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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Tue, 29 Sep 2020 15:29:58 +0200
Message-ID: <CACT4Y+bfKwoZe3SC-BKJkOET1GxGp9tCpLzkae8q1sjWYnmgmw@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 Tue, Sep 29, 2020 at 10:33 AM Borislav Petkov <bp@alien8.de> wrote:
>
> On Mon, Sep 28, 2020 at 10:23:53PM +0200, Borislav Petkov wrote:
> > 2020/09/28 22:21:01 VMs 3, executed 179, corpus cover 11792, corpus signal 10881, max signal 19337, crashes 0, repro 0
>
> Ok, so far triggered two things:
>
> WARNING in f2fs_is_valid_blkaddr        1       2020/09/29 10:27        reproducing
> WARNING in reiserfs_put_super           1       2020/09/28 22:42
>
> you've probably seen them already.
>
> Anyway, next question. Let's say I trigger the corruption: is there a
> way to stop the guest VM which has triggered it so that I'm able to
> examine it with gdb?
>
> What about kdump? Can I dump the guest memory either with kdump or
> through the qemu monitor (I believe there's a command to dump memory) so
> that it can be poked at?
>
> Because as it is, we don't have a reproducer and as I see it, the fuzzing simply
> gets restarted:
>
> 2020/09/29 10:27:03 vm-3: crash: WARNING in f2fs_is_valid_blkaddr
> ...
> 2020/09/29 10:27:05 loop: phase=1 shutdown=false instances=1/4 [3] repro: pending=0 reproducing=1 queued=1
> 2020/09/29 10:27:05 loop: starting instance 3
>
> so it would be good to be able to say, when a vm encounters a crash, it
> should be stopped immediately so that the guest can be examined through
> qemu's gdb interface, i.e.,
>
> -gdb tcp::<portnum>
>
> or so?

Currently there is no such feature. I think some people did it because
something similar was mentioned on the mailing IIRC, but I don't know
how they did it, probably with some local code changes.