linux-kernel - Re: [BUG] Use of probe_kernel_address() in task_rcu_dereference() without checking return value

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Date:   Mon, 2 Sep 2019 15:53:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Chris Metcalf <cmetcalf@...hip.com>,
        Christoph Lameter <cl@...ux.com>,
        Kirill Tkhai <tkhai@...dex.ru>, Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] Use of probe_kernel_address() in task_rcu_dereference()
 without checking return value

On Mon, Sep 02, 2019 at 03:40:03PM +0200, Oleg Nesterov wrote:
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 8dc1811..1f9b021 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1134,7 +1134,10 @@ struct task_struct {
>  
>  	struct tlbflush_unmap_batch	tlb_ubc;
>  
> -	struct rcu_head			rcu;
> +	union {
> +		bool			xxx;
> +		struct rcu_head		rcu;
> +	};
>  
>  	/* Cache last used pipe for splice(): */
>  	struct pipe_inode_info		*splice_pipe;
> diff --git a/kernel/exit.c b/kernel/exit.c
> index a75b6a7..baacfce 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -182,6 +182,11 @@ static void delayed_put_task_struct(struct rcu_head *rhp)
>  	put_task_struct(tsk);
>  }
>  
> +void call_delayed_put_task_struct(struct task_struct *p)
> +{
> +	if (xchg(&p->xxx, 1))
> +		call_rcu(&p->rcu, delayed_put_task_struct);
> +}

I think this is the first usage of xchg() on _Bool, also not all archs
implement xchg8()