This repo is for review of requests for signing shim. To create a request for review:
- clone this repo (preferably fork it)
- edit the template below
- add the shim.efi to be signed
- add build logs
- add any additional binaries/certificates/SHA256 hashes that may be needed
- commit all of that
- tag it with a tag of the form "myorg-shim-arch-YYYYMMDD"
- push it to GitHub
- file an issue at https://github.com/rhboot/shim-review/issues with a link to your tag
- approval is ready when the "accepted" label is added to your issue
Note that we really only have experience with using GRUB2 or systemd-boot on Linux, so asking us to endorse anything else for signing is going to require some convincing on your part.
Hint: check the docs directory in this repo for guidance on submission and getting your shim signed.
Here's the template:
Canonical Ltd.
Ubuntu
What's the justification that this really does need to be signed for the whole world to be able to boot it?
We are a well-known and widely used Linux distro.
We build and sign our own bootloaders and kernels with many custom patches.
The security contacts need to be verified before the shim can be accepted. For subsequent requests, contact verification is only necessary if the security contacts or their PGP keys have changed since the last successful verification.
An authorized reviewer will initiate contact verification by sending each security contact a PGP-encrypted email containing random words.
You will be asked to post the contents of these mails in your shim-review issue to prove ownership of the email addresses and PGP keys.
- Name: Julian Andres Klode
- Position: engineer
- Email address: julian.klode@canonical.com
- PGP key fingerprint: AEE1 C8AA AAF0 B768 4019 C546 021B 361B 6B03 1B00
(Key should be signed by the other security contacts, pushed to a keyserver like keyserver.ubuntu.com, and preferably have signatures that are reasonably well known in the Linux community.)
Secondary contact 1:
- Name: dann frazier
- Position: engineer
- Email address: dannf@ubuntu.com
- PGP key fingerprint: 09F4 7DBF 2D32 EEDC 2443 EBEE 1BF8 3C5E 54FC 8640
Secondary contact 2:
- Name: Mate Kukri
- Position: Software Engineer
- Email address: mate.kukri@canonical.com
- PGP key fingerprint: 9850 FD0C 92D5 2276 794E 4595 5243 F6D8 1246 00EC
(Key should be signed by the other security contacts, pushed to a keyserver like keyserver.ubuntu.com, and preferably have signatures that are reasonably well known in the Linux community.)
Please create your shim binaries starting with the 15.8 shim release tar file: https://github.com/rhboot/shim/releases/download/15.8/shim-15.8.tar.bz2
This matches https://github.com/rhboot/shim/releases/tag/15.8 and contains the appropriate gnu-efi source.
Make sure the tarball is correct by verifying your download's checksum with the following ones:
a9452c2e6fafe4e1b87ab2e1cac9ec00 shim-15.8.tar.bz2
cdec924ca437a4509dcb178396996ddf92c11183 shim-15.8.tar.bz2
a79f0a9b89f3681ab384865b1a46ab3f79d88b11b4ca59aa040ab03fffae80a9 shim-15.8.tar.bz2
30b3390ae935121ea6fe728d8f59d37ded7b918ad81bea06e213464298b4bdabbca881b30817965bd397facc596db1ad0b8462a84c87896ce6c1204b19371cd1 shim-15.8.tar.bz2
Make sure that you've verified that your build process uses that file as a source of truth (excluding external patches) and its checksum matches. Furthermore, there's a detached signature as well - check with the public key that has the fingerprint 8107B101A432AAC9FE8E547CA348D61BC2713E9F that the tarball is authentic. Once you're sure, please confirm this here with a simple yes.
A short guide on verifying public keys and signatures should be available in the docs directory.
The shim-15.8.tar.bz2 is used as the original tarball.
Hint: If you attach all the patches and modifications that are being used to your application, you can point to the URL of your application here (https://github.com/YOUR_ORGANIZATION/shim-review).
You can also point to your custom git servers, where the code is hosted.
https://code.launchpad.net/~ubuntu-uefi-team/+git/shim/+ref/master
Mention all the external patches and build process modifications, which are used during your building process, that make your shim binary be the exact one that you posted as part of this application.
Patches included also previous submission:
- debian/patches/ubuntu-no-addend-vendor-dbx.patch: Stop addending the vendor dbx to the MokListX, ours is too large. Our kernels don't read it anyway, and new ones that will can just embed it themselves.
