linux-kernel - Re: [PATCH 26/35] kasan, arm64: Enable TBI EL1

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Message-ID: <20200908140620.GE25591@gaia>
Date:   Tue, 8 Sep 2020 15:06:20 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Elena Petrova <lenaptr@...gle.com>,
        Branislav Rankov <Branislav.Rankov@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Will Deacon <will.deacon@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 26/35] kasan, arm64: Enable TBI EL1

On Tue, Sep 08, 2020 at 03:18:04PM +0200, Andrey Konovalov wrote:
> On Thu, Aug 27, 2020 at 12:40 PM Catalin Marinas
> <catalin.marinas@....com> wrote:
> >
> > On Fri, Aug 14, 2020 at 07:27:08PM +0200, Andrey Konovalov wrote:
> > > diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
> > > index 152d74f2cc9c..6880ddaa5144 100644
> > > --- a/arch/arm64/mm/proc.S
> > > +++ b/arch/arm64/mm/proc.S
> > > @@ -38,7 +38,7 @@
> > >  /* PTWs cacheable, inner/outer WBWA */
> > >  #define TCR_CACHE_FLAGS      TCR_IRGN_WBWA | TCR_ORGN_WBWA
> > >
> > > -#ifdef CONFIG_KASAN_SW_TAGS
> > > +#if defined(CONFIG_KASAN_SW_TAGS) || defined(CONFIG_KASAN_HW_TAGS)
> > >  #define TCR_KASAN_FLAGS TCR_TBI1
> > >  #else
> > >  #define TCR_KASAN_FLAGS 0
> >
> > I prefer to turn TBI1 on only if MTE is present. So on top of the v8
> > user series, just do this in __cpu_setup.
> 
> Started working on this, but realized that I don't understand what
> exactly is suggested here. TCR_KASAN_FLAGS are used in __cpu_setup(),
> so this already happens in __cpu_setup().
> 
> Do you mean that TBI1 should be enabled when CONFIG_ARM64_MTE is
> enabled, but CONFIG_KASAN_HW_TAGS is disabled?

What I meant is that we should turn TBI1 only when the MTE is present in
hardware (and the ARM64_MTE option is on). But I probably missed the way
MTE is used with KASAN.

So what happens if CONFIG_KASAN_HW_TAGS and CONFIG_ARM64_MTE are both on
but the hardware does not support MTE? Does KASAN still generate tagged
pointers? If yes, then the current patch is fine, we should always set
TBI1.

-- 
Catalin