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

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Message-ID: <20190617171813.GC34565@arrakis.emea.arm.com>
Date:   Mon, 17 Jun 2019 18:18:13 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Evgenii Stepanov <eugenis@...gle.com>
Cc:     Andrey Konovalov <andreyknvl@...gle.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <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,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>,
        Yishai Hadas <yishaih@...lanox.com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Alexander Deucher <Alexander.Deucher@....com>,
        Christian Koenig <Christian.Koenig@....com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Jens Wiklander <jens.wiklander@...aro.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Leon Romanovsky <leon@...nel.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Dave Martin <Dave.Martin@....com>,
        Khalid Aziz <khalid.aziz@...cle.com>, enh <enh@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christoph Hellwig <hch@...radead.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        Lee Smith <Lee.Smith@....com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Robin Murphy <robin.murphy@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Szabolcs Nagy <Szabolcs.Nagy@....com>
Subject: Re: [PATCH v17 03/15] arm64: Introduce prctl() options to control
 the tagged user addresses ABI

On Mon, Jun 17, 2019 at 09:57:36AM -0700, Evgenii Stepanov wrote:
> On Mon, Jun 17, 2019 at 6:56 AM Catalin Marinas <catalin.marinas@....com> wrote:
> > On Wed, Jun 12, 2019 at 01:43:20PM +0200, Andrey Konovalov wrote:
> > > From: Catalin Marinas <catalin.marinas@....com>
> > >
> > > It is not desirable to relax the ABI to allow tagged user addresses into
> > > the kernel indiscriminately. This patch introduces a prctl() interface
> > > for enabling or disabling the tagged ABI with a global sysctl control
> > > for preventing applications from enabling the relaxed ABI (meant for
> > > testing user-space prctl() return error checking without reconfiguring
> > > the kernel). The ABI properties are inherited by threads of the same
> > > application and fork()'ed children but cleared on execve().
> > >
> > > The PR_SET_TAGGED_ADDR_CTRL will be expanded in the future to handle
> > > MTE-specific settings like imprecise vs precise exceptions.
> > >
> > > Signed-off-by: Catalin Marinas <catalin.marinas@....com>
> >
> > A question for the user-space folk: if an application opts in to this
> > ABI, would you want the sigcontext.fault_address and/or siginfo.si_addr
> > to contain the tag? We currently clear it early in the arm64 entry.S but
> > we could find a way to pass it down if needed.
> 
> For HWASan this would not be useful because we instrument memory
> accesses with explicit checks anyway. For MTE, on the other hand, it
> would be very convenient to know the fault address tag without
> disassembling the code.

I could as this differently: does anything break if, once the user
opts in to TBI, fault_address and/or si_addr have non-zero top byte?

Alternatively, we could present the original FAR_EL1 register as a
separate field as we do with ESR_EL1, independently of whether the user
opted in to TBI or not.

-- 
Catalin