- debian/patches/Build-an-additional-NX-shim-mark-MokManager-and-Fallback-.patch: Build two copies of shim for NX rollout. Mark MokManager and Fallback as NX_COMPAT. (Enforcement properties of the two shims are detailed in the next answer.)
The second patch is new, and is part of our NX rollout.
Do you have the NX bit set in your shim? If so, is your entire boot stack NX-compatible and what testing have you done to ensure such compatibility?
See https://techcommunity.microsoft.com/t5/hardware-dev-center/nx-exception-for-shim-community/ba-p/3976522 for more details on the signing of shim without NX bit.
- There are two shims per architecture:
- With the NX_COMPAT bit and MokPolicy set to enforce NX
- Without the NX_COMPAT bit and MokPolicy set to not require NX
- We have implemented NX compatibility in the latest version of GRUB2 in Ubuntu 24.10
- We have had NX compatible kernels for a while
What exact implementation of Secure Boot in GRUB2 do you have? (Either Upstream GRUB2 shim_lock verifier or Downstream RHEL/Fedora/Debian/Canonical-like implementation)
Skip this, if you're not using GRUB2.
- GRUB 2.06 with "Downstream RHEL/Fedora/Debian/Canonical-like implementation"
- GRUB 2.12 with "Upstream GRUB2 shim_lock verifier" with the peimage loader added
Skip this, if you're not using GRUB2, otherwise make sure these are present and confirm with yes.
- 2020 July - BootHole
- Details: https://lists.gnu.org/archive/html/grub-devel/2020-07/msg00034.html
- CVE-2020-10713
- CVE-2020-14308
- CVE-2020-14309
- CVE-2020-14310
- CVE-2020-14311
- CVE-2020-15705
- CVE-2020-15706
- CVE-2020-15707
- March 2021
- Details: https://lists.gnu.org/archive/html/grub-devel/2021-03/msg00007.html
- CVE-2020-14372
- CVE-2020-25632
- CVE-2020-25647
- CVE-2020-27749
- CVE-2020-27779
- CVE-2021-3418 (if you are shipping the shim_lock module)
- CVE-2021-20225
- CVE-2021-20233
- June 2022
- Details: https://lists.gnu.org/archive/html/grub-devel/2022-06/msg00035.html, SBAT increase to 2
- CVE-2021-3695
- CVE-2021-3696
- CVE-2021-3697
- CVE-2022-28733
- CVE-2022-28734
- CVE-2022-28735
- CVE-2022-28736
- CVE-2022-28737
- November 2022
- Details: https://lists.gnu.org/archive/html/grub-devel/2022-11/msg00059.html, SBAT increase to 3
- CVE-2022-2601
- CVE-2022-3775
- October 2023 - NTFS vulnerabilities
- Details: https://lists.gnu.org/archive/html/grub-devel/2023-10/msg00028.html, SBAT increase to 4
- CVE-2023-4693
- CVE-2023-4692
Yes.
If shim is loading GRUB2 bootloader, and if these fixes have been applied, is the upstream global SBAT generation in your GRUB2 binary set to 4?
Skip this, if you're not using GRUB2, otherwise do you have an entry in your GRUB2 binary similar to:
grub,4,Free Software Foundation,grub,GRUB_UPSTREAM_VERSION,https://www.gnu.org/software/grub/?
Yes.
If you had no previous signed shim, say so here. Otherwise a simple yes will do.
- Pre-SBAT shims were revoked in dbx update.
- We use a self-managed CA certificate as the VENDOR_CERT.
- Vulnerable artefacts signed by the CA are revoked via VENDOR_DBX or SBAT.
Is upstream commit 1957a85b0032a81e6482ca4aab883643b8dae06e "efi: Restrict efivar_ssdt_load when the kernel is locked down" applied?
Is upstream commit 75b0cea7bf307f362057cc778efe89af4c615354 "ACPI: configfs: Disallow loading ACPI tables when locked down" applied?
Is upstream commit eadb2f47a3ced5c64b23b90fd2a3463f63726066 "lockdown: also lock down previous kgdb use" applied?
Hint: upstream kernels should have all these applied, but if you ship your own heavily-modified older kernel version, that is being maintained separately from upstream, this may not be the case. If you are shipping an older kernel, double-check your sources; maybe you do not have all the patches, but ship a configuration, that does not expose the issue(s).
All Ubuntu kernels in all currently supported series have the above applied.
