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

[Andrea Parri]

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?


> +	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
>