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

[Christian Brauner <christian.brauner@ubuntu.com>]

On Tue, Oct 08, 2019 at 05:26:59PM +0200, Andrea Parri wrote:
> On Tue, Oct 08, 2019 at 04:24:14PM +0200, Christian Brauner wrote:
> > On Tue, Oct 08, 2019 at 04:20:35PM +0200, Andrea Parri wrote:
> > > 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?
> > 
> > I've done that already in my tree yesterday. I can resend for another
> > review if you'd prefer.
> 
> Oh nice!  No need to resend of course.  ;D FWIW, I can check it if you
> let me know the particular branch/commit (guessing that's somewhere in
> git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git, yes?).

Oh ups, yeah of course :)
https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/log/?h=taskstats_syzbot

Thanks!
Christian