Merge tag 'docs-4.16-2' of git://git.lwn.net/linux

Pull more documentation updates from Jonathan Corbet:
 "A few late-arriving fixes, along with Konstantin's PGP document that
  had no reason to wait another cycle"

* tag 'docs-4.16-2' of git://git.lwn.net/linux:
  Documentation/process: tweak pgp maintainer guide
  Documentation/admin-guide: fixes for thunderbolt.rst
  Documentation: mips: Update AU1xxx_IDE Kconfig dependencies
  Fix broken link in Documentation/process/kernel-docs.rst
  Documentation/process: kernel maintainer PGP guide

5 files changed, 964 insertions, 41 deletions

diff --git a/Documentation/admin-guide/thunderbolt.rst b/Documentation/admin-guide/thunderbolt.rst
index 9b55952039a6..9948ec36a204 100644
--- a/Documentation/admin-guide/thunderbolt.rst
+++ b/Documentation/admin-guide/thunderbolt.rst
@@ -3,13 +3,13 @@
 =============
 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.
+a database of the authorized devices and prompts users 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
+manual work can add following line to
 ``/etc/udev/rules.d/99-local.rules``::
 
   ACTION=="add", SUBSYSTEM=="thunderbolt", ATTR{authorized}=="0", ATTR{authorized}="1"
@@ -20,7 +20,7 @@ vulnerable to DMA attacks.
 
 Security levels and how to use them
 -----------------------------------
-Starting from Intel Falcon Ridge Thunderbolt controller there are 4
+Starting with 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
@@ -37,14 +37,14 @@ The security levels are as follows:
   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.
+    ``/sys/bus/thunderbolt/devices``, the user then can make 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
+    written to the ``key`` sysfs attribute. In BIOS settings this is
     typically called *One time saved key*.
 
   dponly
@@ -78,7 +78,7 @@ When a device is plugged in it will appear in sysfs as follows::
   /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::
+created yet. The user can authorize the device by simply entering::
 
   # echo 1 > /sys/bus/thunderbolt/devices/0-1/authorized
 
@@ -86,7 +86,7 @@ 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
+a random 32-byte value used for authorization and challenging the device in
 future connects::
 
   /sys/bus/thunderbolt/devices/0-3/authorized	- 0
@@ -99,12 +99,12 @@ future connects::
 
 Notice the key is empty by default.
 
-If the user does not want to use secure connect it can just ``echo 1``
+If the user does not want to use secure connect they can just ``echo 1``
 to the ``authorized`` attribute and the PCIe tunnels will be created in
-the same way than in ``user`` security level.
+the same way as in the ``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::
+plugged a key needs to be created and sent to the device::
 
   # key=$(openssl rand -hex 32)
   # echo $key > /sys/bus/thunderbolt/devices/0-3/key
@@ -121,27 +121,27 @@ device using the same key::
 
 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
+However, if the challenge fails 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
+If the user still wants to connect the device they can either approve
+the device without a key or write a 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
+Since most of the functionality is handled in 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
+There is also a central site which has links where to download firmware
 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
+Before you upgrade firmware on a device or host, please make sure it is a
+suitable upgrade. 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.
@@ -151,7 +151,7 @@ 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
+Note an 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.
 
@@ -171,7 +171,7 @@ 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
+We can verify that the new NVM firmware is active by running the following
 commands::
 
   # cat /sys/bus/thunderbolt/devices/0-0/nvm_authenticate
@@ -179,38 +179,38 @@ commands::
   # cat /sys/bus/thunderbolt/devices/0-0/nvm_version
   18.0
 
-If ``nvm_authenticate`` contains anything else than 0x0 it is the error
+If ``nvm_authenticate`` contains anything other 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
+depend 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
+host controller goes into safe mode which means that the only available
+functionality is flashing a new NVM image. When in this mode, 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.
+host controller in the same way it is done in the previous chapter.
 
 Networking over Thunderbolt cable
 ---------------------------------
-Thunderbolt technology allows software communication across two hosts
+Thunderbolt technology allows software communication between two hosts
 connected by a Thunderbolt cable.
 
-It is possible to tunnel any kind of traffic over Thunderbolt link but
+It is possible to tunnel any kind of traffic over a Thunderbolt link but
 currently we only support Apple ThunderboltIP protocol.
 
-If the other host is running Windows or macOS only thing you need to
-do is to connect Thunderbolt cable between the two hosts, the
-``thunderbolt-net`` is loaded automatically. If the other host is also
-Linux you should load ``thunderbolt-net`` manually on one host (it does
-not matter which one)::
+If the other host is running Windows or macOS, the only thing you need to
+do is to connect a Thunderbolt cable between the two hosts; the
+``thunderbolt-net`` driver is loaded automatically. If the other host is
+also Linux you should load ``thunderbolt-net`` manually on one host (it
+does not matter which one)::
 
   # modprobe thunderbolt-net
 
@@ -220,12 +220,12 @@ is built-in to the kernel image, there is no need to do anything.
 The driver will create one virtual ethernet interface per Thunderbolt
 port which are named like ``thunderbolt0`` and so on. From this point
 you can either use standard userspace tools like ``ifconfig`` to
-configure the interface or let your GUI to handle it automatically.
+configure the interface or let your GUI handle it automatically.
 