All vulnerable kernels are disallowed to boot by VENDOR_DBX by their signing cert being revoked in vendor dbx.
Yes but there are like hundred patches and like 80 different kernels, so it's a bit much to include here. There's additional secure boot enforcing patches, hardware enablement, and zfs is built alongside.
Most interesting things are:
df8b92624f UBUNTU: SAUCE: (lockdown) security: lockdown: expose a hook to lock the kernel down
fede732054 UBUNTU: SAUCE: (lockdown) efi: Add an EFI_SECURE_BOOT flag to indicate secure boot mode
438296a598 UBUNTU: SAUCE: (lockdown) efi: Lock down the kernel if booted in secure boot mode
03deb74301 UBUNTU: SAUCE: (lockdown) s390: Lock down the kernel when the IPL secure flag is set
c2952ca438 UBUNTU: SAUCE: (lockdown) KEYS: Make use of platform keyring for module signature verify
9ba951d4e7 UBUNTU: SAUCE: (lockdown) arm64: Allow locking down the kernel under EFI secure boot
01f96e4abc UBUNTU: SAUCE: (lockdown) security: lockdown: Make CONFIG_LOCK_DOWN_IN_EFI_SECURE_BOOT more generic
59a69f2418 UBUNTU: SAUCE: (lockdown) powerpc: lock down kernel in secure boot mode
0db545033f UBUNTU: SAUCE: integrity: Load mokx certs from the EFI MOK config table
7482fcc79c UBUNTU: SAUCE: integrity: add informational messages when revoking certs
9075b83ae9 UBUNTU: [Packaging] Revoke 2012 UEFI signing certificate as built-in
The above ensure that lockdown is enforced when booted with secureboot, MOKX keys are imported into kernel .blacklist keyring, and thus revoked kernels are prohibited from kexec/kdump.
Notable features of our config options:
CONFIG_MODULE_SIG=y
CONFIG_MODULE_SIG_ALL=y
# CONFIG_MODULE_SIG_FORCE is not set
CONFIG_MODULE_SIG_FORMAT=y
CONFIG_MODULE_SIG_HASH="sha512"
CONFIG_MODULE_SIG_KEY="certs/signing_key.pem"
CONFIG_MODULE_SIG_KEY_TYPE_RSA=y
CONFIG_MODULE_SIG_SHA512=y
CONFIG_SYSTEM_REVOCATION_KEYS="debian/canonical-revoked-certs.pem"
CONFIG_SYSTEM_TRUSTED_KEYS="debian/canonical-certs.pem"
The above settings ensure that all drivers are signed with built-time ephemeral signing key. In addition, we trust livepatch & 3rd-party driver signing key for signing modules post kernel build.
Drivers signed with built-in kernel signing key:
-
CONFIG_STAGING=ythat are listed in./drivers/staging/signature-inclusion, currently exfat, realtek wifi drivers only. NB! most importantly android ashmem/binder are not signed -
Vendored at build-time dkms modules listed in
debian/dkms-versions, currently these arezfs-linux,v4l2loopback,backport-iwlwifi-dkmsfor ZFS, webcam and wifi support.
Drivers signed with SYSTEM_TRUSTED_KEYS:
-
Canonical Livepatch Service modules for livepatching security vulnerabilities
-
Detached reproducible builds NVIDIA proprietary driver signatures
Certificates present in CONFIG_SYSTEM_REVOCATION_KEYS:
- The certificates in
CONFIG_SYSTEM_REVOCATION_KEYSare the same as shim'sVENDOR_DBXdiscussed below. This is to ensure that kernel prohibits kexec/kdump of kernels that are distrusted by the shim to boot. This works, even if MOKX mirroring facility fails at runtime, due to shim/platform deficiencies.
If not, please describe how you ensure that one kernel build does not load modules built for another kernel.
Yes.
If you use vendor_db functionality of providing multiple certificates and/or hashes please briefly describe your certificate setup.
If there are allow-listed hashes please provide exact binaries for which hashes are created via file sharing service, available in public with anonymous access for verification.
VENDOR_DB is not used.
If you are re-using the CA certificate from your last shim binary, you will need to add the hashes of the previous GRUB2 binaries exposed to the CVEs mentioned earlier to vendor_dbx in shim. Please describe your strategy.
This ensures that your new shim+GRUB2 can no longer chainload those older GRUB2 binaries with issues.
If this is your first application or you're using a new CA certificate, please say so here.
