308 lines
14 KiB
ReStructuredText
308 lines
14 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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======================
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USB4 and Thunderbolt
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======================
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USB4 is the public specification based on Thunderbolt 3 protocol with
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some differences at the register level among other things. Connection
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manager is an entity running on the host router (host controller)
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responsible for enumerating routers and establishing tunnels. A
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connection manager can be implemented either in firmware or software.
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Typically PCs come with a firmware connection manager for Thunderbolt 3
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and early USB4 capable systems. Apple systems on the other hand use
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software connection manager and the later USB4 compliant devices follow
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the suit.
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The Linux Thunderbolt driver supports both and can detect at runtime which
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connection manager implementation is to be used. To be on the safe side the
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software connection manager in Linux also advertises security level
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``user`` which means PCIe tunneling is disabled by default. The
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documentation below applies to both implementations with the exception that
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the software connection manager only supports ``user`` security level and
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is expected to be accompanied with an IOMMU based DMA protection.
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Security levels and how to use them
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-----------------------------------
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The interface presented here is not meant for end users. Instead there
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should be a userspace tool that handles all the low-level details, keeps
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a database of the authorized devices and prompts users for new connections.
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More details about the sysfs interface for Thunderbolt devices can be
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found in ``Documentation/ABI/testing/sysfs-bus-thunderbolt``.
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Those users who just want to connect any device without any sort of
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manual work can add following line to
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``/etc/udev/rules.d/99-local.rules``::
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ACTION=="add", SUBSYSTEM=="thunderbolt", ATTR{authorized}=="0", ATTR{authorized}="1"
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This will authorize all devices automatically when they appear. However,
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keep in mind that this bypasses the security levels and makes the system
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vulnerable to DMA attacks.
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Starting with Intel Falcon Ridge Thunderbolt controller there are 4
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security levels available. Intel Titan Ridge added one more security level
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(usbonly). The reason for these is the fact that the connected devices can
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be DMA masters and thus read contents of the host memory without CPU and OS
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knowing about it. There are ways to prevent this by setting up an IOMMU but
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it is not always available for various reasons.
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Some USB4 systems have a BIOS setting to disable PCIe tunneling. This is
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treated as another security level (nopcie).
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The security levels are as follows:
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none
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All devices are automatically connected by the firmware. No user
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approval is needed. In BIOS settings this is typically called
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*Legacy mode*.
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user
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User is asked whether the device is allowed to be connected.
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Based on the device identification information available through
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``/sys/bus/thunderbolt/devices``, the user then can make the decision.
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In BIOS settings this is typically called *Unique ID*.
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secure
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User is asked whether the device is allowed to be connected. In
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addition to UUID the device (if it supports secure connect) is sent
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a challenge that should match the expected one based on a random key
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written to the ``key`` sysfs attribute. In BIOS settings this is
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typically called *One time saved key*.
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dponly
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The firmware automatically creates tunnels for Display Port and
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USB. No PCIe tunneling is done. In BIOS settings this is
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typically called *Display Port Only*.
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usbonly
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The firmware automatically creates tunnels for the USB controller and
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Display Port in a dock. All PCIe links downstream of the dock are
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removed.
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nopcie
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PCIe tunneling is disabled/forbidden from the BIOS. Available in some
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USB4 systems.
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The current security level can be read from
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``/sys/bus/thunderbolt/devices/domainX/security`` where ``domainX`` is
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the Thunderbolt domain the host controller manages. There is typically
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one domain per Thunderbolt host controller.
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If the security level reads as ``user`` or ``secure`` the connected
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device must be authorized by the user before PCIe tunnels are created
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(e.g the PCIe device appears).
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Each Thunderbolt device plugged in will appear in sysfs under
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``/sys/bus/thunderbolt/devices``. The device directory carries
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information that can be used to identify the particular device,
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including its name and UUID.
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Authorizing devices when security level is ``user`` or ``secure``
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-----------------------------------------------------------------
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When a device is plugged in it will appear in sysfs as follows::
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/sys/bus/thunderbolt/devices/0-1/authorized - 0
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/sys/bus/thunderbolt/devices/0-1/device - 0x8004
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/sys/bus/thunderbolt/devices/0-1/device_name - Thunderbolt to FireWire Adapter
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/sys/bus/thunderbolt/devices/0-1/vendor - 0x1
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/sys/bus/thunderbolt/devices/0-1/vendor_name - Apple, Inc.
