[PATCH AUTOSEL 6.3 3/3] rcu: Protect rcu_print_task_exp_stall() ->exp_tasks access

Sasha Levin
2023-04-29T01:59:08Z

From: Zqiang <qiang1.zhang@intel.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@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>
---
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 249c2967d9e6c..54e8fb258c98d 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) {
--
2.39.2