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diff --git a/Documentation/process/maintainer-pgp-guide.rst b/Documentation/process/maintainer-pgp-guide.rst new file mode 100644 index 0000000000..f5277993b1 --- /dev/null +++ b/Documentation/process/maintainer-pgp-guide.rst @@ -0,0 +1,919 @@ +.. _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 2.2 or later +---------------------- + +Your distro should already have GnuPG installed by default, you just +need to verify that you are using a reasonably recent version of it. +To check, run:: + + $ gpg --version | head -n1 + +If you have version 2.2 or above, then you are good to go. If you have a +version that is prior than 2.2, then some commands from this guide may +not work. + +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. + +.. _protect_your_key: + +Protect your 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). + +Understanding PGP Subkeys +------------------------- + +A PGP key rarely consists of a single keypair -- usually it is a +collection of independent subkeys that can be used for different +purposes based on their capabilities, assigned at their creation time. +PGP defines four capabilities that a key can have: + +- **[S]** keys can be used for signing +- **[E]** keys can be used for encryption +- **[A]** keys can be used for authentication +- **[C]** keys can be used for certifying other keys + +The key with the **[C]** capability is often called the "master" key, +but this terminology is misleading because it implies that the Certify +key can be used in place of any of other subkey on the same chain (like +a physical "master key" can be used to open locks made for other keys). +Since this is not the case, this guide will refer to it as "the Certify +key" to avoid any ambiguity. + +It is critical to fully understand the following: + +1. All subkeys are fully independent from each other. If you lose a + private subkey, it cannot be restored or recreated from any other + private key on your chain. +2. With the exception of the Certify key, there can be multiple subkeys + with identical capabilities (e.g. you can have 2 valid encryption + subkeys, 3 valid signing subkeys, but only one valid certification + subkey). All subkeys are fully independent -- a message encrypted to + one **[E]** subkey cannot be decrypted with any other **[E]** subkey + you may also have. +3. A single subkey may have multiple capabilities (e.g. your **[C]** key + can also be your **[S]** key). + +The key carrying the **[C]** (certify) capability 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: + +- One subkey 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 ed25519 2022-12-20 [SC] [expires: 2024-12-19] + 000000000000000000000000AAAABBBBCCCCDDDD + uid [ultimate] Alice Dev <adev@kernel.org> + ssb cv25519 2022-12-20 [E] [expires: 2024-12-19] + +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 Certify 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-addkey [fpr] ed25519 sign + +.. note:: ECC support in GnuPG + + Note, that if you intend to use a hardware token that does not + support ED25519 ECC keys, you should choose "nistp256" instead or + "ed25519." See the section below on recommended hardware devices. + + +Back up your Certify 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. + +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 +PGP 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 Certify 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 Certify 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 Certify key:: + + $ gpg --with-keygrip --list-key [fpr] + +The output will be something like this:: + + pub ed25519 2022-12-20 [SC] [expires: 2022-12-19] + 000000000000000000000000AAAABBBBCCCCDDDD + Keygrip = 1111000000000000000000000000000000000000 + uid [ultimate] Alice Dev <adev@kernel.org> + sub cv25519 2022-12-20 [E] [expires: 2022-12-19] + Keygrip = 2222000000000000000000000000000000000000 + sub ed25519 2022-12-20 [S] + Keygrip = 3333000000000000000000000000000000000000 + +Find the keygrip entry that is beneath the ``pub`` line (right under the +Certify 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 Certify key keygrip:: + + $ cd ~/.gnupg/private-keys-v1.d + $ rm 1111000000000000000000000000000000000000.key + +Now, if you issue the ``--list-secret-keys`` command, it will show that +the Certify key is missing (the ``#`` indicates it is not available):: + + $ gpg --list-secret-keys + sec# ed25519 2022-12-20 [SC] [expires: 2024-12-19] + 000000000000000000000000AAAABBBBCCCCDDDD + uid [ultimate] Alice Dev <adev@kernel.org> + ssb cv25519 2022-12-20 [E] [expires: 2024-12-19] + ssb ed25519 2022-12-20 [S] + +You should also remove any ``secring.gpg`` files in the ``~/.gnupg`` +directory, which may be left over from previous 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 Certify 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`_. One of the few available commercial devices that + support ED25519 ECC keys, but offer fewest security features (such as + resistance to tampering or some side-channel attacks). +- `Nitrokey Pro 2`_: Similar to the Nitrokey Start, but more + tamper-resistant and offers