Date:   Mon, 24 Oct 2016 18:08:13 +0200
From:   Roman Pen <roman.penyaev@...fitbricks.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     Roman Pen <roman.penyaev@...fitbricks.com>,
        Andy Lutomirski <luto@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2 1/2] kthread: allocate kthread structure using kmalloc

This patch avoids allocation of kthread structure on a stack, and simply
uses kmalloc.  Allocation on a stack became a huge problem (with memory
corruption and all other not nice consequences) after the commit 2deb4be28
by Andy Lutomirski, which rewinds the stack on oops, thus ooopsed kthread
steps on a garbage memory while completion of task->vfork_done structure
on the following path:

       oops_end()
       rewind_stack_do_exit()
       exit_mm()
       mm_release()
       complete_vfork_done()

Also in this patch two structures 'struct kthread_create_info' and
'struct kthread' are merged into one 'struct kthread' and its freeing
is controlled by a reference counter.

The last reference on kthread is put from a task work, the callback,
which is invoked from do_exit().  The major thing is that the last
put is happens *after* completion_vfork_done() is invoked.

Signed-off-by: Roman Pen <roman.penyaev@...fitbricks.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Tejun Heo <tj@...nel.org>
Cc: linux-kernel@...r.kernel.org
---
v2:
  o let x86/kernel/dumpstack.c rewind a stack, but do not use a stack
    for a structure allocation.

 kernel/kthread.c | 160 +++++++++++++++++++++++++++++++------------------------
 1 file changed, 90 insertions(+), 70 deletions(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 4ab4c37..9ccfe06 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -18,14 +18,19 @@
 #include <linux/freezer.h>
 #include <linux/ptrace.h>
 #include <linux/uaccess.h>
+#include <linux/task_work.h>
 #include <trace/events/sched.h>
 
 static DEFINE_SPINLOCK(kthread_create_lock);
 static LIST_HEAD(kthread_create_list);
 struct task_struct *kthreadd_task;
 
-struct kthread_create_info
-{
+struct kthread {
+	struct list_head list;
+	unsigned long flags;
+	unsigned int cpu;
+	atomic_t refs;
+
 	/* Information passed to kthread() from kthreadd. */
 	int (*threadfn)(void *data);
 	void *data;
@@ -33,15 +38,9 @@ struct kthread_create_info
 
 	/* Result passed back to kthread_create() from kthreadd. */
 	struct task_struct *result;
-	struct completion *done;
-
-	struct list_head list;
-};
 
-struct kthread {
-	unsigned long flags;
-	unsigned int cpu;
-	void *data;
+	struct callback_head put_work;
+	struct completion *started;
 	struct completion parked;
 	struct completion exited;
 };
@@ -69,6 +68,24 @@ static struct kthread *to_live_kthread(struct task_struct *k)
 	return NULL;
 }
 
+static inline void put_kthread(struct kthread *kthread)
+{
+	if (atomic_dec_and_test(&kthread->refs))
+		kfree(kthread);
+}
+
+/**
+ * put_kthread_cb - is called from do_exit() and does likely
+ *                  the final put.
+ */
+static void put_kthread_cb(struct callback_head *work)
+{
+	struct kthread *kthread;
+
+	kthread = container_of(work, struct kthread, put_work);
+	put_kthread(kthread);
+}
+
 /**
  * kthread_should_stop - should this kthread return now?
  *
@@ -174,41 +191,36 @@ void kthread_parkme(void)
 }
 EXPORT_SYMBOL_GPL(kthread_parkme);
 
-static int kthread(void *_create)
+static int kthreadfn(void *_self)
 {
-	/* Copy data: it's on kthread's stack */
-	struct kthread_create_info *create = _create;
-	int (*threadfn)(void *data) = create->threadfn;
-	void *data = create->data;
-	struct completion *done;
-	struct kthread self;
-	int ret;
-
-	self.flags = 0;
-	self.data = data;
-	init_completion(&self.exited);
-	init_completion(&self.parked);
-	current->vfork_done = &self.exited;
-
-	/* If user was SIGKILLed, I release the structure. */
-	done = xchg(&create->done, NULL);
-	if (!done) {
-		kfree(create);
-		do_exit(-EINTR);
+	struct completion *started;
+	struct kthread *self = _self;
+	int ret = -EINTR;
+
+	/* If user was SIGKILLed, put a ref and exit silently. */
+	started = xchg(&self->started, NULL);
+	if (!started) {
+		put_kthread(self);
+		goto exit;
 	}
+	/* Delegate last ref put to a task work, which will happen
+	 * after 'vfork_done' completion.
+	 */
+	init_task_work(&self->put_work, put_kthread_cb);
+	task_work_add(current, &self->put_work, false);
+	current->vfork_done = &self->exited;
+
 	/* OK, tell user we're spawned, wait for stop or wakeup */
 	__set_current_state(TASK_UNINTERRUPTIBLE);
-	create->result = current;
-	complete(done);
+	self->result = current;
+	complete(started);
 	schedule();
 
