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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 1 Oct 2020 12:23:08 +0200
Message-ID: <CACT4Y+amigeTPFtwEgbeOGGTiRXFOVZauhOhQCz9CQ70HM17Ow@mail.gmail.com>
Subject: Re: general protection fault in perf_misc_flags
To:     Borislav Petkov <bp@alien8.de>
Cc:     Alexander Potapenko <glider@google.com>,
        Marco Elver <elver@google.com>,
        syzbot <syzbot+ce179bc99e64377c24bc@syzkaller.appspotmail.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Jiri Olsa <jolsa@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        syzkaller <syzkaller@googlegroups.com>
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On Wed, Sep 30, 2020 at 6:31 PM Borislav Petkov <bp@alien8.de> wrote:
>
> On Wed, Sep 30, 2020 at 06:23:44PM +0200, Dmitry Vyukov wrote:
> > Here is the answer:
> > https://github.com/google/syzkaller/blob/master/tools/create-gce-image.sh#L189
> >
> > # rodata=n: mark_rodata_ro becomes very slow with KASAN (lots of PGDs)
> >
> > I have some vague memory that there was some debug double checking
> > that pages are indeed read-only and that debug check was slow, but it
> > was always executed without rodata=n.
>
> Sounds like debug_checkwx() which is disabled by turning off
> CONFIG_DEBUG_WX.
>
> You could either disable it in your .configs or, provided there's even
> such an option, disable KASAN checking around it until that one-time
> boot test completes and then reenable KASAN.

Thanks!

I've prepared a change that removes rodata=n:
https://github.com/google/syzkaller/pull/2155

I think we will be able to indirectly evaluate if it helps or not over
some period of time based on occurrence of any new similar crashes.