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Date:   Tue, 8 Oct 2019 16:20:35 +0200
From:   Andrea Parri <parri.andrea@gmail.com>
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Christian Brauner <christian.brauner@ubuntu.com>,
        bsingharora@gmail.com, Marco Elver <elver@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        syzbot <syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        stable <stable@vger.kernel.org>
Subject: Re: [PATCH v2] taskstats: fix data-race
Message-ID: <20191008142035.GA13564@andrea.guest.corp.microsoft.com>
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 <20191007131804.GA19242@andrea.guest.corp.microsoft.com>
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On Mon, Oct 07, 2019 at 04:18:26PM +0200, Dmitry Vyukov wrote:
> On Mon, Oct 7, 2019 at 4:14 PM Andrea Parri <parri.andrea@gmail.com> wrote:
> >
> > > > >  static struct taskstats *taskstats_tgid_alloc(struct task_struct *tsk)
> > > > >  {
> > > > >       struct signal_struct *sig = tsk->signal;
> > > > > -     struct taskstats *stats;
> > > > > +     struct taskstats *stats_new, *stats;
> > > > >
> > > > > -     if (sig->stats || thread_group_empty(tsk))
> > > > > -             goto ret;
> > > > > +     /* Pairs with smp_store_release() below. */
> > > > > +     stats = READ_ONCE(sig->stats);
> > > >
> > > > This pairing suggests that the READ_ONCE() is heading an address
> > > > dependency, but I fail to identify it: what is the target memory
> > > > access of such a (putative) dependency?
> > >
> > > I would assume callers of this function access *stats. So the
> > > dependency is between loading stats and accessing *stats.
> >
> > AFAICT, the only caller of the function in 5.4-rc2 is taskstats_exit(),
> > which 'casts' the return value to a boolean (so I really don't see how
> > any address dependency could be carried over/relied upon here).
> 
> This does not make sense.
> 
> But later taskstats_exit does:
> 
> memcpy(stats, tsk->signal->stats, sizeof(*stats));
> 
> Perhaps it's supposed to use stats returned by taskstats_tgid_alloc?

Seems reasonable to me.  If so, replacing the READ_ONCE() in question
with an smp_load_acquire() might be the solution.  Thoughts?

  Andrea