android: binder: Don't get mm from task

Use binder_alloc struct's mm_struct rather than getting
a reference to the mm struct through get_task_mm to
avoid a potential deadlock between lru lock, task lock and
dentry lock, since a thread can be holding the task lock
and the dentry lock while trying to acquire the lru lock.

Acked-by: Arve Hjønnevåg <[e-mail]>
Signed-off-by: Sherry Yang <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/android/binder_alloc.c

···
 		}
 	}
 
-	if (!vma && need_mm)
-		mm = get_task_mm(alloc->tsk);
+	if (!vma && need_mm && mmget_not_zero(alloc->vma_vm_mm))
+		mm = alloc->vma_vm_mm;
 
 	if (mm) {
 		down_write(&mm->mmap_sem);
 		vma = alloc->vma;
-		if (vma && mm != alloc->vma_vm_mm) {
-			pr_err("%d: vma mm and task mm mismatch\n",
-				alloc->pid);
-			vma = NULL;
-		}
 	}
 
 	if (!vma && need_mm) {
···
 	barrier();
 	alloc->vma = vma;
 	alloc->vma_vm_mm = vma->vm_mm;
+	mmgrab(alloc->vma_vm_mm);
 
 	return 0;
 
···
 		vfree(alloc->buffer);
 	}
 	mutex_unlock(&alloc->mutex);
+	if (alloc->vma_vm_mm)
+		mmdrop(alloc->vma_vm_mm);
 
 	binder_alloc_debug(BINDER_DEBUG_OPEN_CLOSE,
 		     "%s: %d buffers %d, pages %d\n",
···
 void binder_alloc_vma_close(struct binder_alloc *alloc)
 {
 	WRITE_ONCE(alloc->vma, NULL);
-	WRITE_ONCE(alloc->vma_vm_mm, NULL);
 }
 
 /**
···
 	page_addr = (uintptr_t)alloc->buffer + index * PAGE_SIZE;
 	vma = alloc->vma;
 	if (vma) {
-		mm = get_task_mm(alloc->tsk);
-		if (!mm)
-			goto err_get_task_mm_failed;
+		if (!mmget_not_zero(alloc->vma_vm_mm))
+			goto err_mmget;
+		mm = alloc->vma_vm_mm;
 		if (!down_write_trylock(&mm->mmap_sem))
 			goto err_down_write_mmap_sem_failed;
 	}
···
 
 err_down_write_mmap_sem_failed:
 	mmput_async(mm);
-err_get_task_mm_failed:
+err_mmget:
 err_page_already_freed:
 	mutex_unlock(&alloc->mutex);
 err_get_alloc_mutex_failed:
···
  */
 void binder_alloc_init(struct binder_alloc *alloc)
 {
-	alloc->tsk = current->group_leader;
 	alloc->pid = current->group_leader->pid;
 	mutex_init(&alloc->mutex);
 	INIT_LIST_HEAD(&alloc->buffers);

drivers/android/binder_alloc.h

···
  */
 struct binder_alloc {
 	struct mutex mutex;
-	struct task_struct *tsk;
 	struct vm_area_struct *vma;
 	struct mm_struct *vma_vm_mm;
 	void *buffer;