more security features. Pro 2 supports ECC + cryptography (NISTP). +- `Yubikey 5`_: 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, among others, and now finally supports NISTP and ED25519 ECC + keys. + +Your choice will depend on cost, shipping availability in your +geographical region, and open/proprietary hardware considerations. + +.. note:: + + If you are listed in MAINTAINERS or have an account at kernel.org, + you `qualify for a free Nitrokey Start`_ courtesy of The Linux + Foundation. + +.. _`Nitrokey Start`: https://shop.nitrokey.com/shop/product/nitrokey-start-6 +.. _`Nitrokey Pro 2`: https://shop.nitrokey.com/shop/product/nkpr2-nitrokey-pro-2-3 +.. _`Yubikey 5`: https://www.yubico.com/products/yubikey-5-overview/ +.. _Gnuk: https://www.fsij.org/doc-gnuk/ +.. _`qualify for a free Nitrokey Start`: https://www.kernel.org/nitrokey-digital-tokens-for-kernel-developers.html + +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. + +.. warning:: + + Some devices may require that you move the subkeys onto the device + before you can change the passphrase. Please check the documentation + provided by the device manufacturer. + +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 ed25519/AAAABBBBCCCCDDDD + created: 2022-12-20 expires: 2024-12-19 usage: SC + trust: ultimate validity: ultimate + ssb cv25519/1111222233334444 + created: 2022-12-20 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# ed25519 2022-12-20 [SC] [expires: 2024-12-19] + 000000000000000000000000AAAABBBBCCCCDDDD + uid [ultimate] Alice Dev <adev@kernel.org> + ssb> cv25519 2022-12-20 [E] [expires: 2024-12-19] + ssb> ed25519 2022-12-20 [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 safe offline storage +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +You will need your Certify 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 Certify 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] 2025-07-01 + +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 gpg-agent over ssh +~~~~~~~~~~~~~~~~~~~~~~~~ + +You can forward your gpg-agent over ssh if you need to sign tags or +commits on a remote system. Please refer to the instructions provided +on the GnuPG wiki: + +- `Agent Forwarding over SSH`_ + +It works more smoothly if you can modify the sshd server settings on the +remote end. + +.. _`Agent Forwarding over SSH`: https://wiki.gnupg.org/AgentForwarding + +.. _pgp_with_git: + +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] + +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 work with signed patches +------------------------------- + +It is possible to use your PGP key to sign patches sent to kernel +developer mailing lists. Since existing email signature mechanisms +(PGP-Mime or PGP-inline) tend to cause problems with regular code +review tasks, you should use the tool kernel.org created for this +purpose that puts cryptographic attestation signatures into message +headers (a-la DKIM): + +- `Patatt Patch Attestation`_ + +.. _`Patatt Patch Attestation`: https://pypi.org/project/patatt/ + +Installing and configuring patatt +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Patatt is packaged for many distributions already, so please check there +first. You can also install it from pypi using "``pip install patatt``". + +If you already have your PGP key configured with git (via the +``user.signingKey`` configuration parameter), then patatt requires no +further configuration. You can start signing your patches by installing +the git-send-email hook in the repository you want:: + + patatt install-hook + +Now any patches you send with ``git send-email`` will be automatically +signed with your cryptographic signature. + +Checking patatt signatures +~~~~~~~~~~~~~~~~~~~~~~~~~~ + +If you are using ``b4`` to retrieve and apply patches, then it will +automatically attempt to verify all DKIM and patatt signatures it +encounters, for example:: + + $ b4 am 20220720205013.890942-1-broonie@kernel.org + [...] + Checking attestation on all messages, may take a moment... + --- + ✓ [PATCH v1 1/3] kselftest/arm64: Correct buffer allocation for SVE Z registers + ✓ [PATCH v1 2/3] arm64/sve: Document our actual ABI for clearing registers on syscall + ✓ [PATCH v1 3/3] kselftest/arm64: Enforce actual ABI for SVE syscalls + --- + ✓ Signed: openpgp/broonie@kernel.org + ✓ Signed: DKIM/kernel.org + +.. note:: + + Patatt and b4 are still in active development and you should check + the latest documentation for these projects for any new or updated + features. + +.. _kernel_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 + +Using the kernel.org web of trust repository +-------------------------------------------- + +Kernel.org maintains a git repository with developers' public keys as a +replacement for replicating keyserver networks that have gone mostly +dark in the past few years. The full documentation for how to set up +that repository as your source of public keys can be found here: + +- `Kernel developer PGP Keyring`_ + +If you are a kernel developer, please consider submitting your key for +inclusion into that keyring. + +.. _`Kernel developer PGP Keyring`: https://korg.docs.kernel.org/pgpkeys.html |