kthread: zero the kthread data structure

kthread() could bail out early before we initialize blkcg_css (if the
kthread is killed very early. Please see xchg() statement in kthread()),
which confuses free_kthread_struct. Instead of moving the blkcg_css
initialization early, we simply zero the whole 'self' data structure,
which doesn't sound much overhead.

Reported-by: syzbot <[email protected]>
Fixes: 05e3db95eb ("kthread: add a mechanism to store cgroup info")
Cc: Andrew Morton <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Signed-off-by: Shaohua Li <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

kernel/kthread.c

@@ -204,7 +204,7 @@ static int kthread(void *_create)
 	struct kthread *self;
 	int ret;
 
-	self = kmalloc(sizeof(*self), GFP_KERNEL);
+	self = kzalloc(sizeof(*self), GFP_KERNEL);
 	set_kthread_struct(self);
 
 	/* If user was SIGKILLed, I release the structure. */
@@ -220,13 +220,9 @@ static int kthread(void *_create)
 		do_exit(-ENOMEM);
 	}
 
-	self->flags = 0;
 	self->data = data;
 	init_completion(&self->exited);
 	init_completion(&self->parked);
-#ifdef CONFIG_BLK_CGROUP
-	self->blkcg_css = NULL;
-#endif
 	current->vfork_done = &self->exited;
 
 	/* OK, tell user we're spawned, wait for stop or wakeup */