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) {