 Forcing power
 -------------
 Many OEMs include a method that can be used to force the power of a
-thunderbolt controller to an "On" state even if nothing is connected.
+Thunderbolt controller to an "On" state even if nothing is connected.
 If supported by your machine this will be exposed by the WMI bus with
 a sysfs attribute called "force_power".
 
diff --git a/Documentation/mips/AU1xxx_IDE.README b/Documentation/mips/AU1xxx_IDE.README
index 52844a58cc8a..ff675a1b1422 100644
--- a/Documentation/mips/AU1xxx_IDE.README
+++ b/Documentation/mips/AU1xxx_IDE.README
@@ -56,8 +56,6 @@ Following extra configs variables are introduced:
 
   CONFIG_BLK_DEV_IDE_AU1XXX_PIO_DBDMA    - enable the PIO+DBDMA mode
   CONFIG_BLK_DEV_IDE_AU1XXX_MDMA2_DBDMA  - enable the MWDMA mode
-  CONFIG_BLK_DEV_IDE_AU1XXX_BURSTABLE_ON - set Burstable FIFO in DBDMA
-                                           controller
 
 
 SUPPORTED IDE MODES
@@ -82,11 +80,9 @@ CONFIG_IDE_GENERIC=y
 CONFIG_BLK_DEV_IDEPCI=y
 CONFIG_BLK_DEV_GENERIC=y
 CONFIG_BLK_DEV_IDEDMA_PCI=y
-CONFIG_IDEDMA_PCI_AUTO=y
 CONFIG_BLK_DEV_IDE_AU1XXX=y
 CONFIG_BLK_DEV_IDE_AU1XXX_MDMA2_DBDMA=y
 CONFIG_BLK_DEV_IDEDMA=y
-CONFIG_IDEDMA_AUTO=y
 
 Also define 'IDE_AU1XXX_BURSTMODE' in 'drivers/ide/mips/au1xxx-ide.c' to enable
 the burst support on DBDMA controller.
@@ -94,16 +90,13 @@ the burst support on DBDMA controller.
 If the used system need the USB support enable the following kernel configs for
 high IDE to USB throughput.
 
-CONFIG_BLK_DEV_IDEDISK=y
 CONFIG_IDE_GENERIC=y
 CONFIG_BLK_DEV_IDEPCI=y
 CONFIG_BLK_DEV_GENERIC=y
 CONFIG_BLK_DEV_IDEDMA_PCI=y
-CONFIG_IDEDMA_PCI_AUTO=y
 CONFIG_BLK_DEV_IDE_AU1XXX=y
 CONFIG_BLK_DEV_IDE_AU1XXX_MDMA2_DBDMA=y
 CONFIG_BLK_DEV_IDEDMA=y
-CONFIG_IDEDMA_AUTO=y
 
 Also undefine 'IDE_AU1XXX_BURSTMODE' in 'drivers/ide/mips/au1xxx-ide.c' to
 disable the burst support on DBDMA controller.
diff --git a/Documentation/process/index.rst b/Documentation/process/index.rst
index a430f6eee756..1c9fe657ed01 100644
--- a/Documentation/process/index.rst
+++ b/Documentation/process/index.rst
@@ -24,6 +24,7 @@ Below are the essential guides that every developer should read.
    development-process
    submitting-patches
    coding-style
+   maintainer-pgp-guide
    email-clients
    kernel-enforcement-statement
    kernel-driver-statement
diff --git a/Documentation/process/kernel-docs.rst b/Documentation/process/kernel-docs.rst
index b8cac85a4001..3fb28de556e4 100644
--- a/Documentation/process/kernel-docs.rst
+++ b/Documentation/process/kernel-docs.rst
@@ -58,7 +58,7 @@ On-line docs
     * Title: **Linux Kernel Mailing List Glossary**
 
