linux-kernel - Re: [PATCH v17 03/15] arm64: Introduce prctl() options to control the tagged user addresses ABI

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Date:   Thu, 13 Jun 2019 16:57:55 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Vincenzo Frascino <vincenzo.frascino@....com>
Cc:     Dave Martin <Dave.Martin@....com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-rdma@...r.kernel.org,
        linux-media@...r.kernel.org, kvm@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Szabolcs Nagy <Szabolcs.Nagy@....com>,
        Will Deacon <will.deacon@....com>,
        Kostya Serebryany <kcc@...gle.com>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Leon Romanovsky <leon@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Evgeniy Stepanov <eugenis@...gle.com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Kees Cook <keescook@...omium.org>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Yishai Hadas <yishaih@...lanox.com>,
        Jens Wiklander <jens.wiklander@...aro.org>,
        Lee Smith <Lee.Smith@....com>,
        Alexander Deucher <Alexander.Deucher@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        enh <enh@...gle.com>, Robin Murphy <robin.murphy@....com>,
        Christian Koenig <Christian.Koenig@....com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>
Subject: Re: [PATCH v17 03/15] arm64: Introduce prctl() options to control
 the tagged user addresses ABI

On Thu, Jun 13, 2019 at 04:45:54PM +0100, Vincenzo Frascino wrote:
> On 13/06/2019 16:35, Catalin Marinas wrote:
> > On Thu, Jun 13, 2019 at 12:16:59PM +0100, Dave P Martin wrote:
> >> On Wed, Jun 12, 2019 at 01:43:20PM +0200, Andrey Konovalov wrote:
> >>> +
> >>> +/*
> >>> + * Control the relaxed ABI allowing tagged user addresses into the kernel.
> >>> + */
> >>> +static unsigned int tagged_addr_prctl_allowed = 1;
> >>> +
> >>> +long set_tagged_addr_ctrl(unsigned long arg)
> >>> +{
> >>> +	if (!tagged_addr_prctl_allowed)
> >>> +		return -EINVAL;
> >>
> >> So, tagging can actually be locked on by having a process enable it and
> >> then some possibly unrelated process clearing tagged_addr_prctl_allowed.
> >> That feels a bit weird.
> > 
> > The problem is that if you disable the ABI globally, lots of
> > applications would crash. This sysctl is meant as a way to disable the
> > opt-in to the TBI ABI. Another option would be a kernel command line
> > option (I'm not keen on a Kconfig option).
> 
> Why you are not keen on a Kconfig option?

Because I don't want to rebuild the kernel/reboot just to be able to
test how user space handles the ABI opt-in. I'm ok with a Kconfig option
to disable this globally in addition to a run-time option (if actually
needed, I'm not sure).

-- 
Catalin