x86/bugs: Expose x86_spec_ctrl_base directly

[mirror_ubuntu-artful-kernel.git] / Documentation / admin-guide / thunderbolt.rst

=============
 Thunderbolt
=============
The interface presented here is not meant for end users. Instead there
should be a userspace tool that handles all the low-level details, keeps
database of the authorized devices and prompts user for new connections.

More details about the sysfs interface for Thunderbolt devices can be
found in ``Documentation/ABI/testing/sysfs-bus-thunderbolt``.

Those users who just want to connect any device without any sort of
manual work, can add following line to
``/etc/udev/rules.d/99-local.rules``::

  ACTION=="add", SUBSYSTEM=="thunderbolt", ATTR{authorized}=="0", ATTR{authorized}="1"

This will authorize all devices automatically when they appear. However,
keep in mind that this bypasses the security levels and makes the system
vulnerable to DMA attacks.

Security levels and how to use them
-----------------------------------
Starting from Intel Falcon Ridge Thunderbolt controller there are 4
security levels available. The reason for these is the fact that the
connected devices can be DMA masters and thus read contents of the host
memory without CPU and OS knowing about it. There are ways to prevent
this by setting up an IOMMU but it is not always available for various
reasons.

The security levels are as follows:

  none
    All devices are automatically connected by the firmware. No user
    approval is needed. In BIOS settings this is typically called
    *Legacy mode*.

  user
    User is asked whether the device is allowed to be connected.
    Based on the device identification information available through
    ``/sys/bus/thunderbolt/devices``. user then can do the decision.
    In BIOS settings this is typically called *Unique ID*.

  secure
    User is asked whether the device is allowed to be connected. In
    addition to UUID the device (if it supports secure connect) is sent
    a challenge that should match the expected one based on a random key
    written to ``key`` sysfs attribute. In BIOS settings this is
    typically called *One time saved key*.

  dponly
    The firmware automatically creates tunnels for Display Port and
    USB. No PCIe tunneling is done. In BIOS settings this is
    typically called *Display Port Only*.

The current security level can be read from
``/sys/bus/thunderbolt/devices/domainX/security`` where ``domainX`` is
the Thunderbolt domain the host controller manages. There is typically
one domain per Thunderbolt host controller.

If the security level reads as ``user`` or ``secure`` the connected
device must be authorized by the user before PCIe tunnels are created
(e.g the PCIe device appears).

Each Thunderbolt device plugged in will appear in sysfs under
``/sys/bus/thunderbolt/devices``. The device directory carries
information that can be used to identify the particular device,
including its name and UUID.

Authorizing devices when security level is ``user`` or ``secure``
-----------------------------------------------------------------
When a device is plugged in it will appear in sysfs as follows::

  /sys/bus/thunderbolt/devices/0-1/authorized	- 0
  /sys/bus/thunderbolt/devices/0-1/device	- 0x8004
  /sys/bus/thunderbolt/devices/0-1/device_name	- Thunderbolt to FireWire Adapter
  /sys/bus/thunderbolt/devices/0-1/vendor	- 0x1
  /sys/bus/thunderbolt/devices/0-1/vendor_name	- Apple, Inc.
  /sys/bus/thunderbolt/devices/0-1/unique_id	- e0376f00-0300-0100-ffff-ffffffffffff

The ``authorized`` attribute reads 0 which means no PCIe tunnels are
created yet. The user can authorize the device by simply::

  # echo 1 > /sys/bus/thunderbolt/devices/0-1/authorized

This will create the PCIe tunnels and the device is now connected.

If the device supports secure connect, and the domain security level is
set to ``secure``, it has an additional attribute ``key`` which can hold
a random 32 byte value used for authorization and challenging the device in
future connects::

  /sys/bus/thunderbolt/devices/0-3/authorized	- 0
  /sys/bus/thunderbolt/devices/0-3/device	- 0x305
  /sys/bus/thunderbolt/devices/0-3/device_name	- AKiTiO Thunder3 PCIe Box
  /sys/bus/thunderbolt/devices/0-3/key		-
  /sys/bus/thunderbolt/devices/0-3/vendor	- 0x41
  /sys/bus/thunderbolt/devices/0-3/vendor_name	- inXtron
  /sys/bus/thunderbolt/devices/0-3/unique_id	- dc010000-0000-8508-a22d-32ca6421cb16

Notice the key is empty by default.

