linux-kernel - Re: [RFC] thunderbolt: Automatically authorize PCIe tunnels when IOMMU is active

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Date:   Wed, 16 Mar 2022 08:30:04 +0200
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     Mario Limonciello <mario.limonciello@....com>
Cc:     Andreas Noever <andreas.noever@...il.com>,
        Michael Jamet <michael.jamet@...el.com>,
        Yehezkel Bernat <YehezkelShB@...il.com>,
        "open list:THUNDERBOLT DRIVER" <linux-usb@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] thunderbolt: Automatically authorize PCIe tunnels when
 IOMMU is active

Hi Mario,

On Tue, Mar 15, 2022 at 04:30:08PM -0500, Mario Limonciello wrote:
> Historically TBT3 in Linux used "Thunderbolt security levels" as a primary
> means of "security" against DMA attacks. This mean that users would need to
> ack any device plugged in via userspace.  In ~2018 machines started to use
> the IOMMU for protection, but instead of dropping security levels a
> convoluted flow was introduced:
> * User hotplugs device
> * Driver discovers supported tunnels
> * Driver emits a uevent to userspace that a PCIe tunnel is present
> * Userspace reads 'iommu_dma_protection' attribute (which currently
>   indicates an Intel IOMMU is present and was enabled pre-boot not that
>   it's active "now")
> * Based on that value userspace then authorizes automatically or prompts
>   the user like how security level based support worked.

There are legitimate reasons to disable PCIe tunneling even if the IOMMU
bits are in place. The ACPI _OSC allows the boot firmware to do so and
our "security levels" allows the userspace policy to do the same. I
would not like to change that unless absolutely necessary.