linux-kernel - Re: [PATCH] rcu: Safe access to rcu_node structure's->exp

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Date:   Mon, 9 Jan 2023 14:21:01 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Zqiang <qiang1.zhang@...el.com>, quic_neeraju@...cinc.com,
        joel@...lfernandes.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: Safe access to rcu_node structure's->exp_tasks

On Thu, Jan 05, 2023 at 07:41:46PM -0800, Paul E. McKenney wrote:
> On Sat, Dec 24, 2022 at 01:25:53PM +0800, Zqiang wrote:
> > For kernels built with CONFIG_PREEMPT_RCU=y, the following scenario
> > can result system oops.
> > 
> >            CPU1                                           CPU2
> > rcu_preempt_deferred_qs_irqrestore                rcu_print_task_exp_stall
> >   if (special.b.blocked)                            READ_ONCE(rnp->exp_tasks) != NULL
> >     raw_spin_lock_rcu_node
> >     np = rcu_next_node_entry(t, rnp)
> >     if (&t->rcu_node_entry == rnp->exp_tasks)
> >       WRITE_ONCE(rnp->exp_tasks, np)
> >       ....
> >       raw_spin_unlock_irqrestore_rcu_node
> >                                                     raw_spin_lock_irqsave_rcu_node
> >                                                     t = list_entry(rnp->exp_tasks->prev,
> >                                                         struct task_struct, rcu_node_entry)
> >                                                     (if rnp->exp_tasks is NULL
> >                                                        will trigger oops)
> > 
> > This problem is that CPU2 accesses rcu_node structure's->exp_tasks
> > without holding the rcu_node structure's ->lock and CPU2 did not
> > observe CPU1's change to rcu_node structure's->exp_tasks in time,
> > if rcu_node structure's->exp_tasks is set null pointer by CPU1, after
> > that CPU2 accesses members of rcu_node structure's->exp_tasks will
> > trigger oops.
> > 
> > This commit therefore allows rcu_node structure's->exp_tasks to be
> > accessed while holding rcu_node structure's ->lock.
> > 
> > Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> 
> Apologies for the delay and thank you for the reminder!
> 
> Please check the wordsmithed version below, which I have queued.
> 
> 						Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 389b0eafd72829fd63548f7ff4e8d6ac90fa1f98
> Author: Zqiang <qiang1.zhang@...el.com>
> Date:   Sat Dec 24 13:25:53 2022 +0800
> 
>     rcu: Protect rcu_print_task_exp_stall() ->exp_tasks access
>     
>     For kernels built with CONFIG_PREEMPT_RCU=y, the following scenario can
>     result in a NULL-pointer dereference:
>     
>                CPU1                                           CPU2
>     rcu_preempt_deferred_qs_irqrestore                rcu_print_task_exp_stall
>       if (special.b.blocked)                            READ_ONCE(rnp->exp_tasks) != NULL
>         raw_spin_lock_rcu_node
>         np = rcu_next_node_entry(t, rnp)
>         if (&t->rcu_node_entry == rnp->exp_tasks)
>           WRITE_ONCE(rnp->exp_tasks, np)
>           ....
>           raw_spin_unlock_irqrestore_rcu_node
>                                                         raw_spin_lock_irqsave_rcu_node
>                                                         t = list_entry(rnp->exp_tasks->prev,
>                                                             struct task_struct, rcu_node_entry)
>                                                         (if rnp->exp_tasks is NULL, this
>                                                            will dereference a NULL pointer)
>     
>     The problem is that CPU2 accesses the rcu_node structure's->exp_tasks
>     field without holding the rcu_node structure's ->lock and CPU2 did
>     not observe CPU1's change to rcu_node structure's ->exp_tasks in time.
>     Therefore, if CPU1 sets rcu_node structure's->exp_tasks pointer to NULL,
>     then CPU2 might dereference that NULL pointer.
>     
>     This commit therefore holds the rcu_node structure's ->lock while
>     accessing that structure's->exp_tasks field.
>     
>     Signed-off-by: Zqiang <qiang1.zhang@...el.com>
>     Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> 
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 7cc4856da0817..902e7c8709c7e 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -803,9 +803,11 @@ static int rcu_print_task_exp_stall(struct rcu_node *rnp)
>  	int ndetected = 0;
>  	struct task_struct *t;
>  
> -	if (!READ_ONCE(rnp->exp_tasks))
> -		return 0;
>  	raw_spin_lock_irqsave_rcu_node(rnp, flags);
> +	if (!READ_ONCE(rnp->exp_tasks)) {

Does it have to be READ_ONCE then?

Thanks.

> +		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> +		return 0;
> +	}
>  	t = list_entry(rnp->exp_tasks->prev,
>  		       struct task_struct, rcu_node_entry);
>  	list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {