RE: Linux guest kernel threat model for Confidential Computing

Reshetova, Elena
2023-01-31T16:34:52Z

> On Tue, 2023-01-31 at 11:31 +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.
>
> You cited this as your example. I'm pointing out it seems to be an
> event of the class we've agreed not to consider because it's an oops
> not an exploit. If there are examples of fixing actual exploits to CC
> VMs, what are they?
>
> This patch is, however, an example of the problem everyone else on the
> thread is complaining about: a patch which adds an unnecessary check to
> the MSI subsystem; unnecessary because it doesn't fix a CC exploit and
> in the real world the tables are correct (or the manufacturer is
> quickly chastened), so it adds overhead to no benefit.

How can you make sure there is no exploit possible using this crash
as a stepping stone into a CC guest? Or are you saying that we are back
to the times when we can merge the fixes for crashes and out of bound errors in
kernel only given that we submit a proof of concept exploit with the
patch for every issue?

>
>
> [...]
> > > 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
>
> Given that we previously agreed that oppses and other DoS attacks are
> out of scope for CC, I really don't think fuzzing, which primarily
> finds oopses, is at all a useful tool unless you filter the results by
> the question "could we exploit this in a CC VM to reveal secrets".
> Without applying that filter you're sending a load of patches which
> don't really do much to reduce the CC attack surface and which do annoy
> non-CC people because they add pointless checks to things they expect
> the cards and config tables to get right.

I don’t think we have agreed that random kernel crashes are out of scope in CC threat model
(controlled safe panic is out of scope, but this is not what we have here).
It all depends if this ops can be used in a successful attack against guest private
memory or not and this is *not* a trivial thing to decide.
That's said, we are mostly focusing on KASAN findings, which
have higher likelihood to be exploitable at least for host -> guest privilege escalation
(which in turn compromised guest private memory confidentiality). Fuzzing has a
long history of find such issues in past (including the ones that have been
exploited after). But even for this ops bug, can anyone guarantee it cannot be chained
with other ones to cause a more complex privilege escalation attack? I wont be making
such a claim, I feel it is safer to fix this vs debating whenever it can be used for an
attack or not.

Best Regards,
Elena.