drm/panfrost: Fix GEM handle creation ref-counting

panfrost_gem_create_with_handle() previously returned a BO but with the
only reference being from the handle, which user space could in theory
guess and release, causing a use-after-free. Additionally if the call to
panfrost_gem_mapping_get() in panfrost_ioctl_create_bo() failed then
a(nother) reference on the BO was dropped.

The _create_with_handle() is a problematic pattern, so ditch it and
instead create the handle in panfrost_ioctl_create_bo(). If the call to
panfrost_gem_mapping_get() fails then this means that user space has
indeed gone behind our back and freed the handle. In which case just
return an error code.
Reported-by: Rob Clark <robdclark@chromium.org>
Fixes: f3ba9122 ("drm/panfrost: Add initial panfrost driver")
Signed-off-by: Steven Price <steven.price@arm.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221219140130.410578-1-steven.price@arm.com

drivers/gpu/drm/panfrost/panfrost_drv.c

@@ -82,6 +82,7 @@ static int panfrost_ioctl_create_bo(struct drm_device *dev, void *data,
 	struct panfrost_gem_object *bo;
 	struct drm_panfrost_create_bo *args = data;
 	struct panfrost_gem_mapping *mapping;
+	int ret;
 
 	if (!args->size || args->pad ||
 	    (args->flags & ~(PANFROST_BO_NOEXEC | PANFROST_BO_HEAP)))
@@ -92,21 +93,29 @@ static int panfrost_ioctl_create_bo(struct drm_device *dev, void *data,
 	    !(args->flags & PANFROST_BO_NOEXEC))
 		return -EINVAL;
 
-	bo = panfrost_gem_create_with_handle(file, dev, args->size, args->flags,
-					     &args->handle);
+	bo = panfrost_gem_create(dev, args->size, args->flags);
 	if (IS_ERR(bo))
 		return PTR_ERR(bo);
 
+	ret = drm_gem_handle_create(file, &bo->base.base, &args->handle);
+	if (ret)
+		goto out;
+
 	mapping = panfrost_gem_mapping_get(bo, priv);
-	if (!mapping) {
-		drm_gem_object_put(&bo->base.base);
-		return -EINVAL;
+	if (mapping) {
+		args->offset = mapping->mmnode.start << PAGE_SHIFT;
+		panfrost_gem_mapping_put(mapping);
+	} else {
+		/* This can only happen if the handle from
+		 * drm_gem_handle_create() has already been guessed and freed
+		 * by user space
+		 */
+		ret = -EINVAL;
 	}
 
-	args->offset = mapping->mmnode.start << PAGE_SHIFT;
-	panfrost_gem_mapping_put(mapping);
-
-	return 0;
+out:
+	drm_gem_object_put(&bo->base.base);
+	return ret;
 }
 
 /**

drivers/gpu/drm/panfrost/panfrost_gem.c

@@ -235,12 +235,8 @@ struct drm_gem_object *panfrost_gem_create_object(struct drm_device *dev, size_t
 }
 
 struct panfrost_gem_object *
-panfrost_gem_create_with_handle(struct drm_file *file_priv,
-				struct drm_device *dev, size_t size,
-				u32 flags,
-				uint32_t *handle)
+panfrost_gem_create(struct drm_device *dev, size_t size, u32 flags)
 {
-	int ret;
 	struct drm_gem_shmem_object *shmem;
 	struct panfrost_gem_object *bo;
 
@@ -256,16 +252,6 @@ panfrost_gem_create_with_handle(struct drm_file *file_priv,
 	bo->noexec = !!(flags & PANFROST_BO_NOEXEC);
 	bo->is_heap = !!(flags & PANFROST_BO_HEAP);
 
-	/*
-	 * Allocate an id of idr table where the obj is registered
-	 * and handle has the id what user can see.
-	 */
-	ret = drm_gem_handle_create(file_priv, &shmem->base, handle);
-	/* drop reference from allocate - handle holds it now. */
-	drm_gem_object_put(&shmem->base);
-	if (ret)
-		return ERR_PTR(ret);
-
 	return bo;
 }
 

drivers/gpu/drm/panfrost/panfrost_gem.h

@@ -69,10 +69,7 @@ panfrost_gem_prime_import_sg_table(struct drm_device *dev,
 				   struct sg_table *sgt);
 
 struct panfrost_gem_object *
-panfrost_gem_create_with_handle(struct drm_file *file_priv,
-				struct drm_device *dev, size_t size,
-				u32 flags,
-				uint32_t *handle);
+panfrost_gem_create(struct drm_device *dev, size_t size, u32 flags);
 
 int panfrost_gem_open(struct drm_gem_object *obj, struct drm_file *file_priv);
 void panfrost_gem_close(struct drm_gem_object *obj,