Re: [PATCH v2] taskstats: fix data-race

From: Dmitry Vyukov <dvyukov@google.com>
To: Andrea Parri <parri.andrea@gmail.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
Date: Mon, 7 Oct 2019 16:18:26 +0200	[thread overview]
Message-ID: <CACT4Y+avbYvtF9mHiX=R8Y2=YsP1_QsN6i_FpjLM7UxCKv6vxA@mail.gmail.com> (raw)
In-Reply-To: <20191007141432.GA22083@andrea.guest.corp.microsoft.com>

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?