linux-kernel - Re: general protection fault in perf_misc

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Date:   Sat, 26 Sep 2020 08:46:54 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        syzbot <syzbot+ce179bc99e64377c24bc@...kaller.appspotmail.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Jiri Olsa <jolsa@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: general protection fault in perf_misc_flags

On Sat, Sep 26, 2020 at 2:32 AM 'Nick Desaulniers' via syzkaller-bugs
<syzkaller-bugs@...glegroups.com> wrote:
> > > > On Wed, Sep 23, 2020 at 11:24:48AM +0200, Dmitry Vyukov wrote:
> > > > > 3. Run syzkaller locally with custom patches.
> > > >
> > > > Let's say I wanna build the kernel with clang-10 using your .config and
> > > > run it in a vm locally. What are the steps in order to reproduce the
> > > > same workload syzkaller runs in the guest on the GCE so that I can at
> > > > least try get as close as possible to reproducing locally?
> > >
> > > It's a random fuzzing workload. You can get this workload by running
> > > syzkaller locally:
> > > https://github.com/google/syzkaller/blob/master/docs/linux/setup_ubuntu-host_qemu-vm_x86-64-kernel.md
>
> These are virtualized guests, right?  Has anyone played with getting
> `rr` working to record traces of guests in QEMU?
>
> I had seen the bug that generated this on github:
> https://julialang.org/blog/2020/09/rr-memory-magic/
>
> That way, even if syzkaller didn't have a reproducer binary, it would
> at least have a replayable trace.

These are virtualized guests, but they run on GCE, not in QEMU.

> Boris, one question I have. Doesn't the kernel mark pages backing
> executable code as read only at some point?  If that were the case,
> then I don't see how the instruction stream could be modified.  I
> guess static key patching would have to undo that permission mapping
> before patching.
>
> You're right about the length shorter than what I would have expected
> from static key patching.  That could very well be a write through
> dangling int pointer...
>
> > >
> > > The exact clang compiler syzbot used is available here:
> > > https://github.com/google/syzkaller/blob/master/docs/syzbot.md#crash-does-not-reproduce
> >
> > I've marked all other similar ones a dup of this one. Now you can see
> > all manifestations on the dashboard:
> > https://syzkaller.appspot.com/bug?extid=ce179bc99e64377c24bc
> >
> > Another possible debugging vector on this:
> > The location of crashes does not seem to be completely random and
> > evenly spread across kernel code. I think there are many more static
> > branches (mm, net), but we have 3 crashes in vdso and 9 in paravirt
> > code + these 6 crashes in perf_misc_flags which looks a bit like an
> > outlier (?). What's special about paravirt/vdso?..
>
>
>
> --
> Thanks,
> ~Nick Desaulniers
>
> --
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