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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:

What organization or people are asking to have this signed?

Organization name and website:
The organization is Cloud Software Group, Inc (website: https://www.cloud.com/)

Cloud Software Group, Inc was formed from a merger between Citrix Systems, Inc (website: https://www.citrix.com) and TIBCO Software which means the company register entries and public certificate details below still refer to Citrix Systems, Inc.

What's the legal data that proves the organization's genuineness?

The reviewers should be able to easily verify, that your organization is a legal entity, to prevent abuse. Provide the information, which can prove the genuineness with certainty.

Company/tax register entries or equivalent:
(a link to the organization entry in your jurisdiction's register will do)

https://search.sunbiz.org/Inquiry/CorporationSearch/SearchResultDetail?inquirytype=EntityName&directionType=Initial&searchNameOrder=CITRIXSYSTEMS%20P299920&aggregateId=forp-p29992-fb5e8d14-6c9a-4783-a1d0-a23ca7776da4&searchTerm=citrix&listNameOrder=CITRIXCOMMUNICATIONS%20M130000033450

The public details of both your organization and the issuer in the EV certificate used for signing .cab files at Microsoft Hardware Dev Center File Signing Services.
(not the CA certificate embedded in your shim binary)

Example:

Issuer: O=MyIssuer, Ltd., CN=MyIssuer EV Code Signing CA
Subject: C=XX, O=MyCompany, Inc., CN=MyCompany, Inc.

Issuer: C=US, O=DigiCert, Inc., CN=DigiCert Trusted G4 Code Signing RSA4096 SHA384 2021 CA1

Subject: C=US, S=Florida, L=Fort Lauderdale, O=Citrix Systems, Inc., CN=Citrix Systems, Inc.

What product or service is this for?

The product is XenServer, a virtualization platform. More details at https://www.xenserver.com/

What's the justification that this really does need to be signed for the whole world to be able to boot it?

XenServer is an operating system that customers can buy, install, and run on their own hardware.

Why are you unable to reuse shim from another distro that is already signed?

XenServer contains custom builds of the boot components (Shim, GRUB, Xen, Linux) signed with our vendor key therefore we need a build of Shim that contains our own vendor key.

Who is the primary contact for security updates, etc.?

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: Ross Lagerwall
  • Position: Principal Systems Software Engineer
  • Email address: [email protected]
  • PGP key fingerprint: 062A A6EC C8CE CDCD FD69 B4FA 0014 F59B E38B E503

(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.)

Who is the secondary contact for security updates, etc.?

  • Name: Andrew Cooper
  • Position: Principal Systems Software Engineer, Xen Project committer, x86 maintainer, and security team member
  • Email address: [email protected]
  • PGP key fingerprint: CF35 495B 7EA6 F70E A77D 2176 65C3 F906 A5D7 9FA0

(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.)

Were these binaries created from the 16.1 shim release tar?

Please create your shim binaries starting with the 16.1 shim release tar file: https://github.com/rhboot/shim/releases/download/16.1/shim-16.1.tar.bz2

This matches https://github.com/rhboot/shim/releases/tag/16.1 and contains the appropriate gnu-efi source.

Make sure the tarball is correct by verifying your download's checksum (SHA256, SHA512) with the following ones:

46319cd228d8f2c06c744241c0f342412329a7c630436fce7f82cf6936b1d603  shim-16.1.tar.bz2
ca5f80e82f3b80b622028f03ef23105c98ee1b6a25f52a59c823080a3202dd4b9962266489296e99f955eb92e36ce13e0b1d57f688350006bba45f2718f159fb  shim-16.1.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. You can also further validate the release by checking the PGP signature: there's a detached signature

The release is signed by the maintainer Peter Jones - his master key has the fingerprint B00B48BC731AA8840FED9FB0EED266B70F4FEF10 and the signing sub-key in the signature here has the fingerprint 02093E0D19DDE0F7DFFBB53C1FD3F540256A1372. A copy of his public key is included here for reference: pjones.asc

Once you're sure that the tarball you are using is correct and authentic, please confirm this here with a simple yes.

