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

[Andrey Konovalov]

On Tue, Sep 8, 2020 at 4:06 PM Catalin Marinas <catalin.marinas@xxxxxxx> 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@xxxxxxx> 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?