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Subject[PATCH] drm/virtio: Fix GEM handle creation UAF
Date
From: Rob Clark <robdclark@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 <robdclark@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
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