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

[Dmitry Vyukov]

On Mon, Oct 7, 2019 at 3:18 PM Andrea Parri <parri.andrea@xxxxxxxxx> wrote:
>
> On Mon, Oct 07, 2019 at 01:01:17PM +0200, Christian Brauner wrote:
> > When assiging and testing taskstats in taskstats_exit() there's a race
> > when writing and reading sig->stats when a thread-group with more than
> > one thread exits:
> >
> > cpu0:
> > thread catches fatal signal and whole thread-group gets taken down
> >  do_exit()
> >  do_group_exit()
> >  taskstats_exit()
> >  taskstats_tgid_alloc()
> > The tasks reads sig->stats holding sighand lock seeing garbage.
>
> You meant "without holding sighand lock" here, right?
>
>
> >
> > cpu1:
> > task calls exit_group()
> >  do_exit()
> >  do_group_exit()
> >  taskstats_exit()
> >  taskstats_tgid_alloc()
> > The task takes sighand lock and assigns new stats to sig->stats.
> >
> > Fix this by using READ_ONCE() and smp_store_release().
> >
> > Reported-by: syzbot+c5d03165a1bd1dead0c1@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Fixes: 34ec12349c8a ("taskstats: cleanup ->signal->stats allocation")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
> > Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > Link: https://lore.kernel.org/r/20191006235216.7483-1-christian.brauner@xxxxxxxxxx
> > ---
> > /* v1 */
> > Link: https://lore.kernel.org/r/20191005112806.13960-1-christian.brauner@xxxxxxxxxx
> >
> > /* v2 */
> > - Dmitry Vyukov <dvyukov@xxxxxxxxxx>, Marco Elver <elver@xxxxxxxxxx>:
> >   - fix the original double-checked locking using memory barriers
> >
> > /* v3 */
> > - Andrea Parri <parri.andrea@xxxxxxxxx>:
> >   - document memory barriers to make checkpatch happy
> > ---
> >  kernel/taskstats.c | 21 ++++++++++++---------
> >  1 file changed, 12 insertions(+), 9 deletions(-)
> >
> > diff --git a/kernel/taskstats.c b/kernel/taskstats.c
> > index 13a0f2e6ebc2..978d7931fb65 100644
> > --- a/kernel/taskstats.c
> > +++ b/kernel/taskstats.c
> > @@ -554,24 +554,27 @@ static int taskstats_user_cmd(struct sk_buff *skb, struct genl_info *info)
> >  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.

> > +     if (stats || thread_group_empty(tsk))
> > +             return stats;
> >
> >       /* No problem if kmem_cache_zalloc() fails */
> > -     stats = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL);
> > +     stats_new = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL);
> >
> >       spin_lock_irq(&tsk->sighand->siglock);
> >       if (!sig->stats) {
> > -             sig->stats = stats;
> > -             stats = NULL;
> > +             /* Pairs with READ_ONCE() above. */
> > +             smp_store_release(&sig->stats, stats_new);
>
> This is intended to 'order' the _zalloc()  (zero initializazion)
> before the update of sig->stats, right?  what else am I missing?
>
> Thanks,
>   Andrea
>
>
> > +             stats_new = NULL;
> >       }
> >       spin_unlock_irq(&tsk->sighand->siglock);
> >
> > -     if (stats)
> > -             kmem_cache_free(taskstats_cache, stats);
> > -ret:
> > +     if (stats_new)
> > +             kmem_cache_free(taskstats_cache, stats_new);
> > +
> >       return sig->stats;
> >  }
> >
> > --
> > 2.23.0
> >