       :Author: various
-      :URL: http://kernelnewbies.org/glossary/
+      :URL: https://kernelnewbies.org/KernelGlossary
       :Date: rolling version
       :Keywords: glossary, terms, linux-kernel.
       :Description: From the introduction: "This glossary is intended as
diff --git a/Documentation/process/maintainer-pgp-guide.rst b/Documentation/process/maintainer-pgp-guide.rst
new file mode 100644
index 000000000000..b453561a7148
--- /dev/null
+++ b/Documentation/process/maintainer-pgp-guide.rst
@@ -0,0 +1,929 @@
+.. _pgpguide:
+
+===========================
+Kernel Maintainer PGP guide
+===========================
+
+:Author: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
+
+This document is aimed at Linux kernel developers, and especially at
+subsystem maintainers. It contains a subset of information discussed in
+the more general "`Protecting Code Integrity`_" guide published by the
+Linux Foundation. Please read that document for more in-depth discussion
+on some of the topics mentioned in this guide.
+
+.. _`Protecting Code Integrity`: https://github.com/lfit/itpol/blob/master/protecting-code-integrity.md
+
+The role of PGP in Linux Kernel development
+===========================================
+
+PGP helps ensure the integrity of the code that is produced by the Linux
+kernel development community and, to a lesser degree, establish trusted
+communication channels between developers via PGP-signed email exchange.
+
+The Linux kernel source code is available in two main formats:
+
+- Distributed source repositories (git)
+- Periodic release snapshots (tarballs)
+
+Both git repositories and tarballs carry PGP signatures of the kernel
+developers who create official kernel releases. These signatures offer a
+cryptographic guarantee that downloadable versions made available via
+kernel.org or any other mirrors are identical to what these developers
+have on their workstations. To this end:
+
+- git repositories provide PGP signatures on all tags
+- tarballs provide detached PGP signatures with all downloads
+
+.. _devs_not_infra:
+
+Trusting the developers, not infrastructure
+-------------------------------------------
+
+Ever since the 2011 compromise of core kernel.org systems, the main
+operating principle of the Kernel Archives project has been to assume
+that any part of the infrastructure can be compromised at any time. For
+this reason, the administrators have taken deliberate steps to emphasize
+that trust must always be placed with developers and never with the code
+hosting infrastructure, regardless of how good the security practices
+for the latter may be.
+
+The above guiding principle is the reason why this guide is needed. We
+want to make sure that by placing trust into developers we do not simply
+shift the blame for potential future security incidents to someone else.
+The goal is to provide a set of guidelines developers can use to create
+a secure working environment and safeguard the PGP keys used to
+establish the integrity of the Linux kernel itself.
+
+.. _pgp_tools:
+
+PGP tools
+=========
+
+Use GnuPG v2
+------------
+
+Your distro should already have GnuPG installed by default, you just
+need to verify that you are using version 2.x and not the legacy 1.4
+release -- many distributions still package both, with the default
+``gpg`` command invoking GnuPG v.1. To check, run::
+
+    $ gpg --version | head -n1
+
+If you see ``gpg (GnuPG) 1.4.x``, then you are using GnuPG v.1. Try the
+``gpg2`` command (if you don't have it, you may need to install the
+gnupg2 package)::
+
+    $ gpg2 --version | head -n1
+
+If you see ``gpg (GnuPG) 2.x.x``, then you are good to go. This guide
+will assume you have the version 2.2 of GnuPG (or later). If you are
+using version 2.0 of GnuPG, then some of the commands in this guide will
+not work, and you should consider installing the latest 2.2 version of
+GnuPG. Versions of gnupg-2.1.11 and later should be compatible for the
+purposes of this guide as well.
+
+If you have both ``gpg`` and ``gpg2`` commands, you should make sure you
+are always using GnuPG v2, not the legacy version. You can enforce this
+by setting the appropriate alias::
+
+    $ alias gpg=gpg2
+
+You can put that in your ``.bashrc`` to make sure it's always the case.
+
+Configure gpg-agent options
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The GnuPG agent is a helper tool that will start automatically whenever
+you use the ``gpg`` command and run in the background with the purpose
+of caching the private key passphrase. There are two options you should
+know in order to tweak when the passphrase should be expired from cache:
+
+- ``default-cache-ttl`` (seconds): If you use the same key again before
+  the time-to-live expires, the countdown will reset for another period.
+  The default is 600 (10 minutes).
+- ``max-cache-ttl`` (seconds): Regardless of how recently you've used
+  the key since initial passphrase entry, if the maximum time-to-live
+  countdown expires, you'll have to enter the passphrase again. The
+  default is 30 minutes.
+
+If you find either of these defaults too short (or too long), you can
+edit your ``~/.gnupg/gpg-agent.conf`` file to set your own values::
+
+    # set to 30 minutes for regular ttl, and 2 hours for max ttl
+    default-cache-ttl 1800
+    max-cache-ttl 7200
+
+.. note::
+
+    It is no longer necessary to start gpg-agent manually at the
+    beginning of your shell session. You may want to check your rc files
+    to remove anything you had in place for older versions of GnuPG, as
+    it may not be doing the right thing any more.
+
+Set up a refresh cronjob
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+You will need to regularly refresh your keyring in order to get the
+latest changes on other people's public keys, which is best done with a
+daily cronjob::
+
+    @daily /usr/bin/gpg2 --refresh >/dev/null 2>&1
+
+Check the full path to your ``gpg`` or ``gpg2`` command and use the
+``gpg2`` command if regular ``gpg`` for you is the legacy GnuPG v.1.
+
+.. _master_key:
+
+Protect your master PGP key
+===========================
+
+This guide assumes that you already have a PGP key that you use for Linux
+kernel development purposes. If you do not yet have one, please see the
+"`Protecting Code Integrity`_" document mentioned earlier for guidance
+on how to create a new one.
