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Message-ID: <20230106034146.GM4028633@paulmck-ThinkPad-P17-Gen-1>
Date: Thu, 5 Jan 2023 19:41:46 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Zqiang <qiang1.zhang@...el.com>
Cc: frederic@...nel.org, quic_neeraju@...cinc.com,
joel@...lfernandes.org, rcu@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: Safe access to rcu_node structure's->exp_tasks
On Sat, Dec 24, 2022 at 01:25:53PM +0800, Zqiang wrote:
> For kernels built with CONFIG_PREEMPT_RCU=y, the following scenario
> can result system oops.
>
> 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
> will trigger oops)
>
> This problem is that CPU2 accesses rcu_node structure's->exp_tasks
> 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,
> if rcu_node structure's->exp_tasks is set null pointer by CPU1, after
> that CPU2 accesses members of rcu_node structure's->exp_tasks will
> trigger oops.
>
> This commit therefore allows rcu_node structure's->exp_tasks to be
> accessed while holding rcu_node structure's ->lock.
>
> Signed-off-by: Zqiang <qiang1.zhang@...el.com>
Apologies for the delay and thank you for the reminder!
Please check the wordsmithed version below, which I have queued.
Thanx, Paul
------------------------------------------------------------------------
commit 389b0eafd72829fd63548f7ff4e8d6ac90fa1f98
Author: Zqiang <qiang1.zhang@...el.com>
Date: Sat Dec 24 13:25:53 2022 +0800
rcu: Protect rcu_print_task_exp_stall() ->exp_tasks access
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.
Signed-off-by: Zqiang <qiang1.zhang@...el.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 7cc4856da0817..902e7c8709c7e 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -803,9 +803,11 @@ 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 (!READ_ONCE(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) {
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