[PATCH] drm/gem: Expose the buffer object handle to userspace last

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From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>

Currently drm_gem_handle_create_tail exposes the handle to userspace
before the buffer object constructions is complete. This allowing
of working against a partially constructed object, which may also be in
the process of having its creation fail, can have a range of negative

A lot of those will depend on what the individual drivers are doing in
their obj->funcs->open() callbacks, and also with a common failure mode
being -ENOMEM from drm_vma_node_allow.

We can make sure none of this can happen by allocating a handle last,
although with a downside that more of the function now runs under the

Looking into the individual drivers open() hooks, we have
amdgpu_gem_object_open which seems like it could have a potential security
issue without this change.

A couple drivers like qxl_gem_object_open and vmw_gem_object_open
implement no-op hooks so no impact for them.

A bunch of other require a deeper look by individual owners to asses for
impact. Those are lima_gem_object_open, nouveau_gem_object_open,
panfrost_gem_open, radeon_gem_object_open and virtio_gpu_gem_object_open.

Putting aside the risk assesment of the above, some common scenarios to
think about are along these lines:

Userspace closes a handle by speculatively "guessing" it from a second

This results in an unreachable buffer object so, a memory leak.

Same as 1), but object is in the process of getting closed (failed

The second thread is then able to re-cycle the handle and idr_remove would
in the first thread would then remove the handle it does not own from the

Going back to the earlier per driver problem space - individual impact
assesment of allowing a second thread to access and operate on a partially
constructed handle / object. (Can something crash? Leak information?)

In terms of identifying when the problem started I will tag some patches
as references, but not all, if even any, of them actually point to a
broken state. I am just identifying points at which more opportunity for
issues to arise was added.

References: 304eda32920b ("drm/gem: add hooks to notify driver when object handle is created/destroyed")
References: ca481c9b2a3a ("drm/gem: implement vma access management")
References: b39b5394fabc ("drm/gem: Add drm_gem_object_funcs")
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: Rob Clark <robdclark@xxxxxxxxxxxx>
Cc: Ben Skeggs <bskeggs@xxxxxxxxxx>
Cc: David Herrmann <dh.herrmann@xxxxxxxxx>
Cc: Noralf Trønnes <noralf@xxxxxxxxxxx>
Cc: David Airlie <airlied@xxxxxxxxx>
Cc: Daniel Vetter <daniel@xxxxxxxx>
Cc: amd-gfx@xxxxxxxxxxxxxxxxxxxxx
Cc: lima@xxxxxxxxxxxxxxxxxxxxx
Cc: nouveau@xxxxxxxxxxxxxxxxxxxxx
Cc: Steven Price <steven.price@xxxxxxx>
Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
Cc: spice-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: Zack Rusin <zackr@xxxxxxxxxx>
 drivers/gpu/drm/drm_gem.c | 48 +++++++++++++++++++--------------------
 1 file changed, 24 insertions(+), 24 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index aa15c52ae182..e3d897bca0f2 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -356,52 +356,52 @@ drm_gem_handle_create_tail(struct drm_file *file_priv,
 			   u32 *handlep)
 	struct drm_device *dev = obj->dev;
-	u32 handle;
 	int ret;
 	if (obj->handle_count++ == 0)
+	ret = drm_vma_node_allow(&obj->vma_node, file_priv);
+	if (ret)
+		goto err_put;
+	if (obj->funcs->open) {
+		ret = obj->funcs->open(obj, file_priv);
+		if (ret)
+			goto err_revoke;
+	}
-	 * Get the user-visible handle using idr.  Preload and perform
-	 * allocation under our spinlock.
+	 * Get the user-visible handle using idr as the _last_ step.
+	 * Preload and perform allocation under our spinlock.
 	ret = idr_alloc(&file_priv->object_idr, obj, 1, 0, GFP_NOWAIT);
-	mutex_unlock(&dev->object_name_lock);
 	if (ret < 0)
-		goto err_unref;
-	handle = ret;
+		goto err_close;
-	ret = drm_vma_node_allow(&obj->vma_node, file_priv);
-	if (ret)
-		goto err_remove;
+	mutex_unlock(&dev->object_name_lock);
-	if (obj->funcs->open) {
-		ret = obj->funcs->open(obj, file_priv);
-		if (ret)
-			goto err_revoke;
-	}
+	*handlep = ret;
-	*handlep = handle;
 	return 0;
+	if (obj->funcs->close)
+		obj->funcs->close(obj, file_priv);
 	drm_vma_node_revoke(&obj->vma_node, file_priv);
-	spin_lock(&file_priv->table_lock);
-	idr_remove(&file_priv->object_idr, handle);
-	spin_unlock(&file_priv->table_lock);
-	drm_gem_object_handle_put_unlocked(obj);
+	if (--obj->handle_count == 0)
+		drm_gem_object_put(obj);
+	mutex_unlock(&dev->object_name_lock);
 	return ret;

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