netdev - [patch 2/2] proc_fork_connector: a lockless ->real

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Message-Id: <201107282054.p6SKsPtJ007979@imap1.linux-foundation.org>
Date:	Thu, 28 Jul 2011 13:54:25 -0700
From:	akpm@...ux-foundation.org
To:	davem@...emloft.net
Cc:	netdev@...r.kernel.org, akpm@...ux-foundation.org, oleg@...hat.com,
	johnpol@....mipt.ru, vzapolskiy@...il.com, zbr@...emap.net
Subject: [patch 2/2] proc_fork_connector: a lockless ->real_parent usage is not safe

From: Oleg Nesterov <oleg@...hat.com>

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>
Cc: Vladimir Zapolskiy <vzapolskiy@...il.com>
Cc: "David S. Miller" <davem@...emloft.net>
Cc: Evgeniy Polyakov <zbr@...emap.net>
Cc: Evgeniy Polyakov <johnpol@....mipt.ru>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
---

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

diff -puN drivers/connector/cn_proc.c~proc_fork_connector-a-lockless-real_parent-usage-is-not-safe drivers/connector/cn_proc.c
--- a/drivers/connector/cn_proc.c~proc_fork_connector-a-lockless-real_parent-usage-is-not-safe
+++ a/drivers/connector/cn_proc.c
@@ -57,6 +57,7 @@ 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;
@@ -67,8 +68,11 @@ 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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