drm/i915: Pin pages whilst allocating for dma-buf vmap()

During the vmap() routine for the dma-buf, we first grab the pages and
then try to allocate a temporary array to pass to the vmap(). However,
the shrinker can and will reap any object that is unbound if the
allocation for the array first fails. This includes the object which we
are attempting to vmap(). The solution is to mark the object's pages as
pinned whilst we try the allocation to prevent the use-after-free
introduced by the potential shrinkage.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_dmabuf.c

@@ -125,13 +125,15 @@ static void *i915_gem_dmabuf_vmap(struct dma_buf *dma_buf)
 
 	ret = i915_gem_object_get_pages(obj);
 	if (ret)
-		goto error;
+		goto err;
+
+	i915_gem_object_pin_pages(obj);
 
 	ret = -ENOMEM;
 
 	pages = drm_malloc_ab(obj->base.size >> PAGE_SHIFT, sizeof(*pages));
 	if (pages == NULL)
-		goto error;
+		goto err_unpin;
 
 	i = 0;
 	for_each_sg_page(obj->pages->sgl, &sg_iter, obj->pages->nents, 0)
@@ -141,15 +143,16 @@ static void *i915_gem_dmabuf_vmap(struct dma_buf *dma_buf)
 	drm_free_large(pages);
 
 	if (!obj->dma_buf_vmapping)
-		goto error;
+		goto err_unpin;
 
 	obj->vmapping_count = 1;
-	i915_gem_object_pin_pages(obj);
 out_unlock:
 	mutex_unlock(&dev->struct_mutex);
 	return obj->dma_buf_vmapping;
 
-error:
+err_unpin:
+	i915_gem_object_unpin_pages(obj);
+err:
 	mutex_unlock(&dev->struct_mutex);
 	return ERR_PTR(ret);
 }