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/sys/bus/thunderbolt/devices/0-1/unique_id - e0376f00-0300-0100-ffff-ffffffffffff
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The ``authorized`` attribute reads 0 which means no PCIe tunnels are
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created yet. The user can authorize the device by simply entering::
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# echo 1 > /sys/bus/thunderbolt/devices/0-1/authorized
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This will create the PCIe tunnels and the device is now connected.
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If the device supports secure connect, and the domain security level is
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set to ``secure``, it has an additional attribute ``key`` which can hold
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a random 32-byte value used for authorization and challenging the device in
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future connects::
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/sys/bus/thunderbolt/devices/0-3/authorized - 0
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/sys/bus/thunderbolt/devices/0-3/device - 0x305
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/sys/bus/thunderbolt/devices/0-3/device_name - AKiTiO Thunder3 PCIe Box
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/sys/bus/thunderbolt/devices/0-3/key -
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/sys/bus/thunderbolt/devices/0-3/vendor - 0x41
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/sys/bus/thunderbolt/devices/0-3/vendor_name - inXtron
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/sys/bus/thunderbolt/devices/0-3/unique_id - dc010000-0000-8508-a22d-32ca6421cb16
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Notice the key is empty by default.
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If the user does not want to use secure connect they can just ``echo 1``
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to the ``authorized`` attribute and the PCIe tunnels will be created in
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the same way as in the ``user`` security level.
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If the user wants to use secure connect, the first time the device is
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plugged a key needs to be created and sent to the device::
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# key=$(openssl rand -hex 32)
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# echo $key > /sys/bus/thunderbolt/devices/0-3/key
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# echo 1 > /sys/bus/thunderbolt/devices/0-3/authorized
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Now the device is connected (PCIe tunnels are created) and in addition
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the key is stored on the device NVM.
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Next time the device is plugged in the user can verify (challenge) the
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device using the same key::
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# echo $key > /sys/bus/thunderbolt/devices/0-3/key
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# echo 2 > /sys/bus/thunderbolt/devices/0-3/authorized
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If the challenge the device returns back matches the one we expect based
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on the key, the device is connected and the PCIe tunnels are created.
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However, if the challenge fails no tunnels are created and error is
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returned to the user.
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If the user still wants to connect the device they can either approve
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the device without a key or write a new key and write 1 to the
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``authorized`` file to get the new key stored on the device NVM.
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De-authorizing devices
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----------------------
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It is possible to de-authorize devices by writing ``0`` to their
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``authorized`` attribute. This requires support from the connection
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manager implementation and can be checked by reading domain
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``deauthorization`` attribute. If it reads ``1`` then the feature is
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supported.
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When a device is de-authorized the PCIe tunnel from the parent device
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PCIe downstream (or root) port to the device PCIe upstream port is torn
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down. This is essentially the same thing as PCIe hot-remove and the PCIe
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toplogy in question will not be accessible anymore until the device is
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authorized again. If there is storage such as NVMe or similar involved,
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there is a risk for data loss if the filesystem on that storage is not
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properly shut down. You have been warned!
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DMA protection utilizing IOMMU
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------------------------------
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Recent systems from 2018 and forward with Thunderbolt ports may natively
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support IOMMU. This means that Thunderbolt security is handled by an IOMMU
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so connected devices cannot access memory regions outside of what is
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allocated for them by drivers. When Linux is running on such system it
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automatically enables IOMMU if not enabled by the user already. These
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systems can be identified by reading ``1`` from
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``/sys/bus/thunderbolt/devices/domainX/iommu_dma_protection`` attribute.
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The driver does not do anything special in this case but because DMA
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protection is handled by the IOMMU, security levels (if set) are
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redundant. For this reason some systems ship with security level set to
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``none``. Other systems have security level set to ``user`` in order to
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support downgrade to older OS, so users who want to automatically
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authorize devices when IOMMU DMA protection is enabled can use the
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following ``udev`` rule::
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ACTION=="add", SUBSYSTEM=="thunderbolt", ATTRS{iommu_dma_protection}=="1", ATTR{authorized}=="0", ATTR{authorized}="1"
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Upgrading NVM on Thunderbolt device, host or retimer
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----------------------------------------------------
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Since most of the functionality is handled in firmware running on a
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host controller or a device, it is important that the firmware can be
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upgraded to the latest where possible bugs in it have been fixed.