+
+You should also make a new key if your current one is weaker than 2048 bits
+(RSA).
+
+Master key vs. Subkeys
+----------------------
+
+Subkeys are fully independent PGP keypairs that are tied to the "master"
+key using certifying key signatures (certificates). It is important to
+understand the following:
+
+1. There are no technical differences between the "master key" and "subkeys."
+2. At creation time, we assign functional limitations to each key by
+   giving it specific capabilities.
+3. A PGP key can have 4 capabilities:
+
+   - **[S]** key can be used for signing
+   - **[E]** key can be used for encryption
+   - **[A]** key can be used for authentication
+   - **[C]** key can be used for certifying other keys
+
+4. A single key may have multiple capabilities.
+5. A subkey is fully independent from the master key. A message
+   encrypted to a subkey cannot be decrypted with the master key. If you
+   lose your private subkey, it cannot be recreated from the master key
+   in any way.
+
+The key carrying the **[C]** (certify) capability is considered the
+"master" key because it is the only key that can be used to indicate
+relationship with other keys. Only the **[C]** key can be used to:
+
+- add or revoke other keys (subkeys) with S/E/A capabilities
+- add, change or revoke identities (uids) associated with the key
+- add or change the expiration date on itself or any subkey
+- sign other people's keys for web of trust purposes
+
+By default, GnuPG creates the following when generating new keys:
+
+- A master key carrying both Certify and Sign capabilities (**[SC]**)
+- A separate subkey with the Encryption capability (**[E]**)
+
+If you used the default parameters when generating your key, then that
+is what you will have. You can verify by running ``gpg --list-secret-keys``,
+for example::
+
+    sec   rsa2048 2018-01-23 [SC] [expires: 2020-01-23]
+          000000000000000000000000AAAABBBBCCCCDDDD
+    uid           [ultimate] Alice Dev <adev@kernel.org>
+    ssb   rsa2048 2018-01-23 [E] [expires: 2020-01-23]
+
+Any key carrying the **[C]** capability is your master key, regardless
+of any other capabilities it may have assigned to it.
+
+The long line under the ``sec`` entry is your key fingerprint --
+whenever you see ``[fpr]`` in the examples below, that 40-character
+string is what it refers to.
+
+Ensure your passphrase is strong
+--------------------------------
+
+GnuPG uses passphrases to encrypt your private keys before storing them on
+disk. This way, even if your ``.gnupg`` directory is leaked or stolen in
+its entirety, the attackers cannot use your private keys without first
+obtaining the passphrase to decrypt them.
+
+It is absolutely essential that your private keys are protected by a
+strong passphrase. To set it or change it, use::
+
+    $ gpg --change-passphrase [fpr]
+
+Create a separate Signing subkey
+--------------------------------
+
+Our goal is to protect your master key by moving it to offline media, so
+if you only have a combined **[SC]** key, then you should create a separate
+signing subkey::
+
+    $ gpg --quick-add-key [fpr] ed25519 sign
+
+Remember to tell the keyservers about this change, so others can pull down
+your new subkey::
+
+    $ gpg --send-key [fpr]
+
+.. note:: ECC support in GnuPG
+
+    GnuPG 2.1 and later has full support for Elliptic Curve
+    Cryptography, with ability to combine ECC subkeys with traditional
+    RSA master keys. The main upside of ECC cryptography is that it is
+    much faster computationally and creates much smaller signatures when
+    compared byte for byte with 2048+ bit RSA keys. Unless you plan on
+    using a smartcard device that does not support ECC operations, we
+    recommend that you create an ECC signing subkey for your kernel
+    work.
+
+    If for some reason you prefer to stay with RSA subkeys, just replace
+    "ed25519" with "rsa2048" in the above command.
+
+
+Back up your master key for disaster recovery
+---------------------------------------------
+
+The more signatures you have on your PGP key from other developers, the
+more reasons you have to create a backup version that lives on something
+other than digital media, for disaster recovery reasons.
+
+The best way to create a printable hardcopy of your private key is by
+using the ``paperkey`` software written for this very purpose. See ``man
+paperkey`` for more details on the output format and its benefits over
+other solutions. Paperkey should already be packaged for most
+distributions.
+
+Run the following command to create a hardcopy backup of your private
+key::
+
+    $ gpg --export-secret-key [fpr] | paperkey -o /tmp/key-backup.txt
+
+Print out that file (or pipe the output straight to lpr), then take a
+pen and write your passphrase on the margin of the paper. **This is
+strongly recommended** because the key printout is still encrypted with
+that passphrase, and if you ever change it you will not remember what it
+used to be when you had created the backup -- *guaranteed*.
+
+Put the resulting printout and the hand-written passphrase into an envelope
+and store in a secure and well-protected place, preferably away from your
+home, such as your bank vault.
+
+.. note::
+
+    Your printer is probably no longer a simple dumb device connected to
+    your parallel port, but since the output is still encrypted with
+    your passphrase, printing out even to "cloud-integrated" modern
+    printers should remain a relatively safe operation. One option is to
+    change the passphrase on your master key immediately after you are
+    done with paperkey.
+
+Back up your whole GnuPG directory
+----------------------------------
+
+.. warning::
+
+    **!!!Do not skip this step!!!**
+
+It is important to have a readily available backup of your PGP keys
+should you need to recover them. This is different from the
+disaster-level preparedness we did with ``paperkey``. You will also rely
+on these external copies whenever you need to use your Certify key --
+such as when making changes to your own key or signing other people's
+keys after conferences and summits.
+
+Start by getting a small USB "thumb" drive (preferably two!) that you
+will use for backup purposes. You will need to encrypt them using LUKS
+-- refer to your distro's documentation on how to accomplish this.
+
