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From: Matthew Garrett <>
To: Laszlo Ersek <>
Cc: Ard Biesheuvel <>,
	linux-efi <>,
	"the arch/x86 maintainers" <>,
	linux-pci <>,
	Linux Kernel Mailing List <>
Subject: Re: [PATCH] [EFI,PCI] Allow disabling PCI busmastering on bridges during boot
Date: Tue, 3 Dec 2019 11:36:14 -0800
Message-ID: <> (raw)
In-Reply-To: <>

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

  reply index

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-03  0:40 Matthew Garrett
2019-12-03  0:42 ` Matthew Garrett
2019-12-03 11:54   ` Ard Biesheuvel
2019-12-03 13:38     ` Laszlo Ersek
2019-12-03 19:36       ` Matthew Garrett [this message]
2019-12-03 19:40     ` Matthew Garrett
2019-12-04  7:11       ` Laszlo Ersek
2019-12-04 19:29         ` Matthew Garrett
2019-12-03 15:30 ` Andy Lutomirski
2019-12-03 16:33   ` Ard Biesheuvel
2019-12-03 19:41   ` Matthew Garrett
2019-12-04 19:50     ` Andy Lutomirski
2019-12-04 19:56       ` Matthew Garrett
2019-12-12 15:46         ` Ard Biesheuvel
2019-12-13 21:24           ` Matthew Garrett
2019-12-03 18:23 ` kbuild test robot
2019-12-05 13:04 ` kbuild test robot

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