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

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Message-ID: <20190523170026.nso2me5qnrrjbrdr@mbp>
Date:   Thu, 23 May 2019 18:00:27 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     enh <enh@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Evgenii Stepanov <eugenis@...gle.com>,
        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>,
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        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 Thu, May 23, 2019 at 08:44:12AM -0700, enh wrote:
> On Thu, May 23, 2019 at 7:45 AM Catalin Marinas <catalin.marinas@....com> wrote:
> > On Wed, May 22, 2019 at 01:47:36PM -0700, Kees Cook wrote:
> > > For userspace, how would a future binary choose TBI over MTE? If it's
> > > a library issue, we can't use an ELF bit, since the choice may be
> > > "late" after ELF load (this implies the need for a prctl().) If it's
> > > binary-only ("built with HWKASan") then an ELF bit seems sufficient.
> > > And without the marking, I'd expect the kernel to enforce MTE when
> > > there are high bits.
> >
> > The current plan is that a future binary issues a prctl(), after
> > checking the HWCAP_MTE bit (as I replied to Elliot, the MTE instructions
> > are not in the current NOP space). I'd expect this to be done by the
> > libc or dynamic loader under the assumption that the binaries it loads
> > do _not_ use the top pointer byte for anything else.
> 
> yeah, it sounds like to support hwasan and MTE, the dynamic linker
> will need to not use either itself.
> 
> > With hwasan compiled objects this gets more confusing (any ELF note
> > to identify them?).
> 
> no, at the moment code that wants to know checks for the presence of
> __hwasan_init. (and bionic doesn't actually look at any ELF notes
> right now.) but we can always add something if we need to.

It's a userspace decision to make. In the kernel, we are proposing that
bionic calls a prctl() to enable MTE explicitly. It could first check
for the presence of __hwasan_init.

-- 
Catalin