:26:11 +0000
From:   Will Deacon <will.deacon@....com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-arm-kernel@...ts.infradead.org, catalin.marinas@....com,
        ard.biesheuvel@...aro.org, marc.zyngier@....com,
        lorenzo.pieralisi@....com, christoffer.dall@...aro.org,
        linux-kernel@...r.kernel.org, shankerd@...eaurora.org,
        jnair@...iumnetworks.com
Subject: Re: [PATCH v3 02/13] arm64: Kconfig: Reword UNMAP_KERNEL_AT_EL0
 kconfig entry

On Tue, Jan 09, 2018 at 09:17:00AM -0800, Christoph Hellwig wrote:
> On Mon, Jan 08, 2018 at 05:32:27PM +0000, Will Deacon wrote:
> > Although CONFIG_UNMAP_KERNEL_AT_EL0 does make KASLR more robust, it's
> > actually more useful as a mitigation against speculation attacks that
> > can leak arbitrary kernel data to userspace through speculation.
> > 
> > Reword the Kconfig help message to reflect this, and make the option
> > depend on EXPERT so that it is on by default for the majority of users.
> 
> I still haven't heard an anwer on why this isn't using
> CONFIG_PAGE_TABLE_ISOLATION but instead reinvents its own symbol.

Mainly because this code was written before CONFIG_PAGE_TABLE_ISOLATION had
been proposed and I wanted to avoid confusion with the ongoing backports
just to align on the naming for an arch-specific config option. We could
CONFIG_PAGE_TABLE_ISOLATION and make it select CONFIG_UNMAP_KERNEL_AT_EL) if
you like, but worth noting that this is default 'y' anyway and depends on
EXPERT.

Will