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Recent systems with Thunderbolt ports may support IOMMU natively. In practice this means that Thunderbolt connected devices are placed behind an IOMMU during the whole time it is connected (including during boot) making Thunderbolt security levels redundant. This is called Kernel DMA protection [1] by Microsoft. Some of these systems still have Thunderbolt security level set to "user" in order to support OS downgrade (the older version of the OS might not support IOMMU based DMA protection so connecting a device still relies on user approval). Export this information to userspace by introducing a new sysfs attribute (iommu_dma_protection). Based on it userspace tools can make more accurate decision whether or not authorize the connected device. In addition update Thunderbolt documentation regarding IOMMU based DMA protection. [1] https://docs.microsoft.com/en-us/windows/security/information-protection/kernel-dma-protection-for-thunderbolt Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com> Reviewed-by: Yehezkel Bernat <YehezkelShB@gmail.com>
264 lines
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ReStructuredText
264 lines
11 KiB
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=============
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Thunderbolt
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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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Security levels and how to use them
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-----------------------------------
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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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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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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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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 or host
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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 or host, please make sure it is a
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suitable upgrade. Failing to do that may render the device (or host) in a
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state where it cannot be used properly anymore without special 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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