linux-kernel - Re: [PATCH 5/5] kasan: suppress recursive reports for HW

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Message-ID: <CA+fCnZf=t50u+5z-e9kHUqe=7aAWJpkNgt=aS0n_9R_r2jBSHA@mail.gmail.com>
Date:   Tue, 14 Mar 2023 18:56:15 +0100
From:   Andrey Konovalov <andreyknvl@...il.com>
To:     Marco Elver <elver@...gle.com>
Cc:     andrey.konovalov@...ux.dev,
        Catalin Marinas <catalin.marinas@....com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        kasan-dev@...glegroups.com,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Peter Collingbourne <pcc@...gle.com>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Weizhao Ouyang <ouyangweizhao@...u.com>,
        linux-kernel@...r.kernel.org,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH 5/5] kasan: suppress recursive reports for HW_TAGS

On Mon, Mar 13, 2023 at 12:20 PM Marco Elver <elver@...gle.com> wrote:
>
> > + * 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 report, as
> > +        * hw_suppress_tag_checks_start() disables checks on the current 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!