If the user does not want to use secure connect it can just ``echo 1``
to the ``authorized`` attribute and the PCIe tunnels will be created in
the same way than in ``user`` security level.

If the user wants to use secure connect, the first time the device is
plugged a key needs to be created and send to the device::

  # key=$(openssl rand -hex 32)
  # echo $key > /sys/bus/thunderbolt/devices/0-3/key
  # echo 1 > /sys/bus/thunderbolt/devices/0-3/authorized

Now the device is connected (PCIe tunnels are created) and in addition
the key is stored on the device NVM.

Next time the device is plugged in the user can verify (challenge) the
device using the same key::

  # echo $key > /sys/bus/thunderbolt/devices/0-3/key
  # echo 2 > /sys/bus/thunderbolt/devices/0-3/authorized

If the challenge the device returns back matches the one we expect based
on the key, the device is connected and the PCIe tunnels are created.
However, if the challenge failed no tunnels are created and error is
returned to the user.

If the user still wants to connect the device it can either approve
the device without a key or write new key and write 1 to the
``authorized`` file to get the new key stored on the device NVM.

Upgrading NVM on Thunderbolt device or host
-------------------------------------------
Since most of the functionality is handled in a firmware running on a
host controller or a device, it is important that the firmware can be
upgraded to the latest where possible bugs in it have been fixed.
Typically OEMs provide this firmware from their support site.

There is also a central site which has links where to download firmwares
for some machines:

  `Thunderbolt Updates <https://thunderbolttechnology.net/updates>`_

Before you upgrade firmware on a device or host, please make sure it is
the suitable. Failing to do that may render the device (or host) in a
state where it cannot be used properly anymore without special tools!

Host NVM upgrade on Apple Macs is not supported.

Once the NVM image has been downloaded, you need to plug in a
Thunderbolt device so that the host controller appears. It does not
matter which device is connected (unless you are upgrading NVM on a
device - then you need to connect that particular device).

Note OEM-specific method to power the controller up ("force power") may
be available for your system in which case there is no need to plug in a
Thunderbolt device.

After that we can write the firmware to the non-active parts of the NVM
of the host or device. As an example here is how Intel NUC6i7KYK (Skull
Canyon) Thunderbolt controller NVM is upgraded::

  # dd if=KYK_TBT_FW_0018.bin of=/sys/bus/thunderbolt/devices/0-0/nvm_non_active0/nvmem

Once the operation completes we can trigger NVM authentication and
upgrade process as follows::

  # echo 1 > /sys/bus/thunderbolt/devices/0-0/nvm_authenticate

If no errors are returned, the host controller shortly disappears. Once
it comes back the driver notices it and initiates a full power cycle.
After a while the host controller appears again and this time it should
be fully functional.

We can verify that the new NVM firmware is active by running following
commands::

  # cat /sys/bus/thunderbolt/devices/0-0/nvm_authenticate
  0x0
  # cat /sys/bus/thunderbolt/devices/0-0/nvm_version
  18.0

If ``nvm_authenticate`` contains anything else than 0x0 it is the error
code from the last authentication cycle, which means the authentication
of the NVM image failed.

Note names of the NVMem devices ``nvm_activeN`` and ``nvm_non_activeN``
depends on the order they are registered in the NVMem subsystem. N in
the name is the identifier added by the NVMem subsystem.

Upgrading NVM when host controller is in safe mode
--------------------------------------------------
If the existing NVM is not properly authenticated (or is missing) the
host controller goes into safe mode which means that only available
functionality is flashing new NVM image. When in this mode the reading
``nvm_version`` fails with ``ENODATA`` and the device identification
information is missing.

To recover from this mode, one needs to flash a valid NVM image to the
host host controller in the same way it is done in the previous chapter.