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Typically OEMs provide this firmware from their support site.
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There is also a central site which has links where to download firmware
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for some machines:
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`Thunderbolt Updates <https://thunderbolttechnology.net/updates>`_
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Before you upgrade firmware on a device, host or retimer, please make
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sure it is a suitable upgrade. Failing to do that may render the device
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in a state where it cannot be used properly anymore without special
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tools!
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Host NVM upgrade on Apple Macs is not supported.
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Once the NVM image has been downloaded, you need to plug in a
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Thunderbolt device so that the host controller appears. It does not
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matter which device is connected (unless you are upgrading NVM on a
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device - then you need to connect that particular device).
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Note an OEM-specific method to power the controller up ("force power") may
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be available for your system in which case there is no need to plug in a
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Thunderbolt device.
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After that we can write the firmware to the non-active parts of the NVM
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of the host or device. As an example here is how Intel NUC6i7KYK (Skull
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Canyon) Thunderbolt controller NVM is upgraded::
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# dd if=KYK_TBT_FW_0018.bin of=/sys/bus/thunderbolt/devices/0-0/nvm_non_active0/nvmem
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Once the operation completes we can trigger NVM authentication and
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upgrade process as follows::
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# echo 1 > /sys/bus/thunderbolt/devices/0-0/nvm_authenticate
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If no errors are returned, the host controller shortly disappears. Once
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it comes back the driver notices it and initiates a full power cycle.
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After a while the host controller appears again and this time it should
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be fully functional.
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We can verify that the new NVM firmware is active by running the following
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commands::
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# cat /sys/bus/thunderbolt/devices/0-0/nvm_authenticate
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0x0
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# cat /sys/bus/thunderbolt/devices/0-0/nvm_version
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18.0
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If ``nvm_authenticate`` contains anything other than 0x0 it is the error
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code from the last authentication cycle, which means the authentication
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of the NVM image failed.
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Note names of the NVMem devices ``nvm_activeN`` and ``nvm_non_activeN``
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depend on the order they are registered in the NVMem subsystem. N in
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the name is the identifier added by the NVMem subsystem.
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Upgrading NVM when host controller is in safe mode
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--------------------------------------------------
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If the existing NVM is not properly authenticated (or is missing) the
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host controller goes into safe mode which means that the only available
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functionality is flashing a new NVM image. When in this mode, reading
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``nvm_version`` fails with ``ENODATA`` and the device identification
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information is missing.
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To recover from this mode, one needs to flash a valid NVM image to the
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host controller in the same way it is done in the previous chapter.
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Networking over Thunderbolt cable
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---------------------------------
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Thunderbolt technology allows software communication between two hosts
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connected by a Thunderbolt cable.
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It is possible to tunnel any kind of traffic over a Thunderbolt link but
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currently we only support Apple ThunderboltIP protocol.
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If the other host is running Windows or macOS, the only thing you need to
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do is to connect a Thunderbolt cable between the two hosts; the
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``thunderbolt-net`` driver is loaded automatically. If the other host is
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also Linux you should load ``thunderbolt-net`` manually on one host (it
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does not matter which one)::
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# modprobe thunderbolt-net
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This triggers module load on the other host automatically. If the driver
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is built-in to the kernel image, there is no need to do anything.
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The driver will create one virtual ethernet interface per Thunderbolt
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port which are named like ``thunderbolt0`` and so on. From this point
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you can either use standard userspace tools like ``ifconfig`` to
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configure the interface or let your GUI handle it automatically.
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Forcing power
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-------------
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Many OEMs include a method that can be used to force the power of a
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Thunderbolt controller to an "On" state even if nothing is connected.
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If supported by your machine this will be exposed by the WMI bus with
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a sysfs attribute called "force_power".
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For example the intel-wmi-thunderbolt driver exposes this attribute in:
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/sys/bus/wmi/devices/86CCFD48-205E-4A77-9C48-2021CBEDE341/force_power
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To force the power to on, write 1 to this attribute file.
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To disable force power, write 0 to this attribute file.
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Note: it's currently not possible to query the force power state of a platform.
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