Unless you're distributing, I don't see how anybody could do anything. Personal (or company wide) use has always allowed the mixing of basically any licenses.
The worst case scenarios would be something like Ubuntu being unable to provide compiled modules, but dkms would still be fine. Or the very unlikely ZFS on Linux getting sued, but that would involve a lengthy trial that would allow you to move away from Open ZFS.
The danger is specifically to the copyright holders of Linux - the authors who have code in the kernel. If they do not defend their copyright, then it is not strong and can be broken in certain scenarios.
"Linux copyright holders in the GPL Compliance Project for Linux Developers believe that distribution of ZFS binaries is a GPL violation and infringes Linux's copyright."
Linux bundling ZFS code would bring this text against the GPL: "You may not offer or impose any terms on any Covered Software in Source Code form that alters or restricts the applicable version of [the CDDL]."
Ubuntu distributes ZFS as an out of tree module, which taints the kernel at immediately at installation. Hopefully, this is enough to prevent a great legal challenge.
I don't think anything Oracle can do would change the usability of OpenZFS.
I'm also not positive how easily they could fix the distribution problems given OpenZFS has over a decade of work after the split, but as a whole I'm only discussing use, not distribution.
Oracle could do what they did with DTrace in 2017, and release it under the GPL in addition to the CDDL. Then the OpenZFS folks would have to track down every contributor to OpenZFS, and get their permission to re-license their contribution. Contributions from folks who refused or couldn't be found would have to be re-implemented or dropped.
It is a pain in the ass, but it has been done before.
The worst case scenarios would be something like Ubuntu being unable to provide compiled modules, but dkms would still be fine. Or the very unlikely ZFS on Linux getting sued, but that would involve a lengthy trial that would allow you to move away from Open ZFS.