[PATCH 1/2] kthread: add barriers to set_kthread_struct() and to_kthread()

[Tejun Heo <tj@kernel.org>]

Until now, all to_kthread() users are interlocked with kthread
creation and there's no need to have explicit barriers when setting
the kthread pointer or dereferencing it.

However, There is a race condition where userland can interfere with a
kthread while it's being initialized.  To close it, to_kthread() needs
to be used from an unsynchronized context.

This patch moves struct kthread initialization before
set_kthread_struct() and adds matching barriers in
set_kthread_struct() and to_kthread(), so that dereferencing
to_kthread() always returns initialized fields.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Chris Mason <clm@fb.com>
Cc: stable@vger.kernel.org # v4.3+ (we can't close the race < v4.3)
---
 kernel/kthread.c |   30 +++++++++++++++++++++++-------
 1 file changed, 23 insertions(+), 7 deletions(-)

--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -57,6 +57,9 @@ enum KTHREAD_BITS {
 
 static inline void set_kthread_struct(void *kthread)
 {
+	/* paired with smp_read_data_barrier_depends() in to_kthread() */
+	smp_wmb();
+
 	/*
 	 * We abuse ->set_child_tid to avoid the new member and because it
 	 * can't be wrongly copied by copy_process(). We also rely on fact
@@ -67,8 +70,19 @@ static inline void set_kthread_struct(vo
 
 static inline struct kthread *to_kthread(struct task_struct *k)
 {
+	void *ptr;
+
 	WARN_ON(!(k->flags & PF_KTHREAD));
-	return (__force void *)k->set_child_tid;
+
+	ptr = (__force void *)k->set_child_tid;
+
+	/*
+	 * Paired with smp_wmb() in set_kthread_struct() and ensures that
+	 * the caller sees initialized content of the returned kthread.
+	 */
+	smp_read_barrier_depends();
+
+	return ptr;
 }
 
 void free_kthread_struct(struct task_struct *k)
@@ -196,6 +210,14 @@ static int kthread(void *_create)
 	int ret;
 
 	self = kmalloc(sizeof(*self), GFP_KERNEL);
+	if (self) {
+		self->flags = 0;
+		self->data = data;
+		init_completion(&self->exited);
+		init_completion(&self->parked);
+		current->vfork_done = &self->exited;
+	}
+
 	set_kthread_struct(self);
 
 	/* If user was SIGKILLed, I release the structure. */
@@ -211,12 +233,6 @@ static int kthread(void *_create)
 		do_exit(-ENOMEM);
 	}
 
-	self->flags = 0;
-	self->data = data;
-	init_completion(&self->exited);
-	init_completion(&self->parked);
-	current->vfork_done = &self->exited;
-
 	/* OK, tell user we're spawned, wait for stop or wakeup */
 	__set_current_state(TASK_UNINTERRUPTIBLE);
 	create->result = current;