Re: [PATCH] [EFI,PCI] Allow disabling PCI busmastering on bridges during boot

[Matthew Garrett]

On Tue, Dec 3, 2019 at 5:38 AM Laszlo Ersek <lersek@xxxxxxxxxx> wrote:

> (2) I'm not 100% convinced this threat model -- I hope I'm using the
> right term -- is useful. A PCI device will likely not "itself" set up
> DMA (maliciously or not) without a matching driver.

A malicious PCI device can absolutely set up DMA itself without a
matching driver. There's a couple of cases:

1) A device that's entirely under the control of an attacker. Using
external Thunderbolt devices to overwrite OS data has been
demonstrated on multiple occasions.

2) A device that's been compromised in some way. The UEFI driver is a
long way from the only software that's related to the device -
discrete GPUs boot themselves even in the absence of a driver, and if
that on-board code can be compromised in any persistent way then they
can be used to attack the OS.

> Is this a scenario where we trust the device driver that comes from the
> device's ROM BAR (let's say after the driver passes Secure Boot
> verification and after we measure it into the TPM), but don't trust the
> silicon jammed in the motherboard that presents the driver?

Yes, though it's not just internal devices that we need to worry about.

> (3) I never understood why the default behavior (or rather, "only"
> behavior) for system firmware wrt. the IOMMU at EBS was "whitelist
> everything". Why not "blacklist everything"?
>
> I understand the compat perspective, but the OS should at least be able
> to request such a full blackout through OsIndications or whatever. (With
> the SEV IOMMU driver in OVMF, that's what we do -- we set everything to
> encrypted.)

I'm working on that, but it would be nice to have an approach for
existing systems.