drm/vmwgfx: Fix up user_dmabuf refcounting
commit 54c12bc3 upstream. If user space calls unreference on a user_dmabuf it will typically kill the struct ttm_base_object member which is responsible for the user-space visibility. However the dmabuf part may still be alive and refcounted. In some situations, like for shared guest-backed surface referencing/opening, the driver may try to reference the struct ttm_base_object member again, causing an immediate kernel warning and a later kernel NULL pointer dereference. Fix this by always maintaining a reference on the struct ttm_base_object member, in situations where it might subsequently be referenced. Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com> Reviewed-by: Brian Paul <brianp@vmware.com> Reviewed-by: Sinclair Yeh <syeh@vmware.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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