+For the encryption passphrase, you can use the same one as on your
+master key.
+
+Once the encryption process is over, re-insert the USB drive and make
+sure it gets properly mounted. Copy your entire ``.gnupg`` directory
+over to the encrypted storage::
+
+    $ cp -a ~/.gnupg /media/disk/foo/gnupg-backup
+
+You should now test to make sure everything still works::
+
+    $ gpg --homedir=/media/disk/foo/gnupg-backup --list-key [fpr]
+
+If you don't get any errors, then you should be good to go. Unmount the
+USB drive, distinctly label it so you don't blow it away next time you
+need to use a random USB drive, and put in a safe place -- but not too
+far away, because you'll need to use it every now and again for things
+like editing identities, adding or revoking subkeys, or signing other
+people's keys.
+
+Remove the master key from  your homedir
+----------------------------------------
+
+The files in our home directory are not as well protected as we like to
+think.  They can be leaked or stolen via many different means:
+
+- by accident when making quick homedir copies to set up a new workstation
+- by systems administrator negligence or malice
+- via poorly secured backups
+- via malware in desktop apps (browsers, pdf viewers, etc)
+- via coercion when crossing international borders
+
+Protecting your key with a good passphrase greatly helps reduce the risk
+of any of the above, but passphrases can be discovered via keyloggers,
+shoulder-surfing, or any number of other means. For this reason, the
+recommended setup is to remove your master key from your home directory
+and store it on offline storage.
+
+.. warning::
+
+    Please see the previous section and make sure you have backed up
+    your GnuPG directory in its entirety. What we are about to do will
+    render your key useless if you do not have a usable backup!
+
+First, identify the keygrip of your master key::
+
+    $ gpg --with-keygrip --list-key [fpr]
+
+The output will be something like this::
+
+    pub   rsa2048 2018-01-24 [SC] [expires: 2020-01-24]
+          000000000000000000000000AAAABBBBCCCCDDDD
+          Keygrip = 1111000000000000000000000000000000000000
+    uid           [ultimate] Alice Dev <adev@kernel.org>
+    sub   rsa2048 2018-01-24 [E] [expires: 2020-01-24]
+          Keygrip = 2222000000000000000000000000000000000000
+    sub   ed25519 2018-01-24 [S]
+          Keygrip = 3333000000000000000000000000000000000000
+
+Find the keygrip entry that is beneath the ``pub`` line (right under the
+master key fingerprint). This will correspond directly to a file in your
+``~/.gnupg`` directory::
+
+    $ cd ~/.gnupg/private-keys-v1.d
+    $ ls
+    1111000000000000000000000000000000000000.key
+    2222000000000000000000000000000000000000.key
+    3333000000000000000000000000000000000000.key
+
+All you have to do is simply remove the .key file that corresponds to
+the master keygrip::
+
+    $ cd ~/.gnupg/private-keys-v1.d
+    $ rm 1111000000000000000000000000000000000000.key
+
+Now, if you issue the ``--list-secret-keys`` command, it will show that
+the master key is missing (the ``#`` indicates it is not available)::
+
+    $ gpg --list-secret-keys
+    sec#  rsa2048 2018-01-24 [SC] [expires: 2020-01-24]
+          000000000000000000000000AAAABBBBCCCCDDDD
+    uid           [ultimate] Alice Dev <adev@kernel.org>
+    ssb   rsa2048 2018-01-24 [E] [expires: 2020-01-24]
+    ssb   ed25519 2018-01-24 [S]
+
+You should also remove any ``secring.gpg`` files in the ``~/.gnupg``
+directory, which are left over from earlier versions of GnuPG.
+
+If you don't have the "private-keys-v1.d" directory
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If you do not have a ``~/.gnupg/private-keys-v1.d`` directory, then your
+secret keys are still stored in the legacy ``secring.gpg`` file used by
+GnuPG v1. Making any changes to your key, such as changing the
+passphrase or adding a subkey, should automatically convert the old
+``secring.gpg`` format to use ``private-keys-v1.d`` instead.
+
+Once you get that done, make sure to delete the obsolete ``secring.gpg``
+file, which still contains your private keys.
+
+.. _smartcards:
+
+Move the subkeys to a dedicated crypto device
+=============================================
+
+Even though the master key is now safe from being leaked or stolen, the
+subkeys are still in your home directory. Anyone who manages to get
+their hands on those will be able to decrypt your communication or fake
+your signatures (if they know the passphrase). Furthermore, each time a
+GnuPG operation is performed, the keys are loaded into system memory and
+can be stolen from there by sufficiently advanced malware (think
+Meltdown and Spectre).
+
+The best way to completely protect your keys is to move them to a
+specialized hardware device that is capable of smartcard operations.
+
+The benefits of smartcards
+--------------------------
+
+A smartcard contains a cryptographic chip that is capable of storing
+private keys and performing crypto operations directly on the card
+itself. Because the key contents never leave the smartcard, the
+operating system of the computer into which you plug in the hardware
+device is not able to retrieve the private keys themselves. This is very
+different from the encrypted USB storage device we used earlier for
+backup purposes -- while that USB device is plugged in and mounted, the
+operating system is able to access the private key contents.
+
+Using external encrypted USB media is not a substitute to having a
+smartcard-capable device.
+
+Available smartcard devices
+---------------------------
+
+Unless all your laptops and workstations have smartcard readers, the
+easiest is to get a specialized USB device that implements smartcard
+functionality.  There are several options available:
+
+- `Nitrokey Start`_: Open hardware and Free Software, based on FSI
+  Japan's `Gnuk`_. Offers support for ECC keys, but fewest security
+  features (such as resistance to tampering or some side-channel
+  attacks).
+- `Nitrokey Pro`_: Similar to the Nitrokey Start, but more
+  tamper-resistant and offers more security features, but no ECC
+  support.
+- `Yubikey 4`_: proprietary hardware and software, but cheaper than
+  Nitrokey Pro and comes available in the USB-C form that is more useful
+  with newer laptops. Offers additional security features such as FIDO
+  U2F, but no ECC.
+
