From: Zqiang <qiang1.zhang@intel.com>

Date: Sat, 24 Dec 2022 13:25:53 +0800

Subject: [PATCH] rcu: Protect rcu_print_task_exp_stall() ->exp_tasks access

References: bsc#1012628

Patch-mainline: 6.3.4

Git-commit: 3c1566bca3f8349f12b75d0a2d5e4a20ad6262ec

[ Upstream commit 3c1566bca3f8349f12b75d0a2d5e4a20ad6262ec ]

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.

[ paulmck: Apply Frederic Weisbecker feedback. ]

Acked-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Signed-off-by: Zqiang <qiang1.zhang@intel.com>

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Signed-off-by: Sasha Levin <sashal@kernel.org>

Signed-off-by: Jiri Slaby <jslaby@suse.cz>

---

 kernel/rcu/tree_exp.h | 6 ++++--

 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h

index 249c2967..54e8fb25 100644

--- a/kernel/rcu/tree_exp.h

+++ b/kernel/rcu/tree_exp.h

@@ -802,9 +802,11 @@ 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 (!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) {

-- 

2.35.3