Re: [PATCH v2 08/18] PCI/CMA: Authenticate devices on enumeration

Damien Le Moal
2024-07-15T22:18:00Z

On 7/16/24 07:02, Jason Gunthorpe wrote:
> On Mon, Jul 15, 2024 at 01:36:32PM -0700, Dan Williams wrote:
>> Jason Gunthorpe wrote:
>>> On Mon, Jul 15, 2024 at 10:21:48AM -0700, Kees Cook wrote:
>>>
>>>> Anyway, following the threat model, it doesn't seem like half measures
>>>> make any sense. If the threat model is "we cannot trust bus members" and
>>>> authentication is being used to establish trust, then anything else must
>>>> be explicitly excluded. If this can only be done via the described
>>>> firewalling, then that really does seem to be the right choice.
>>>
>>> There is supposed to be a state machine here, devices start up at VM
>>> time 0 unable to DMA to secure guest memory under any conditions. This
>>> property must be enforced by the trusted platform.
>>>
>>> Further the trusted plaform is supposed to prevent "replacement"
>>> attacks, so once the VM says it trusts a device it cannot be replaced
>>> with something else.
>>>
>>> When the guest decides it would like the device to reach secure memory
>>> the trusted platform is part of making that happen.
>>>
>>> From a kernel and lifecycle perspective we need a bunch of new options
>>> for PCI devices that should be triggered after userspace has had a
>>> look at the device.
>>>
>>> - A device is just forbidden from anything using it
>>> - A device used only with untrusted memory
>>> - A device is usable with trusted memory
>>>
>>> IMHO this determination needs to be made before the device driver is
>>> bound.
>>
>> Yes, and it depends on the device. Some devices should be filtered
>> early, some devices need to be operated against untrusted memory just
>> to get to the point where they can complete the acceptance flow into the
>> TCB.
>
> Operating a device with both trusted and untrusted iommu
> configurations is complex to manage and depends on how the trusted
> iommu HW works.
>
>> The motivation for the security policy is "there is trusted memory to
>> protect". Absent trusted memory, the status quo for the device-driver
>> model applies.
>
> From what I can see on some platforms/configurations if the device is
> trusted capable then it MUST only issue trusted DMA as that is the
> only IO translation that will work.
>
> Meaning the decision to operate a device as trusted or not really has
> to be done before any driver is probed and probably needs to involve
> the iommu layer to try and do something about this mess in some way.

As I commented already, for storage device using SPDM-over-storage (scsi/ata and
nvme devices), the device authentication requires the device driver since we
need a gendisk and request queue to be able to issue the commands transporting
SPDM messages. So this is applicable only to PCI devices.

Of note though is that in the case of SCSI/ATA storage, the device (the HDD or
SSD) is not the one doing DMA directly to the host/guest memory. That is the
adapter (the HBA). So we could ask ourselves if it makes sense to authenticate
storage devices without the HBA being authenticated first.

And for PCI nvme devices that can support SPDM either through either PCI DOE or
SPDM-over-storage (SECURITY SEND/RECV commands), then I guess we need some
special handling/config to allow (or not) SPDM-over-storage authentication as
that will require the device driver to be loaded and to execute some commands
before authentication can happen.

> I have yet to see a complete plan for how these details should work :)
>
> And I only know in detail how the iommu works for one platform, not
> the others, so I don't know how prevalent these concerns are..
>
> Jason

--
Damien Le Moal
Western Digital Research