+`LWN has a good review`_ of some of the above models, as well as several
+others. If you want to use ECC keys, your best bet among commercially
+available devices is the Nitrokey Start.
+
+.. _`Nitrokey Start`: https://shop.nitrokey.com/shop/product/nitrokey-start-6
+.. _`Nitrokey Pro`: https://shop.nitrokey.com/shop/product/nitrokey-pro-3
+.. _`Yubikey 4`: https://www.yubico.com/product/yubikey-4-series/
+.. _Gnuk: http://www.fsij.org/doc-gnuk/
+.. _`LWN has a good review`: https://lwn.net/Articles/736231/
+
+Configure your smartcard device
+-------------------------------
+
+Your smartcard device should Just Work (TM) the moment you plug it into
+any modern Linux workstation. You can verify it by running::
+
+    $ gpg --card-status
+
+If you see full smartcard details, then you are good to go.
+Unfortunately, troubleshooting all possible reasons why things may not
+be working for you is way beyond the scope of this guide. If you are
+having trouble getting the card to work with GnuPG, please seek help via
+usual support channels.
+
+To configure your smartcard, you will need to use the GnuPG menu system, as
+there are no convenient command-line switches::
+
+    $ gpg --card-edit
+    [...omitted...]
+    gpg/card> admin
+    Admin commands are allowed
+    gpg/card> passwd
+
+You should set the user PIN (1), Admin PIN (3), and the Reset Code (4).
+Please make sure to record and store these in a safe place -- especially
+the Admin PIN and the Reset Code (which allows you to completely wipe
+the smartcard).  You so rarely need to use the Admin PIN, that you will
+inevitably forget what it is if you do not record it.
+
+Getting back to the main card menu, you can also set other values (such
+as name, sex, login data, etc), but it's not necessary and will
+additionally leak information about your smartcard should you lose it.
+
+.. note::
+
+    Despite having the name "PIN", neither the user PIN nor the admin
+    PIN on the card need to be numbers.
+
+Move the subkeys to your smartcard
+----------------------------------
+
+Exit the card menu (using "q") and save all changes. Next, let's move
+your subkeys onto the smartcard. You will need both your PGP key
+passphrase and the admin PIN of the card for most operations::
+
+    $ gpg --edit-key [fpr]
+
+    Secret subkeys are available.
+
+    pub  rsa2048/AAAABBBBCCCCDDDD
+         created: 2018-01-23  expires: 2020-01-23  usage: SC
+         trust: ultimate      validity: ultimate
+    ssb  rsa2048/1111222233334444
+         created: 2018-01-23  expires: never       usage: E
+    ssb  ed25519/5555666677778888
+         created: 2017-12-07  expires: never       usage: S
+    [ultimate] (1). Alice Dev <adev@kernel.org>
+
+    gpg>
+
+Using ``--edit-key`` puts us into the menu mode again, and you will
+notice that the key listing is a little different. From here on, all
+commands are done from inside this menu mode, as indicated by ``gpg>``.
+
+First, let's select the key we'll be putting onto the card -- you do
+this by typing ``key 1`` (it's the first one in the listing, the **[E]**
+subkey)::
+
+    gpg> key 1
+
+In the output, you should now see ``ssb*`` on the **[E]** key. The ``*``
+indicates which key is currently "selected." It works as a *toggle*,
+meaning that if you type ``key 1`` again, the ``*`` will disappear and
+the key will not be selected any more.
+
+Now, let's move that key onto the smartcard::
+
+    gpg> keytocard
+    Please select where to store the key:
+       (2) Encryption key
+    Your selection? 2
+
+Since it's our **[E]** key, it makes sense to put it into the Encryption
+slot.  When you submit your selection, you will be prompted first for
+your PGP key passphrase, and then for the admin PIN. If the command
+returns without an error, your key has been moved.
+
+**Important**: Now type ``key 1`` again to unselect the first key, and
+``key 2`` to select the **[S]** key::
+
+    gpg> key 1
+    gpg> key 2
+    gpg> keytocard
+    Please select where to store the key:
+       (1) Signature key
+       (3) Authentication key
+    Your selection? 1
+
+You can use the **[S]** key both for Signature and Authentication, but
+we want to make sure it's in the Signature slot, so choose (1). Once
+again, if your command returns without an error, then the operation was
+successful::
+
+    gpg> q
+    Save changes? (y/N) y
+
+Saving the changes will delete the keys you moved to the card from your
+home directory (but it's okay, because we have them in our backups
+should we need to do this again for a replacement smartcard).
+
+Verifying that the keys were moved
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If you perform ``--list-secret-keys`` now, you will see a subtle
+difference in the output::
+
+    $ gpg --list-secret-keys
+    sec#  rsa2048 2018-01-24 [SC] [expires: 2020-01-24]
+          000000000000000000000000AAAABBBBCCCCDDDD
+    uid           [ultimate] Alice Dev <adev@kernel.org>
+    ssb>  rsa2048 2018-01-24 [E] [expires: 2020-01-24]
+    ssb>  ed25519 2018-01-24 [S]
+
+The ``>`` in the ``ssb>`` output indicates that the subkey is only
+available on the smartcard. If you go back into your secret keys
+directory and look at the contents there, you will notice that the
+``.key`` files there have been replaced with stubs::
+
+    $ cd ~/.gnupg/private-keys-v1.d
+    $ strings *.key | grep 'private-key'
+
+The output should contain ``shadowed-private-key`` to indicate that
+these files are only stubs and the actual content is on the smartcard.
+
+Verifying that the smartcard is functioning
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To verify that the smartcard is working as intended, you can create a
+signature::
+
+    $ echo "Hello world" | gpg --clearsign > /tmp/test.asc
+    $ gpg --verify /tmp/test.asc
+
+This should ask for your smartcard PIN on your first command, and then
+show "Good signature" after you run ``gpg --verify``.
+
+Congratulations, you have successfully made it extremely difficult to
+steal your digital developer identity!
+
+Other common GnuPG operations
+-----------------------------
+
+Here is a quick reference for some common operations you'll need to do
+with your PGP key.
+
+Mounting your master key offline storage
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You will need your master key for any of the operations below, so you
