linux-kernel - [PATCH 1/1] proc_fork_connector: a lockless ->real

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Date:	Thu, 21 Jul 2011 20:35:11 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Vladimir Zapolskiy <vzapolskiy@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Evgeniy Polyakov <zbr@...emap.net>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 1/1] proc_fork_connector: a lockless ->real_parent usage is
	not safe

proc_fork_connector() uses ->real_parent lockless. This is not safe
if copy_process() was called with CLONE_THREAD or CLONE_PARENT, in
this case the parent != current can go away at any moment.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---

 drivers/connector/cn_proc.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

--- ts/drivers/connector/cn_proc.c~proc_fork_connector_parent	2011-04-06 21:33:43.000000000 +0200
+++ ts/drivers/connector/cn_proc.c	2011-07-21 20:24:09.000000000 +0200
@@ -55,6 +55,7 @@ void proc_fork_connector(struct task_str
 	struct proc_event *ev;
 	__u8 buffer[CN_PROC_MSG_SIZE];
 	struct timespec ts;
+	struct task_struct *parent;
 
 	if (atomic_read(&proc_event_num_listeners) < 1)
 		return;
@@ -65,8 +66,11 @@ void proc_fork_connector(struct task_str
 	ktime_get_ts(&ts); /* get high res monotonic timestamp */
 	put_unaligned(timespec_to_ns(&ts), (__u64 *)&ev->timestamp_ns);
 	ev->what = PROC_EVENT_FORK;
-	ev->event_data.fork.parent_pid = task->real_parent->pid;
-	ev->event_data.fork.parent_tgid = task->real_parent->tgid;
+	rcu_read_lock();
+	parent = rcu_dereference(task->real_parent);
+	ev->event_data.fork.parent_pid = parent->pid;
+	ev->event_data.fork.parent_tgid = parent->tgid;
+	rcu_read_unlock();
 	ev->event_data.fork.child_pid = task->pid;
 	ev->event_data.fork.child_tgid = task->tgid;
 

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