A short guide on verifying public keys and signatures should be available in the docs directory.

yes

URL for a repo that contains the exact code which was built to result in your binary:

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://github.com/xenserver/shim-review

What patches are being applied and why:

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.

  • 0001-pe-Fix-PF-in-GRUB-after-memattrs-call.patch

This fixes a page fault caused by invalid update_mem_attrs() calls in Shim.

A PR is open here: rhboot/shim#772

  • ignore-mm-missing.patch

This adds a build option "IGNORE_MM_MISSING" which allows Shim to tolerate MokManager being missing. XenServer has no use for MokManager so if it is missing it should not result in a hard failure. We pass "IGNORE_MM_MISSING=1" to the make call.

A PR is open here: rhboot/shim#759

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.

Yes, the NX_COMPAT bit is set in Shim. Yes, the entire boot stack is NX-compatible.

To test this, we have built the latest stable tag of OVMF (edk2-stable202505) (which supports the Memory Attributes protocol on x86_64) and enabled strict memory protections (--pcd gEfiMdeModulePkgTokenSpaceGuid.PcdDxeNxMemoryProtectionPolicy=0xC000000000007FD5), then used that to test booting XenServer with and without Secure Boot enabled.

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.

Upstream GRUB2 shim_lock verifier.

Do you have fixes for all the following GRUB2 CVEs applied?

Skip this, if you're not using GRUB2, otherwise make sure these are present and confirm with yes.

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/?

The SBAT generation is set to 5 since it includes fixes for the February 2025 GRUB CVEs.

Were old shims hashes provided to Microsoft for verification and to be added to future DBX updates?

Does your new chain of trust disallow booting old GRUB2 builds affected by the CVEs?

If you had no previous signed shim, say so here. Otherwise a simple yes will do.

No previous signed shim.

If your boot chain of trust includes a Linux kernel:

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).

Yes.

How does your signed kernel enforce lockdown when your system runs

with Secure Boot enabled?

Hint: If it does not, we are not likely to sign your shim.

In our build, the upstream kernel's lockdown mode is enabled by default and can be disabled using a command-line option if and only if Secure Boot is disabled.

Do you build your signed kernel with additional local patches? What do they do?

Yes. As with many distros, we have a number of patches to tailor the kernel to our needs, fix bugs, etc. Most of these are not relevant to Secure Boot. The full patchqueue is included here. These patches are related to Secure Boot:

Do you use an ephemeral key for signing kernel modules?

If not, please describe how you ensure that one kernel build does not load modules built for another kernel.

An ephemeral key is used to sign in-tree kernel modules.

The kernel also has additional certificates embedded to verify out-of-tree drivers and live patches.

In most cases, a newer kernel can load out-of-tree drivers built against an older kernel. If there is a security vulnerability, the vulnerable module can be revoked by hash or the existing certificate can be removed from the new kernel (requiring a new key to be generated and modules re-signed).

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.

We are not using vendor_db functionality.

In our setup, shimx64.efi has a single embedded key which is used to verify shim_certificate_0.efi and revocations.efi.

shim_certificate_0.efi contains 3 separate certificates for verifying GRUB, Xen, and Linux.

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.

First application

Is the Dockerfile in your repository the recipe for reproducing the building of your shim binary?

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.

Yes. Run podman build --no-cache . (or an equivalent command) to reproduce the exact shim binary.

Which files in this repo are the logs for your build?

This should include logs for creating the buildroots, applying patches, doing the build, creating the archives, etc.

See the build for the full log output from Koji. In particular, build/x86_64/build.log contains the actual compilation logs.

What changes were made in the distro's secure boot chain since your SHIM was last signed?

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.

No previous signed Shim.

What is the SHA256 hash of your final shim binary?

1ee16ab634fbde69f933a89bba241da9750c30d4a87479f1c5223b82c9848b58

How do you manage and protect the keys used in your shim?

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 keys are stored in an HSM with offline backup in a tamperproof bag in a physical safe. Access to the HSM is restricted such that only relevant builds with appropriate approvals can be signed.

Do you use EV certificates as embedded certificates in the shim?

A yes or no will do. There's no penalty for the latter.

