linux-kernel - Re: [PATCH v15 00/17] arm64: untag user pointers passed to the kernel

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Date:   Thu, 23 May 2019 08:34:25 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     enh <enh@...gle.com>
Cc:     Evgenii Stepanov <eugenis@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Khalid Aziz <khalid.aziz@...cle.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>,
        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>,
        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>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Dave Martin <Dave.Martin@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Szabolcs Nagy <Szabolcs.Nagy@....com>
Subject: Re: [PATCH v15 00/17] arm64: untag user pointers passed to the kernel

On Wed, May 22, 2019 at 04:09:31PM -0700, enh wrote:
> On Wed, May 22, 2019 at 4:03 PM Evgenii Stepanov <eugenis@...gle.com> wrote:
> > On Wed, May 22, 2019 at 1:47 PM Kees Cook <keescook@...omium.org> wrote:
> > > On Wed, May 22, 2019 at 05:35:27PM +0100, Catalin Marinas wrote:
> > > > I would also expect the C library or dynamic loader to check for the
> > > > presence of a HWCAP_MTE bit before starting to tag memory allocations,
> > > > otherwise it would get SIGILL on the first MTE instruction it tries to
> > > > execute.
> > >
> > > I've got the same question as Elliot: aren't MTE instructions just NOP
> > > to older CPUs? I.e. if the CPU (or kernel) don't support it, it just
> > > gets entirely ignored: checking is only needed to satisfy curiosity
> > > or behavioral expectations.
> >
> > MTE instructions are not NOP. Most of them have side effects (changing
> > register values, zeroing memory).
> 
> no, i meant "they're encoded in a space that was previously no-ops, so
> running on MTE code on old hardware doesn't cause SIGILL".

It does result in SIGILL, there wasn't enough encoding left in the NOP
space for old/current CPU implementations (in hindsight, we should have
reserved a bigger NOP space).

As Evgenii said, the libc needs to be careful when tagging the heap as
it would cause a SIGILL if the HWCAP_MTE is not set. The standard
application doesn't need to be recompiled as it would not issue MTE
colouring instructions, just standard LDR/STR.

Stack tagging is problematic if you want to colour each frame
individually, the function prologue would need the non-NOP MTE
instructions. The best we can do here is just having the (thread) stacks
of different colours.

-- 
Catalin