linux-kernel - [PATCH AUTOSEL 5.10 2/2] rcu: Protect rcu_print_task_exp_stall() ->exp

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Date:   Fri, 28 Apr 2023 21:59:16 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Zqiang <qiang1.zhang@...el.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Sasha Levin <sashal@...nel.org>, frederic@...nel.org,
        quic_neeraju@...cinc.com, josh@...htriplett.org,
        rcu@...r.kernel.org
Subject: [PATCH AUTOSEL 5.10 2/2] rcu: Protect rcu_print_task_exp_stall() ->exp_tasks access

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

[ 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@...lfernandes.org>
Signed-off-by: Zqiang <qiang1.zhang@...el.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 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 ef6570137dcd5..401c1f331cafa 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -707,9 +707,11 @@ 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) {
-- 
2.39.2