We are shipping VENDOR_DBX that includes all previously used certificates.
A reviewer should always be able to run docker build . to get the exact binary you attached in your application.
Hint: Prefer using frozen packages for your toolchain, since an update to GCC, binutils, gnu-efi may result in building a shim binary with a different checksum.
If your shim binaries can't be reproduced using the provided Dockerfile, please explain why that's the case, what the differences would be and what build environment (OS and toolchain) is being used to reproduce this build? In this case please write a detailed guide, how to setup this build environment from scratch.
The shim binaries were built in Ubuntu 24.04 LTS (Noble Numbat).
The provided Dockerfile should reproduce the binaries, this is also demonstrated by a GitHub workflow.
This should include logs for creating the buildroots, applying patches, doing the build, creating the archives, etc.
The buildlog_* files.
For example, signing new kernel's variants, UKI, systemd-boot, new certs, new CA, etc..
Skip this, if this is your first application for having shim signed.
We have an NX compatible shim now.
$ sha256sum shim*.efi
cbb8344f28251666fdf72b5441b0f8baa6acaa54ccf3ba7a22be1c322396761b shimaa64.efi
b638835c84d03d7bcf9f7dcca84d2c3d1cac010c5a627283335882aeeae95222 shimaa64.nx.efi
e0998956d4af07192246ffe45ba80351dea4457d2b55b523f42562715fae9fa3 shimx64.efi
a52e66a6d58f923ae3621ff34e89f922c49210390f15fdf174c26ab1a34cdd1d shimx64.nx.efi
Describe the security strategy that is used for key protection. This can range from using hardware tokens like HSMs or Smartcards, air-gapped vaults, physical safes to other good practices.
The CA certificate used as VENDOR_CERT is always stored offline, split using Shamir's Secret Sharing into 7 fragments distributed globally, 3 of which are required to assemble the cert.
Thus we require international travel to be available to assemble it and issue new certificates.
A yes or no will do. There's no penalty for the latter.
No
Do you add a vendor-specific SBAT entry to the SBAT section in each binary that supports SBAT metadata ( GRUB2, fwupd, fwupdate, systemd-boot, systemd-stub, shim + all child shim binaries )?
Hint: The history of SBAT and more information on how it works can be found here. That document is large, so for just some examples check out SBAT.example.md
If you are using a downstream implementation of GRUB2 (e.g. from Fedora or Debian), make sure you have their SBAT entries preserved and that you append your own (don't replace theirs) to simplify revocation.
Remember to post the entries of all the binaries. Apart from your bootloader, you may also be shipping e.g. a firmware updater, which will also have these.
Hint: run objcopy --only-section .sbat -O binary YOUR_EFI_BINARY /dev/stdout to get these entries. Paste them here. Preferably surround each listing with three backticks (```), so they render well.
shim, fb, mm:
sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
shim,4,UEFI shim,shim,1,https://github.com/rhboot/shim
shim.ubuntu,1,Ubuntu,shim,15.8-0ubuntu2,https://www.ubuntu.com/
grub: (template, versions and peimage presence vary per series)
sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
grub,4,Free Software Foundation,grub,@UPSTREAM_VERSION@,https://www.gnu.org/software/grub/
grub.ubuntu,2,Ubuntu,grub2,@DEB_VERSION@,https://www.ubuntu.com/
grub.peimage,2,Canonical,grub2,@DEB_VERSION@,https://salsa.debian.org/grub-team/grub/-/blob/master/debian/patches/secure-boot/efi-use-peimage-shim.patch
fwupd (versions vary per series):
sbat,1,UEFI shim,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
fwupd,1,Firmware update daemon,fwupd,$UPSTREAM_VERSION$,https://github.com/fwupd/fwupd
fwupd.ubuntu,1,Ubuntu,fwupd,$PACKAGE_VERSION$,https://launchpad.net/ubuntu/+source/fwupd
kernel.efi (versions vary per series):
sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
systemd,1,The systemd Developers,systemd,$UPSTREAM_VERSION$,https://www.freedesktop.org/wiki/Software/systemd
systemd.ubuntu,1,Ubuntu,systemd,$PACKAGE_VERSION$,https://bugs.launchpad.net/ubuntu/
Skip this, if you're not using GRUB2.
