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Date: Thu, 14 Dec 2017 15:26:38 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Greg KH <greg@kroah.com>
cc: Kees Cook <keescook@chromium.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Dmitry Vyukov <dvyukov@google.com>, security@kernel.org,
        Akinobu Mita <akinobu.mita@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>,
        syzkaller-bugs@googlegroups.com,
        Vegard Nossum <vegard.nossum@oracle.com>
Subject: Re: general protection fault in show_timer
In-Reply-To: <20171212083343.GG22935@kroah.com>
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On Tue, 12 Dec 2017, Greg KH wrote:

> On Tue, Dec 05, 2017 at 11:58:07AM -0800, Kees Cook wrote:
> > On Thu, Nov 30, 2017 at 4:57 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> > > On Thu, 30 Nov 2017, Alexey Dobriyan wrote:
> > >
> > >> [cc security@]
> > >> 100% oops with interrupts disabled by nobody
> > >> or kernel memory read
> > >> [nods]
> > >> you named the bug already
> > >>
> > >> "notify" directly comes from userspace struct sigevent::sigev_notify
> > >> without adult supervision.
> > >>
> > >> Reproducer is timer_create + read(/proc/self/timers)
> > >
> > > Bah. That's a really old one.
> > >
> > > Tentative fix below. That needs more though but looking at the existing
> > > check there is only one valid combo with SIGEV_THREAD_ID.
> > >
> > > Thanks,
> > >
> > >         tglx
> > >
> > > 8<----------------
> > > --- a/kernel/time/posix-timers.c
> > > +++ b/kernel/time/posix-timers.c
> > > @@ -434,6 +434,16 @@ static struct pid *good_sigevent(sigeven
> > >  {
> > >         struct task_struct *rtn = current->group_leader;
> > >
> > > +       switch (event->sigev_notify) {
> > > +       case SIGEV_NONE:
> > > +       case SIGEV_SIGNAL:
> > > +       case SIGEV_SIGNAL | SIGEV_THREAD_ID:
> > > +       case SIGEV_THREAD:
> > > +               break;
> > > +       default:
> > > +               return NULL;
> > > +       }
> > > +
> > >         if ((event->sigev_notify & SIGEV_THREAD_ID ) &&
> > >                 (!(rtn = find_task_by_vpid(event->sigev_notify_thread_id)) ||
> > >                  !same_thread_group(rtn, current) ||
> > 
> > FWIW, this looks correct to me.
> 
> Did this ever go anywhere?  I don't see it in Linus's tree yet...

I learned yesterday that syzboz is understuffed and cannot test patches, so
I need to find a minute to run the reproducer myself and verify that the
patch is correct.

Thanks,

	tglx