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

[Christian Brauner]

On Mon, Oct 07, 2019 at 03:18:04PM +0200, Andrea Parri 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?

Correct, thanks for noticing!

> 
> 
> > 
> > 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?
> 
> 
> > +	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?

Right, I should've mentioned that. I'll change the comment.
But I thought this also paired with smp_read_barrier_depends() that's
placed alongside READ_ONCE()?

Christian