linux-kernel - Re: [PATCH 2/2] arm: apply more __ro_after

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Date:	Thu, 11 Aug 2016 18:02:15 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Russell King - ARM Linux <linux@...linux.org.uk>,
	linux-arch@...r.kernel.org, Kees Cook <keescook@...omium.org>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathias Krause <minipli@...glemail.com>,
	Mark Brown <broonie@...nel.org>,
	Daniel Thompson <daniel.thompson@...aro.org>
Subject: Re: [PATCH 2/2] arm: apply more __ro_after_init

On Thursday, August 11, 2016 12:02:42 AM CEST Russell King - ARM Linux wrote:
> On Wed, Aug 10, 2016 at 09:31:05PM +0200, Arnd Bergmann wrote:
> > On Wednesday, August 10, 2016 11:12:53 AM CEST Russell King - ARM Linux wrote:
> > > There's the TLS emulation too, but that writes via the vectors mapping
> > > at 0xffff0ff0.
> > 
> > Ok, so that should be safe. Can we change the fiq code to also use the
> > high mapping and then take the __ro_after_init patch on top?
> 
> We can't - if the kernel is configured without the kuser helpers in
> the vectors page, it's mapped read-only. I'm not sure what the
> intersection is between platforms that can have FIQs and platforms
> that can disable the kuser helpers.

>From Kconfig logic and callers of set_fiq_handler(), theoretically
there is just i.MX3, but I think they never use fiq in their
audio drivers in practice already, and Mark Brown mentioned
that we could remove fiq support in the imx audio driver (don't
remember the details at the moment).

If we can prove that i.MX3 PCM FIQ support is never used, then the
intersection is empty, and all machines that use FIQ require kuser
helpers.

This may change with Daniel Thompson's patches that use the FIQ
for NMI backtrace.

	Arnd