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

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Date:   Tue, 3 Dec 2019 11:36:14 -0800
From:   Matthew Garrett <mjg59@...gle.com>
To:     Laszlo Ersek <lersek@...hat.com>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [EFI,PCI] Allow disabling PCI busmastering on bridges
 during boot

On Tue, Dec 3, 2019 at 5:38 AM Laszlo Ersek <lersek@...hat.com> 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.