Hint: this is about those modules that are in the binary itself, not the .mod files in your filesystem.
basic:
all_video
boot
btrfs
cat
chain
configfile
echo
efifwsetup
efinet
ext2
fat
font
gettext
gfxmenu
gfxterm
gfxterm_background
gzio
halt
help
hfsplus
iso9660
jpeg
keystatus
loadenv
loopback
linux
ls
lsefi
lsefimmap
lsefisystab
lssal
memdisk
minicmd
normal
ntfs
part_apple
part_msdos
part_gpt
password_pbkdf2
png
probe
reboot
regexp
search
search_fs_uuid
search_fs_file
search_label
sleep
smbios
squash4
test
true
video
xfs
zfs
zfscrypt
zfsinfo
amd64-only:
cpuid
linuxefi
play
tpm
installed grub:
cryptodisk
gcry_arcfour
gcry_blowfish
gcry_camellia
gcry_cast5
gcry_crc
gcry_des
gcry_dsa
gcry_idea
gcry_md4
gcry_md5
gcry_rfc2268
gcry_rijndael
gcry_rmd160
gcry_rsa
gcry_seed
gcry_serpent
gcry_sha1
gcry_sha256
gcry_sha512
gcry_tiger
gcry_twofish
gcry_whirlpool
luks
lvm
mdraid09
mdraid1x
raid5rec
raid6rec
network grub image:
http
tftp
If you are using systemd-boot on arm64 or riscv, is the fix for unverified Devicetree Blob loading included?
We only use systemd-stub, not systemd-boot.
Building / Publishing https://launchpad.net/ubuntu/+source/grub2-unsigned - same signed grub binaries for all series
currently building next one (first one signed with it in):
https://launchpad.net/~ubuntu-uefi-team/+archive/ubuntu/build/+packages
Git managed source code https://code.launchpad.net/~ubuntu-core-dev/grub/+git/ubuntu/+ref/ubuntu
Note patches debian/patches
If your shim launches any other components apart from your bootloader, please provide further details on what is launched.
Hint: The most common case here will be a firmware updater like fwupd.
We load various UKIs which use systemd-boot stub to combine kernels and initrds into a single binary.
We use fwupd as a firmware updater.
If your GRUB2 or systemd-boot launches any other binaries that are not the Linux kernel in SecureBoot mode, please provide further details on what is launched and how it enforces Secureboot lockdown.
Skip this, if you're not using GRUB2 or systemd-boot.
GRUB2 may launch Windows Bootmgr on dual boot systems. Nebooted shim+grub2 may chainloader load shim+grub2 again from disk, which will verify things again as usual. (https://maas.io usecase).
Summarize in one or two sentences, how your secure bootchain works on higher level.
fwupd verifies capsule signatures; kernel implements lockdown.
Our kernels also check MokListXRT for revocations for kexec.
Does your shim load any loaders that support loading unsigned kernels (e.g. certain GRUB2 configurations)?
No, our GRUB enforces lockdown & uses shim protocol to verify next component.
linux, various versions. They include lockdown patches & ACPI patches, lockdown is enforced when booted with SecureBoot, config enforces kernel module signatures under lockdown.
The reviewing process is meant to be a peer-review effort and the best way to have your application reviewed faster is to help with reviewing others. We are in most cases volunteers working on this venue in our free time, rather than being employed and paid to review the applications during our business hours.
A reasonable timeframe of waiting for a review can reach 2-3 months. Helping us is the best way to shorten this period. The more help we get, the faster and the smoother things will go.
For newcomers, the applications labeled as easy to review are recommended to start the contribution process.
- Julian Andres Klode has done shim reviews in the past.
- Mate Kukri has done some unofficial shim reviews during the initial 15.8 rollout.
-
VENDOR_DBX file is included as
canonical-dbx-20221103.esl. One can unpack them usingsig-list-to-certsutility. -
We have disabled the ExitBootServices check:
- In the case of GRUB 2.06, in order to allow chainloading EFI executables. GRUB 2.06 uses the older linuxefi loader for verifying kernels, and it verifies chainloaded EFI executables via the firmware only.
- In order to allow booting artefacts directly from shim that do not have the need to do further verifications. For instance, we build CVM cloud images that directly boot UKIs from shim that do not need to do further verifications.
-
We have disabled the unacceptable 5s boot delay in fallback when TPM is present, as it impacts bootspeed for the noninteractive cloud instances that have vTPM & SecureBoot.
-
We currently use shim itself to roll out SbatLevel.
revocations.efiisn't used currently. This might change in the future.