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

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Date:   Wed, 16 Mar 2022 16:39:30 +0200
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     "Limonciello, Mario" <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

On Wed, Mar 16, 2022 at 01:48:49PM +0000, Limonciello, Mario wrote:
> [Public]
> 
> > > > 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.
> > >
> > > Actually I intentionally left that in the RFC patch, to only do this based off
> > > of tb_acpi_may_tunnel_pcie, so I think that should still work as you
> > described
> > > if boot firmware turned off PCIe tunneling.
> > 
> > Right but if the user still wants to disable it, like say you are
> > travelling and you want to be sure that no PCIe devices get attached
> > while your laptop is charging from a public "charging station" (whatever
> > is the right term).
> 
> So wouldn't you flip the default in BIOS setup to disable PCIe tunnels then for
> this use case?

What if you are on Chromebook? Or something where this is not user
configurable?

> Otherwise with how it is today you end up with the PCIe tunnel created in the
> boot FW and then coming into the OS if it's the same path the tunnel stays
> in place with no opportunity for userspace to authorize it, no?

The boot FW does not need to support CM capabilites nor does it need to
provide the ACPI _OSC.