-	ret = -EINTR;
-
-	if (!test_bit(KTHREAD_SHOULD_STOP, &self.flags)) {
-		__kthread_parkme(&self);
-		ret = threadfn(data);
+	if (!test_bit(KTHREAD_SHOULD_STOP, &self->flags)) {
+		__kthread_parkme(self);
+		ret = self->threadfn(self->data);
 	}
-	/* we can't just return, we must preserve "self" on stack */
+exit:
 	do_exit(ret);
 }
 
@@ -222,25 +234,24 @@ int tsk_fork_get_node(struct task_struct *tsk)
 	return NUMA_NO_NODE;
 }
 
-static void create_kthread(struct kthread_create_info *create)
+static void create_kthread(struct kthread *kthread)
 {
+	struct completion *started;
 	int pid;
 
 #ifdef CONFIG_NUMA
-	current->pref_node_fork = create->node;
+	current->pref_node_fork = kthread->node;
 #endif
 	/* We want our own signal handler (we take no signals by default). */
-	pid = kernel_thread(kthread, create, CLONE_FS | CLONE_FILES | SIGCHLD);
+	pid = kernel_thread(kthreadfn, kthread,
+			    CLONE_FS | CLONE_FILES | SIGCHLD);
 	if (pid < 0) {
-		/* If user was SIGKILLed, I release the structure. */
-		struct completion *done = xchg(&create->done, NULL);
-
-		if (!done) {
-			kfree(create);
-			return;
+		started = xchg(&kthread->started, NULL);
+		if (started) {
+			kthread->result = ERR_PTR(pid);
+			complete(started);
 		}
-		create->result = ERR_PTR(pid);
-		complete(done);
+		put_kthread(kthread);
 	}
 }
 
@@ -272,20 +283,26 @@ struct task_struct *kthread_create_on_node(int (*threadfn)(void *data),
 					   const char namefmt[],
 					   ...)
 {
-	DECLARE_COMPLETION_ONSTACK(done);
+	DECLARE_COMPLETION_ONSTACK(started);
 	struct task_struct *task;
-	struct kthread_create_info *create = kmalloc(sizeof(*create),
-						     GFP_KERNEL);
+	struct kthread *kthread;
 
-	if (!create)
+	kthread = kmalloc(sizeof(*kthread), GFP_KERNEL);
+	if (!kthread)
 		return ERR_PTR(-ENOMEM);
-	create->threadfn = threadfn;
-	create->data = data;
-	create->node = node;
-	create->done = &done;
+	/* One ref for us and one ref for a new kernel thread. */
+	atomic_set(&kthread->refs, 2);
+	kthread->flags = 0;
+	kthread->cpu = 0;
+	kthread->threadfn = threadfn;
+	kthread->data = data;
+	kthread->node = node;
+	kthread->started = &started;
+	init_completion(&kthread->exited);
+	init_completion(&kthread->parked);
 
 	spin_lock(&kthread_create_lock);
-	list_add_tail(&create->list, &kthread_create_list);
+	list_add_tail(&kthread->list, &kthread_create_list);
 	spin_unlock(&kthread_create_lock);
 
 	wake_up_process(kthreadd_task);
@@ -294,21 +311,23 @@ struct task_struct *kthread_create_on_node(int (*threadfn)(void *data),
 	 * the OOM killer while kthreadd is trying to allocate memory for
 	 * new kernel thread.
 	 */
-	if (unlikely(wait_for_completion_killable(&done))) {
+	if (unlikely(wait_for_completion_killable(&started))) {
 		/*
 		 * If I was SIGKILLed before kthreadd (or new kernel thread)
-		 * calls complete(), leave the cleanup of this structure to
-		 * that thread.
+		 * calls complete(), put a ref and return an error.
 		 */
-		if (xchg(&create->done, NULL))
+		if (xchg(&kthread->started, NULL)) {
+			put_kthread(kthread);
+
 			return ERR_PTR(-EINTR);
+		}
 		/*
 		 * kthreadd (or new kernel thread) will call complete()
 		 * shortly.
 		 */
-		wait_for_completion(&done);
+		wait_for_completion(&started);
 	}
-	task = create->result;
+	task = kthread->result;
 	if (!IS_ERR(task)) {
 		static const struct sched_param param = { .sched_priority = 0 };
 		va_list args;
@@ -323,7 +342,8 @@ struct task_struct *kthread_create_on_node(int (*threadfn)(void *data),
 		sched_setscheduler_nocheck(task, SCHED_NORMAL, &param);
 		set_cpus_allowed_ptr(task, cpu_all_mask);
 	}
-	kfree(create);
+	put_kthread(kthread);
+
 	return task;
 }
 EXPORT_SYMBOL(kthread_create_on_node);
@@ -523,14 +543,14 @@ int kthreadd(void *unused)
 
 		spin_lock(&kthread_create_lock);
 		while (!list_empty(&kthread_create_list)) {
-			struct kthread_create_info *create;
+			struct kthread *kthread;
 
-			create = list_entry(kthread_create_list.next,
-					    struct kthread_create_info, list);
-			list_del_init(&create->list);
+			kthread = list_entry(kthread_create_list.next,
+					     struct kthread, list);
+			list_del_init(&kthread->list);
 			spin_unlock(&kthread_create_lock);
 
-			create_kthread(create);
+			create_kthread(kthread);
 
 			spin_lock(&kthread_create_lock);
 		}
-- 
2.9.3

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