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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Mon, 6 Nov 2017 12:56:09 +0100
Message-ID: <CACT4Y+aPcirEC5dPx8FLRAyD6NdWooXeW9KurQRuMYuP6OWdrQ@mail.gmail.com>
Subject: Re: WARNING in task_participate_group_stop
To:     Jamie Iles <jamie.iles@oracle.com>
Cc:     Oleg Nesterov <oleg@redhat.com>,
        syzbot 
        <bot+c9f0eb0d2a5576ece331a767528e6b52b4ff1815@syzkaller.appspotmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Arvind Yadav <arvind.yadav.cs@gmail.com>,
        Mark Brown <broonie@kernel.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        =?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= <fweisbec@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        mchehab@kernel.org, Ingo Molnar <mingo@kernel.org>,
        mpe@ellerman.id.au, syzkaller-bugs@googlegroups.com,
        Al Viro <viro@zeniv.linux.org.uk>, Kyle Huey <me@kylehuey.com>,
        Kees Cook <keescook@chromium.org>
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On Mon, Nov 6, 2017 at 12:25 PM, Jamie Iles <jamie.iles@oracle.com> wrote:
> Hi Dmitry,
>
> On Mon, Nov 06, 2017 at 12:02:19PM +0100, Dmitry Vyukov wrote:
>> On Thu, Nov 2, 2017 at 6:01 PM, Oleg Nesterov <oleg@redhat.com> wrote:
>> > On 11/01, Dmitry Vyukov wrote:
>> >>
>> >> On Tue, Oct 31, 2017 at 7:34 PM, Oleg Nesterov <oleg@redhat.com> wrote:
>> >> > Hmm. I do not see reproducer in this email...
>> >>
>> >> Ah, sorry. You can see full thread with attachments here:
>> >> https://groups.google.com/forum/#!topic/syzkaller-bugs/EUmYZU4m5gU
>> >
>> > Heh. I can't say I enjoyed reading the reproducer ;)
>> >
>> >> >> > WARNING: CPU: 0 PID: 1 at kernel/signal.c:340
>> >> >> > task_participate_group_stop+0x1ce/0x230 kernel/signal.c:340
>> >> >> > Kernel panic - not syncing: panic_on_warn set ...
>> >> >> >
>> >> >> > CPU: 0 PID: 1 Comm: init Not tainted 4.13.0-mm1+ #5
>> >> >
>> >> > So this is init process with SIGNAL_UNKILLABLE flag set. And I hope it has
>> >> > the pending SIGKILL, otherwise there is something else.
>> >
>> > From repro.c
>> >
>> >         line 111    r[8] = syscall(__NR_ptrace, 0x10ul, r[7]);
>> >
>> > this is PTRACE_ATTACH
>> >
>> >         line 115        syscall(__NR_ptrace, 0x4200ul, r[7], 0x40000012ul, 0x100012ul);
>> >
>> > this is PTRACE_SETOPTIONS and "data" includes PTRACE_O_EXITKILL.
>> >
>> > r[7] is initialized at
>> >
>> >         line 110      r[7] = *(uint32_t*)0x20f9cffc;
>> >
>> > so if it is eq to 1 then it can attach to init and in this case the problem
>> > can be explained by the wrong SIGNAL_UNKILLABLE/SIGKILL logic.
>> >
>> > But how *(uint32_t*)0x20f9cffc can be 1 ?
>> >
>> >         line 108    r[6] = syscall(__NR_fcntl, r[1], 0x10ul, 0x20f9cff8ul);
>> >
>> > this is F_GETOWN_EX, addr = 0x20f9cff8 == 0x20f9cffc + 4, so if fcntl()
>> > actually succeeds then r[7] == f_owner_ex->pid.
>> >
>> > It _can_ be 1, but the reproducer doesn't work for me. If you can reproduce,
>> > could you try the patch below?
>>
>> Hi,
>>
>> I would like to understand why you were not able to reproduce it. I
>> won't be sitting here all the time, and we are tracking hundreds of
>> bugs across different linux kernels and other OSes, so it's
>> problematic to do any extensive work on all of them. That's why we try
>> to provide reproducers.
>>
>> I've just tried the repro on the latest upstream
>> (39dae59d66acd86d1de24294bd2f343fd5e7a625) and it triggered the
>> WARNING within a second.
>> Did you use the config provided? Did you use qemu or real hardware?
>> Can you try in qemu (with -smp>1)?
>
> I'm unable to reproduce the warning in qemu with SMP (on a 32 CPU VM).
> Instead I get the following instant traceback which is different to what
> you report when run as root:


Uh, it seems to be racy. I am getting either the WARNING or "attempt
to kill init" in ~1/5 proportion.
Please try this simplified program, it triggers the WARNING all the time for me:


// autogenerated by syzkaller (http://github.com/google/syzkaller)

#define _GNU_SOURCE
#include <sys/syscall.h>
#include <unistd.h>
#include <stdint.h>
#include <string.h>

int main()
{
  long r[11];
  memset(r, -1, sizeof(r));
  r[0] = syscall(__NR_mmap, 0x20000000ul, 0xfec000ul, 0x3ul, 0x32ul,
                 0xfffffffffffffffful, 0x0ul);
  r[1] = syscall(__NR_inotify_init1, 0x80000ul);
  *(uint32_t*)0x20feb000 = (uint32_t)0xc;
  r[3] = syscall(__NR_getsockopt, 0xfffffffffffffffful, 0x1ul, 0x11ul,
                 0x2003cff4ul, 0x20feb000ul);
  if (r[3] != -1)
    r[4] = *(uint32_t*)0x2003cff4;
  r[5] = syscall(__NR_fcntl, r[1], 0x8ul, r[4]);
  r[6] = syscall(__NR_fcntl, r[1], 0x10ul, 0x20f9cff8ul);
  if (r[6] != -1)
    r[7] = *(uint32_t*)0x20f9cffc;
  r[8] = syscall(__NR_ptrace, 0x10ul, r[7]);
  r[9] =
      syscall(__NR_ioctl, 0xfffffffffffffffful, 0x4b6aul, 0x20f9e000ul);
  r[10] =
      syscall(__NR_ptrace, 0x4200ul, r[7], 0x40000012ul, 0x100012ul);
  return 0;
}

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