RE: [PATCH] rcu: Safe access to rcu_node structure's->exp_tasks

[Zhang, Qiang1]

> 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@xxxxxxxxx>
>
>Apologies for the delay and thank you for the reminder!
>
>Please check the wordsmithed version below, which I have queued.


Thanks for wordsmithed, this description is more clear.

Thanks
Zqiang


>
>						Thanx, Paul
>
>------------------------------------------------------------------------
>
>commit 389b0eafd72829fd63548f7ff4e8d6ac90fa1f98
>Author: Zqiang <qiang1.zhang@xxxxxxxxx>
>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@xxxxxxxxx>
>    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
>
>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)) {
>+		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) {