linux-kernel - Re: [PATCH 24/35] arm64: mte: Switch GCR

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Date:   Tue, 8 Sep 2020 16:16:23 +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 24/35] arm64: mte: Switch GCR_EL1 in kernel entry and exit

On Tue, Sep 08, 2020 at 03:58:07PM +0200, Andrey Konovalov wrote:
> On Thu, Aug 27, 2020 at 12:38 PM Catalin Marinas
> <catalin.marinas@....com> wrote:
> >
> > On Fri, Aug 14, 2020 at 07:27:06PM +0200, Andrey Konovalov wrote:
> > > @@ -957,6 +984,7 @@ SYM_FUNC_START(cpu_switch_to)
> > >       mov     sp, x9
> > >       msr     sp_el0, x1
> > >       ptrauth_keys_install_kernel x1, x8, x9, x10
> > > +     mte_restore_gcr 1, x1, x8, x9
> > >       scs_save x0, x8
> > >       scs_load x1, x8
> > >       ret
> >
> > Since we set GCR_EL1 on exception entry and return, why is this needed?
> > We don't have a per-kernel thread GCR_EL1, it's global to all threads,
> > so I think cpu_switch_to() should not be touched.
> 
> Dropping this line from the diff leads to many false-positives... I'll
> leave this to Vincenzo.

I wouldn't expect this to have any effect but maybe the
mte_thread_switch() code still touches GCR_EL1 (it does this in the
user-space support, Vincenzo's patches should move that to exception
entry/return).

-- 
Catalin