+will first need to mount your backup offline storage and tell GnuPG to
+use it::
+
+    $ export GNUPGHOME=/media/disk/foo/gnupg-backup
+    $ gpg --list-secret-keys
+
+You want to make sure that you see ``sec`` and not ``sec#`` in the
+output (the ``#`` means the key is not available and you're still using
+your regular home directory location).
+
+Extending key expiration date
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The master key has the default expiration date of 2 years from the date
+of creation. This is done both for security reasons and to make obsolete
+keys eventually disappear from keyservers.
+
+To extend the expiration on your key by a year from current date, just
+run::
+
+    $ gpg --quick-set-expire [fpr] 1y
+
+You can also use a specific date if that is easier to remember (e.g.
+your birthday, January 1st, or Canada Day)::
+
+    $ gpg --quick-set-expire [fpr] 2020-07-01
+
+Remember to send the updated key back to keyservers::
+
+    $ gpg --send-key [fpr]
+
+Updating your work directory after any changes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+After you make any changes to your key using the offline storage, you will
+want to import these changes back into your regular working directory::
+
+    $ gpg --export | gpg --homedir ~/.gnupg --import
+    $ unset GNUPGHOME
+
+
+Using PGP with Git
+==================
+
+One of the core features of Git is its decentralized nature -- once a
+repository is cloned to your system, you have full history of the
+project, including all of its tags, commits and branches. However, with
+hundreds of cloned repositories floating around, how does anyone verify
+that their copy of linux.git has not been tampered with by a malicious
+third party?
+
+Or what happens if a backdoor is discovered in the code and the "Author"
+line in the commit says it was done by you, while you're pretty sure you
+had `nothing to do with it`_?
+
+To address both of these issues, Git introduced PGP integration. Signed
+tags prove the repository integrity by assuring that its contents are
+exactly the same as on the workstation of the developer who created the
+tag, while signed commits make it nearly impossible for someone to
+impersonate you without having access to your PGP keys.
+
+.. _`nothing to do with it`: https://github.com/jayphelps/git-blame-someone-else
+
+Configure git to use your PGP key
+---------------------------------
+
+If you only have one secret key in your keyring, then you don't really
+need to do anything extra, as it becomes your default key.  However, if
+you happen to have multiple secret keys, you can tell git which key
+should be used (``[fpr]`` is the fingerprint of your key)::
+
+    $ git config --global user.signingKey [fpr]
+
+**IMPORTANT**: If you have a distinct ``gpg2`` command, then you should
+tell git to always use it instead of the legacy ``gpg`` from version 1::
+
+    $ git config --global gpg.program gpg2
+
+How to work with signed tags
+----------------------------
+
+To create a signed tag, simply pass the ``-s`` switch to the tag
+command::
+
+    $ git tag -s [tagname]
+
+Our recommendation is to always sign git tags, as this allows other
+developers to ensure that the git repository they are pulling from has
+not been maliciously altered.
+
+How to verify signed tags
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To verify a signed tag, simply use the ``verify-tag`` command::
+
+    $ git verify-tag [tagname]
+
+If you are pulling a tag from another fork of the project repository,
+git should automatically verify the signature at the tip you're pulling
+and show you the results during the merge operation::
+
+    $ git pull [url] tags/sometag
+
+The merge message will contain something like this::
+
+    Merge tag 'sometag' of [url]
+
+    [Tag message]
+
+    # gpg: Signature made [...]
+    # gpg: Good signature from [...]
+
+If you are verifying someone else's git tag, then you will need to
+import their PGP key. Please refer to the
+":ref:`verify_identities`" section below.
+
+Configure git to always sign annotated tags
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Chances are, if you're creating an annotated tag, you'll want to sign
+it. To force git to always sign annotated tags, you can set a global
+configuration option::
+
+    $ git config --global tag.forceSignAnnotated true
+
+How to work with signed commits
+-------------------------------
+
+It is easy to create signed commits, but it is much more difficult to
+use them in Linux kernel development, since it relies on patches sent to
+the mailing list, and this workflow does not preserve PGP commit
+signatures. Furthermore, when rebasing your repository to match
+upstream, even your own PGP commit signatures will end up discarded. For
+this reason, most kernel developers don't bother signing their commits
+and will ignore signed commits in any external repositories that they
+rely upon in their work.
+
+However, if you have your working git tree publicly available at some
+git hosting service (kernel.org, infradead.org, ozlabs.org, or others),
+then the recommendation is that you sign all your git commits even if
+upstream developers do not directly benefit from this practice.
+
+We recommend this for the following reasons:
+
+1. Should there ever be a need to perform code forensics or track code
+   provenance, even externally maintained trees carrying PGP commit
+   signatures will be valuable for such purposes.
+2. If you ever need to re-clone your local repository (for example,
+   after a disk failure), this lets you easily verify the repository
+   integrity before resuming your work.
+3. If someone needs to cherry-pick your commits, this allows them to
+   quickly verify their integrity before applying them.
+
+Creating signed commits
+~~~~~~~~~~~~~~~~~~~~~~~
+
+To create a signed commit, you just need to pass the ``-S`` flag to the
+``git commit`` command (it's capital ``-S`` due to collision with
+another flag)::
+
+    $ git commit -S
+
+Configure git to always sign commits
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You can tell git to always sign commits::
+
+    git config --global commit.gpgSign true
+
+.. note::
+
+    Make sure you configure ``gpg-agent`` before you turn this on.
+
+.. _verify_identities:
+
+How to verify kernel developer identities
+=========================================
