[PATCH] drm/virtio: Fix GEM handle creation UAF

[Rob Clark]

From: Rob Clark <robdcl...@chromium.org>

Userspace can guess the handle value and try to race GEM object creation
with handle close, resulting in a use-after-free if we dereference the
object after dropping the handle's reference.  For that reason, dropping
the handle's reference must be done *after* we are done dereferencing
the object.

Signed-off-by: Rob Clark <robdcl...@chromium.org>
---
 drivers/gpu/drm/virtio/virtgpu_ioctl.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/virtio/virtgpu_ioctl.c 
b/drivers/gpu/drm/virtio/virtgpu_ioctl.c
index a5cccccb4998..f1c55c1630ca 100644
--- a/drivers/gpu/drm/virtio/virtgpu_ioctl.c
+++ b/drivers/gpu/drm/virtio/virtgpu_ioctl.c
@@ -366,10 +366,18 @@ static int virtio_gpu_resource_create_ioctl(struct 
drm_device *dev, void *data,
                drm_gem_object_release(obj);
                return ret;
        }
-       drm_gem_object_put(obj);
 
        rc->res_handle = qobj->hw_res_handle; /* similiar to a VM address */
        rc->bo_handle = handle;
+
+       /*
+        * The handle owns the reference now.  But we must drop our
+        * remaining reference *after* we no longer need to dereference
+        * the obj.  Otherwise userspace could guess the handle and
+        * race closing it from another thread.
+        */
+       drm_gem_object_put(obj);
+
        return 0;
 }
 
@@ -731,11 +739,18 @@ static int virtio_gpu_resource_create_blob_ioctl(struct 
drm_device *dev,
                drm_gem_object_release(obj);
                return ret;
        }
-       drm_gem_object_put(obj);
 
        rc_blob->res_handle = bo->hw_res_handle;
        rc_blob->bo_handle = handle;
 
+       /*
+        * The handle owns the reference now.  But we must drop our
+        * remaining reference *after* we no longer need to dereference
+        * the obj.  Otherwise userspace could guess the handle and
+        * race closing it from another thread.
+        */
+       drm_gem_object_put(obj);
+
        return 0;
 }
 
-- 
2.38.1