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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Wed, 30 Sep 2020 18:29:26 +0200
Message-ID: <CACT4Y+ZKp9z90a9xF5cC-X5_7D8zZOoB+337Hh1rt5PysLfbJw@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 6:23 PM Dmitry Vyukov <dvyukov@google.com> wrote:
> > Hi,
> >
> > one more thing I just spotted. The default install of syzkaller here
> > runs the guest with this on the kernel command line:
> >
> > 2020/09/30 17:56:18 running command: qemu-system-x86_64 []string{"-m", "2048",
> > "-smp", "2", "-display", ... "-append", "earlyprintk=serial oops=panic ...
> >  nmi_watchdog=panic panic_on_warn=1 panic=1 ftrace_dump_on_oops=orig_cpu rodata=n
> >                                                                         ^^^^^^^^^^
> >
> > which basically leaves guest kernel's memory RW and it gets caught
> > immediately on vm boot by CONFIG_DEBUG_WX.
> >
> > This pretty much explains why kernel text can get corrupted with a stray
> > pointer write or so. So what's the use case for rodata=n?
> >
> > [    2.478136] Kernel memory protection disabled.
> >                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
>
> Ha!
>
> 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.


I don't see this is still the case. Diff between 2 boots:

[   11.985152][    T1] Freeing unused kernel image (initmem) memory: 3432K
[   11.986129][    T1] Write protecting the kernel read-only data: 147456k
[   11.990863][    T1] Freeing unused kernel image (text/rodata gap)
memory: 2012K
[   11.992797][    T1] Freeing unused kernel image (rodata/data gap)
memory: 1324K
[   11.993895][    T1] Run /sbin/init as init process

[   11.910396][    T1] Freeing unused kernel image (initmem) memory: 3432K
[   11.911277][    T1] Kernel memory protection disabled.
[   11.911984][    T1] Run /sbin/init as init process

Was it fixed at some point? Was it backported to stable?