[PATCH v10 11/11] Documentation: add USB authorization document to driver-api

[Allen Webb <allenwebb@google.com>]

There is a user-facing USB authorization document, but it is midding
details a driver should have developer, so add them in a new document.

Signed-off-by: Allen Webb <allenwebb@google.com>
---
 .../driver-api/usb/authorization.rst          | 71 +++++++++++++++++++
 Documentation/driver-api/usb/index.rst        |  1 +
 2 files changed, 72 insertions(+)
 create mode 100644 Documentation/driver-api/usb/authorization.rst

diff --git a/Documentation/driver-api/usb/authorization.rst b/Documentation/driver-api/usb/authorization.rst
new file mode 100644
index 000000000000..383dcc037a15
--- /dev/null
+++ b/Documentation/driver-api/usb/authorization.rst
@@ -0,0 +1,71 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+====================
+Device Authorization
+====================
+
+This document is intended for driver developers. See
+Documentation/usb/authorization.rst if you are looking for how to use
+USB authorization.
+
+Authorization provides userspace a way to allow or block configuring
+devices early during enumeration before any modules are probed for the
+device. While it is possible to block a device by not loading the
+required modules, this also prevents other devices from using the
+module as well. For example someone might have an unattended computer
+downloading installation media to a USB drive. Presumably this computer
+would be locked to make it more difficult for a bad actor to access the
+computer. Since USB storage devices are not needed to interact with the
+lock screen, the authorized_default sysfs attribute can be set to not
+authorize new USB devices by default. A userspace tool like USBGuard
+can then vet the devices. Mice, keyboards, etc can be allowed by
+writing to their authorized sysfs attribute so that the lock screen can
+still be used (this important in cases like suspend+resume or docks)
+while other devices can be blocked as long as the lock screen is shown.
+
+Sysfs Attributes
+================
+
+Userspace can control USB device authorization through the
+authorized_default and authorized sysfs attributes.
+
+authorized_default
+------------------
+
+Defined in ``drivers/usb/core/hcd.c``
+
+The authorized_default sysfs attribute is only present for host
+controllers. It determines the initial state of the authorized sysfs
+attribute of USB devices newly connected to the corresponding host
+controller. It can take on the following values:
+
++---------------------------------------------------+
+| Value | Behavior                                  |
++=======+===========================================+
+|    -1 | Authorize all devices except wireless USB |
++-------+-------------------------------------------+
+|     0 | Do not authorize new devices              |
++-------+-------------------------------------------+
+|     1 | Authorize new devices                     |
++-------+-------------------------------------------+
+|     2 | Authorize new internal devices only       |
++---------------------------------------------------+
+
+Note that firmware platform code determines if a device is internal or
+not and this is reported as the connect_type sysfs attribute of the USB
+port. This is currently supported by ACPI, but device tree still needs
+an implementation. Authorizing new internal devices only can be useful
+to work around issues with devices that misbehave if there are delays
+in probing their module.
+
+authorized
+----------
+
+Defined in ``drivers/usb/core/sysfs.c``
+
+Every USB device has an authorized sysfs attribute which can take the
+values 0 and 1. When authorized is 0, the device still is present in
+sysfs, but none of its interfaces can be associated with drivers and
+modules will not be probed. When authorized is 1 (or set to one) a
+configuration is chosen for the device and its interfaces are
+registered allowing drivers to bind to them.
diff --git a/Documentation/driver-api/usb/index.rst b/Documentation/driver-api/usb/index.rst
index cfa8797ea614..ffe37916f99f 100644
--- a/Documentation/driver-api/usb/index.rst
+++ b/Documentation/driver-api/usb/index.rst
@@ -7,6 +7,7 @@ Linux USB API
    usb
    gadget
    anchors
+   authorization
    bulk-streams
    callbacks
    dma
-- 
2.39.2