No.

Are you embedding a CA certificate in your shim?

A yes or no will do. There's no penalty for the latter. However, if yes: does that certificate include the X509v3 Basic Constraints to say that it is a CA? See the docs for more guidance about this.

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 )?

Please provide the exact SBAT entries for all binaries you are booting directly through shim.

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.

Yes, each SBAT-supporting binary contains a vendor-specific SBAT section. fwupd/fwupdate is not supported.

Shim (shimx64.efi):

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.xs,1,Cloud Software Group,shim,16.1-4.xs9,mailto:[email protected]

Fallback (fbx64.efi):

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.xs,1,Cloud Software Group,shim,16.1-4.xs9,mailto:[email protected]

MokManager (mmx64.efi):

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.xs,1,Cloud Software Group,shim,16.1-4.xs9,mailto:[email protected]

GRUB (grubx64.efi):

sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
grub,5,Free Software Foundation,grub,2.12,https://www.gnu.org/software/grub/
grub.xs,1,Cloud Software Group,grub,2.12-14.xs9,mailto:[email protected]

Xen (xen-4.20.1-4.efi:

sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
xen.xs,1,Cloud Software Group,xen,4.20.1-4.xs9,mailto:[email protected]

Linux (vmlinuz-6.6.98+0):

sbat,1,SBAT Version,sbat,1,https://github.com/rhboot/shim/blob/main/SBAT.md
linux.xs,1,Cloud Software Group,linux,1,mailto:[email protected]

If shim is loading GRUB2 bootloader, which modules are built into your signed GRUB2 image?

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.

  • acpi
  • archelp
  • boot
  • bufio
  • chain
  • configfile
  • crypto
  • cryptodisk
  • datetime
  • disk
  • efifwsetup
  • efinet
  • elf
  • ext2
  • extcmd
  • fat
  • fshelp
  • gcry_crc
  • gettext
  • gzio
  • iso9660
  • key_protector
  • linux
  • loadenv
  • minicmd
  • mmap
  • multiboot2
  • normal
  • part_gpt
  • priority_queue
  • procfs
  • reboot
  • relocator
  • search
  • search_fs_file
  • search_fs_uuid
  • search_label
  • serial
  • terminal
  • terminfo
  • test
  • tftp
  • video
  • xen_boot

If you are using systemd-boot on arm64 or riscv, is the fix for unverified Devicetree Blob loading included?

N/A since XenServer only supports x86_64.

What is the origin and full version number of your bootloader (GRUB2 or systemd-boot or other)?

Upstream GRUB 2.12+ (commit id 0e367796c0f41cb77562aa30282d85d0d2b3480a)

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.

None.

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.

Our GRUB2 boots Xen instead of Linux. Please see the XenServer Secure Boot document for how boot works and enforces Secure Boot.

How do the launched components prevent execution of unauthenticated code?

Summarize in one or two sentences, how your secure bootchain works on higher level.

Our bootchain looks like this:

Shim -> GRUB -> Xen -> Linux (dom0)

Shim verifies and then chainloads GRUB. GRUB uses the Shim protocol to verify and then boot Xen. Xen uses the Shim protocol to verify the dom0 kernel before starting it. The dom0 kernel verifies any modules it loads using a built-in key. Please see XenServer Secure Boot for the full details.

Does your shim load any loaders that support loading unsigned kernels (e.g. certain GRUB2 configurations)?

No.

What kernel are you using? Which patches and configuration does it include to enforce Secure Boot?

Linux 6.6.98. The patches are detailed in an earlier question.

These configuration options are enabled to ensure that modules' signatures are verified and to enable lockdown mode:

  • CONFIG_MODULE_SIG_FORCE=y
  • CONFIG_SECURITY_LOCKDOWN_LSM=y
  • CONFIG_SECURITY_LOCKDOWN_LSM_EARLY=y

What contributions have you made to help us review the applications of other applicants?

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.

I have done some reviews:

I intend to continue reviewing applications.

Add any additional information you think we may need to validate this shim signing application.

I realize this is a bit different from the usual application so if more specific details are needed, I can provide them.

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