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

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Date:   Tue, 8 Sep 2020 15:41:41 +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 04:12:49PM +0200, Andrey Konovalov wrote:
> On Tue, Sep 8, 2020 at 4:06 PM Catalin Marinas <catalin.marinas@....com> wrote:
> > 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.
> 
> No, the tag is always 0xFF when MTE is not supported.
> 
> Should we then only enable TBI1 if system_supports_mte() or something
> like that?

You could add it do this block in __cpu_setup:

https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/tree/arch/arm64/mm/proc.S?h=for-next/mte#n429

It needs a few changes to have "mov_q x10, TCR_..." before the MTE
check so that you can add the TBI1 bit in there.

system_supports_mte() would be called too late, you want this set before
the MMU is turned on.

-- 
Catalin