linux-kernel - Re: [PATCH v3 5/6] kasan: Unset panic_on

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Date:   Thu, 16 Jan 2020 06:23:01 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Elena Petrova <lenaptr@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-hardening@...ts.openwall.com,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: [PATCH v3 5/6] kasan: Unset panic_on_warn before calling panic()

On Thu, Jan 16, 2020 at 2:24 AM Kees Cook <keescook@...omium.org> wrote:
>
> As done in the full WARN() handler, panic_on_warn needs to be cleared
> before calling panic() to avoid recursive panics.
>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  mm/kasan/report.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> index 621782100eaa..844554e78893 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -92,8 +92,16 @@ static void end_report(unsigned long *flags)
>         pr_err("==================================================================\n");
>         add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
>         spin_unlock_irqrestore(&report_lock, *flags);
> -       if (panic_on_warn)
> +       if (panic_on_warn) {
> +               /*
> +                * This thread may hit another WARN() in the panic path.
> +                * Resetting this prevents additional WARN() from panicking the
> +                * system on this thread.  Other threads are blocked by the
> +                * panic_mutex in panic().

I don't understand part about other threads.
Other threads are not necessary inside of panic(). And in fact since
we reset panic_on_warn, they will not get there even if they should.
If I am reading this correctly, once one thread prints a warning and
is going to panic, other threads may now print infinite amounts of
warning and proceed past them freely. Why is this the behavior we
want?

> +                */
> +               panic_on_warn = 0;
>                 panic("panic_on_warn set ...\n");
> +       }
>         kasan_enable_current();
>  }