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From:   Andrey Konovalov <andreyknvl@gmail.com>
Date:   Tue, 14 Mar 2023 18:56:15 +0100
Message-ID: <CA+fCnZf=t50u+5z-e9kHUqe=7aAWJpkNgt=aS0n_9R_r2jBSHA@mail.gmail.com>
Subject: Re: [PATCH 5/5] kasan: suppress recursive reports for HW_TAGS
To:     Marco Elver <elver@google.com>
Cc:     andrey.konovalov@linux.dev,
        Catalin Marinas <catalin.marinas@arm.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        kasan-dev@googlegroups.com,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Will Deacon <will@kernel.org>,
        linux-arm-kernel@lists.infradead.org,
        Peter Collingbourne <pcc@google.com>,
        Evgenii Stepanov <eugenis@google.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        Weizhao Ouyang <ouyangweizhao@zeku.com>,
        linux-kernel@vger.kernel.org,
        Andrey Konovalov <andreyknvl@google.com>
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On Mon, Mar 13, 2023 at 12:20=E2=80=AFPM Marco Elver <elver@google.com> wro=
te:
>
> > + * Hardware Tag-Based KASAN instead relies on:
> > + * For #1: Resetting tags via kasan_reset_tag().
> > + * For #2: Supression of tag checks via CPU, see report_suppress_start=
/end().
>
> Typo: "Suppression"

Will fix in v2.

> > +static void report_suppress_start(void)
> > +{
> > +#ifdef CONFIG_KASAN_HW_TAGS
> > +       /*
> > +        * Disable migration for the duration of printing a KASAN repor=
t, as
> > +        * hw_suppress_tag_checks_start() disables checks on the curren=
t CPU.
> > +        */
> > +       migrate_disable();
>
> This still allows this task to be preempted by another task. If the
> other task is scheduled in right after hw_suppress_tag_checks_start()
> then there won't be any tag checking in that task. If HW-tags KASAN is
> used as a mitigation technique, that may unnecessarily weaken KASAN,
> because right after report_suppress_start(), it does
> spin_lock_irqsave() which disables interrupts (and thereby preemption)
> anyway.
>
> Why not just use preempt_disable()?

Ah, yes, I intended to do that but forgot to make the change.

I'll wait for comments from arm64 maintainers on the other patches and
then send v2 with a fix.

Thank you, Marco!