> 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) {
