Re: Linux guest kernel threat model for Confidential Computing

Jeremi Piotrowski
2023-02-02T14:51:54Z

On Tue, Jan 31, 2023 at 11:31:28AM +0000, Reshetova, Elena wrote:
> > On Mon, 2023-01-30 at 07:42 +0000, Reshetova, Elena wrote:
> > [...]
> > > > The big threat from most devices (including the thunderbolt
> > > > classes) is that they can DMA all over memory.  However, this isn't
> > > > really a threat in CC (well until PCI becomes able to do encrypted
> > > > DMA) because the device has specific unencrypted buffers set aside
> > > > for the expected DMA. If it writes outside that CC integrity will
> > > > detect it and if it reads outside that it gets unintelligible
> > > > ciphertext.  So we're left with the device trying to trick secrets
> > > > out of us by returning unexpected data.
> > >
> > > Yes, by supplying the input that hasn’t been expected. This is
> > > exactly the case we were trying to fix here for example:
> > > https://lore.kernel.org/all/20230119170633.40944-2-
> > alexander.shishkin@linux.intel.com/
> > > I do agree that this case is less severe when others where memory
> > > corruption/buffer overrun can happen, like here:
> > > https://lore.kernel.org/all/20230119135721.83345-6-
> > alexander.shishkin@linux.intel.com/
> > > But we are trying to fix all issues we see now (prioritizing the
> > > second ones though).
> >
> > I don't see how MSI table sizing is a bug in the category we've
> > defined. The very text of the changelog says "resulting in a kernel
> > page fault in pci_write_msg_msix()." which is a crash, which I thought
> > we were agreeing was out of scope for CC attacks?
>
> As I said this is an example of a crash and on the first look
> might not lead to the exploitable condition (albeit attackers are creative).
> But we noticed this one while fuzzing and it was common enough
> that prevented fuzzer going deeper into the virtio devices driver fuzzing.
> The core PCI/MSI doesn’t seem to have that many easily triggerable
> Other examples in virtio patchset are more severe.
>
> >
> > > >
> > > > If I set this as the problem, verifying device correct operation is
> > > > a possible solution (albeit hugely expensive) but there are likely
> > > > many other cheaper ways to defeat or detect a device trying to
> > > > trick us into revealing something.
> > >
> > > What do you have in mind here for the actual devices we need to
> > > enable for CC cases?
> >
> > Well, the most dangerous devices seem to be the virtio set a CC system
> > will rely on to boot up. After that, there are other ways (like SPDM)
> > to verify a real PCI device is on the other end of the transaction.
>
> Yes, it the future, but not yet. Other vendors will not necessary be
> using virtio devices at this point, so we will have non-virtio and not
> CC enabled devices that we want to securely add to the guest.
>
> >
> > > We have been using here a combination of extensive fuzzing and static
> > > code analysis.
> >
> > by fuzzing, I assume you mean fuzzing from the PCI configuration space?
> > Firstly I'm not so sure how useful a tool fuzzing is if we take Oopses
> > off the table because fuzzing primarily triggers those
>
> If you enable memory sanitizers you can detect more server conditions like
> out of bounds accesses and such. I think given that we have a way to
> verify that fuzzing is reaching the code locations we want it to reach, it
> can be pretty effective method to find at least low-hanging bugs. And these
> will be the bugs that most of the attackers will go after at the first place.
> But of course it is not a formal verification of any kind.
>
> so its hard to
> > see what else it could detect given the signal will be smothered by
> > oopses and secondly I think the PCI interface is likely the wrong place
> > to begin and you should probably begin on the virtio bus and the
> > hypervisor generated configuration space.
>
> This is exactly what we do. We don’t fuzz from the PCI config space,
> we supply inputs from the host/vmm via the legitimate interfaces that it can
> inject them to the guest: whenever guest requests a pci config space
> (which is controlled by host/hypervisor as you said) read operation,
> it gets input injected by the kafl fuzzer. Same for other interfaces that
> are under control of host/VMM (MSRs, port IO, MMIO, anything that goes
> via #VE handler in our case). When it comes to virtio, we employ
> two different fuzzing techniques: directly injecting kafl fuzz input when
> virtio core or virtio drivers gets the data received from the host
> (via injecting input in functions virtio16/32/64_to_cpu and others) and
> directly fuzzing DMA memory pages using kfx fuzzer.
> More information can be found in https://intel.github.io/ccc-linux-guest-hardening-docs/tdx-guest-hardening.html#td-guest-fuzzing
>
> Best Regards,
> Elena.

Hi Elena,

I think it might be a good idea to narrow down a configuration that *can*
reasonably be hardened to be suitable for confidential computing, before
proceeding with fuzzing. Eg. a lot of time was spent discussing PCI devices
in the context of virtualization, but what about taking PCI out of scope
completely by switching to virtio-mmio devices?

Jeremi