+
+Signing tags and commits is easy, but how does one go about verifying
+that the key used to sign something belongs to the actual kernel
+developer and not to a malicious imposter?
+
+Configure auto-key-retrieval using WKD and DANE
+-----------------------------------------------
+
+If you are not already someone with an extensive collection of other
+developers' public keys, then you can jumpstart your keyring by relying
+on key auto-discovery and auto-retrieval. GnuPG can piggyback on other
+delegated trust technologies, namely DNSSEC and TLS, to get you going if
+the prospect of starting your own Web of Trust from scratch is too
+daunting.
+
+Add the following to your ``~/.gnupg/gpg.conf``::
+
+    auto-key-locate wkd,dane,local
+    auto-key-retrieve
+
+DNS-Based Authentication of Named Entities ("DANE") is a method for
+publishing public keys in DNS and securing them using DNSSEC signed
+zones. Web Key Directory ("WKD") is the alternative method that uses
+https lookups for the same purpose. When using either DANE or WKD for
+looking up public keys, GnuPG will validate DNSSEC or TLS certificates,
+respectively, before adding auto-retrieved public keys to your local
+keyring.
+
+Kernel.org publishes the WKD for all developers who have kernel.org
+accounts. Once you have the above changes in your ``gpg.conf``, you can
+auto-retrieve the keys for Linus Torvalds and Greg Kroah-Hartman (if you
+don't already have them)::
+
+    $ gpg --locate-keys torvalds@kernel.org gregkh@kernel.org
+
+If you have a kernel.org account, then you should `add the kernel.org
+UID to your key`_ to make WKD more useful to other kernel developers.
+
+.. _`add the kernel.org UID to your key`: https://korg.wiki.kernel.org/userdoc/mail#adding_a_kernelorg_uid_to_your_pgp_key
+
+Web of Trust (WOT) vs. Trust on First Use (TOFU)
+------------------------------------------------
+
+PGP incorporates a trust delegation mechanism known as the "Web of
+Trust." At its core, this is an attempt to replace the need for
+centralized Certification Authorities of the HTTPS/TLS world. Instead of
+various software makers dictating who should be your trusted certifying
+entity, PGP leaves this responsibility to each user.
+
+Unfortunately, very few people understand how the Web of Trust works.
+While it remains an important aspect of the OpenPGP specification,
+recent versions of GnuPG (2.2 and above) have implemented an alternative
+mechanism called "Trust on First Use" (TOFU). You can think of TOFU as
+"the SSH-like approach to trust." With SSH, the first time you connect
+to a remote system, its key fingerprint is recorded and remembered. If
+the key changes in the future, the SSH client will alert you and refuse
+to connect, forcing you to make a decision on whether you choose to
+trust the changed key or not. Similarly, the first time you import
+someone's PGP key, it is assumed to be valid. If at any point in the
+future GnuPG comes across another key with the same identity, both the
+previously imported key and the new key will be marked as invalid and
+you will need to manually figure out which one to keep.
+
+We recommend that you use the combined TOFU+PGP trust model (which is
+the new default in GnuPG v2). To set it, add (or modify) the
+``trust-model`` setting in ``~/.gnupg/gpg.conf``::
+
+    trust-model tofu+pgp
+
+How to use keyservers (more) safely
+-----------------------------------
+
+If you get a "No public key" error when trying to validate someone's
+tag, then you should attempt to lookup that key using a keyserver. It is
+important to keep in mind that there is absolutely no guarantee that the
+key you retrieve from PGP keyservers belongs to the actual person --
+that much is by design. You are supposed to use the Web of Trust to
+establish key validity.
+
+How to properly maintain the Web of Trust is beyond the scope of this
+document, simply because doing it properly requires both effort and
+dedication that tends to be beyond the caring threshold of most human
+beings. Here are some shortcuts that will help you reduce the risk of
+importing a malicious key.
+
+First, let's say you've tried to run ``git verify-tag`` but it returned
+an error saying the key is not found::
+
+    $ git verify-tag sunxi-fixes-for-4.15-2
+    gpg: Signature made Sun 07 Jan 2018 10:51:55 PM EST
+    gpg:                using RSA key DA73759BF8619E484E5A3B47389A54219C0F2430
+    gpg:                issuer "wens@...org"
+    gpg: Can't check signature: No public key
+
+Let's query the keyserver for more info about that key fingerprint (the
+fingerprint probably belongs to a subkey, so we can't use it directly
+without finding out the ID of the master key it is associated with)::
+
+    $ gpg --search DA73759BF8619E484E5A3B47389A54219C0F2430
+    gpg: data source: hkp://keys.gnupg.net
+    (1) Chen-Yu Tsai <wens@...org>
+          4096 bit RSA key C94035C21B4F2AEB, created: 2017-03-14, expires: 2019-03-15
+    Keys 1-1 of 1 for "DA73759BF8619E484E5A3B47389A54219C0F2430".  Enter number(s), N)ext, or Q)uit > q
+
+Locate the ID of the master key in the output, in our example
+``C94035C21B4F2AEB``. Now display the key of Linus Torvalds that you
+have on your keyring::
+
+    $ gpg --list-key torvalds@kernel.org
+    pub   rsa2048 2011-09-20 [SC]
+          ABAF11C65A2970B130ABE3C479BE3E4300411886
+    uid           [ unknown] Linus Torvalds <torvalds@kernel.org>
+    sub   rsa2048 2011-09-20 [E]
+
+Next, open the `PGP pathfinder`_. In the "From" field, paste the key
+fingerprint of Linus Torvalds from the output above. In the "To" field,
+paste they key-id you found via ``gpg --search`` of the unknown key, and
+check the results:
+
+- `Finding paths to Linus`_
+
+If you get a few decent trust paths, then it's a pretty good indication
+that it is a valid key. You can add it to your keyring from the
+keyserver now::
+
+    $ gpg --recv-key C94035C21B4F2AEB
+
+This process is not perfect, and you are obviously trusting the
+administrators of the PGP Pathfinder service to not be malicious (in
+fact, this goes against :ref:`devs_not_infra`). However, if you
+do not carefully maintain your own web of trust, then it is a marked
+improvement over blindly trusting keyservers.
+
+.. _`PGP pathfinder`: https://pgp.cs.uu.nl/
+.. _`Finding paths to Linus`: https://pgp.cs.uu.nl/paths/79BE3E4300411886/to/C94035C21B4F2AEB.html