[PATCH 5.15 16/86] drm/virtio: Fix GEM handle creation UAF

[Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>]

From: Rob Clark <robdclark@xxxxxxxxxxxx>

commit 52531258318ed59a2dc5a43df2eaf0eb1d65438e upstream.

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 <robdclark@xxxxxxxxxxxx>
Reviewed-by: Chia-I Wu <olvaffe@xxxxxxxxx>
Fixes: 62fb7a5e1096 ("virtio-gpu: add 3d/virgl support")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
Link: https://patchwork.freedesktop.org/patch/msgid/20221216233355.542197-2-robdclark@xxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/gpu/drm/virtio/virtgpu_ioctl.c |   19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

--- a/drivers/gpu/drm/virtio/virtgpu_ioctl.c
+++ b/drivers/gpu/drm/virtio/virtgpu_ioctl.c
@@ -292,10 +292,18 @@ static int virtio_gpu_resource_create_io
 		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;
 }
 
@@ -656,11 +664,18 @@ static int virtio_gpu_resource_create_bl
 		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;
 }