rcu: Protect rcu_print_task_exp_stall() ->exp_tasks access

[ 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>

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 60732264a7d0b56673e307914cb76222d888f71a..e25321dbb068e5e63222f55956a84626dae3affc 100644 (file)
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -800,9 +800,11 @@ 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) {