Asahi Linux Installation – Security Features Explained

I also have to place the MacBook into "permissive" security mode via recovery to complete the installation.

This is a misconception. You are not placing the MacBook into "permissive" security mode. You are placing the Asahi Linux install into permissive mode.

Apple Silicon Macs implement a unique secure boot mechanism that is unlike other personal computing devices. Typically devices are either wide open, locked down to manufacturer-approved software only, or can be toggled between those states through some (sometimes more, sometimes less secure) mechanism. Apple Silicon machines instead implement the security mode separately for each installed OS as part of a framework called Boot Policy, and this is what the available security modes mean:

• Full security: Will only boot Apple-approved OSes, and installation requires phoning home to Apple (which allows them to revoke old vulnerable software versions from being installed again)
• Reduced security: Will only boot Apple-approved OSes, but any such OS ever released can be installed without any phoning home.
• Permissive security: Will only boot User-approved OSes, where approval requires physical presence and authentication. Secure Boot is still enabled, but now it is you who chooses what to authorize, not Apple.

Due to the way the mechanism is designed, new OSes must first be installed in Full security (from macOS) or Reduced security (from an existing recoveryOS) mode before being downgraded to Permissive security later. This must be done from a strictly controlled recoveryOS instance, and there are many requirements:

• It must be an Apple-signed recoveryOS
• It must be the same version (from the point of view of the machine) as the OS being downgraded
• It must be an instance installed with and paired with the OS being downgraded
• It must have been launched by a very specific procedure, involving holding down the power button from a cold shutdown state to prove user presence
• The user must enter their credentials, which must have been authorized as a machine owner (typically an admin user of an existing macOS install).

At that point, security of that specific OS can be downgraded, and the actual custom OS bootloader can be installed. This process locally signs the new OS bootloader with a cryptographic signature, so replacing it can only be done by following this process again. This gives you the ability to have a fully user-controlled, securely booted, third-party OS. Of course, such third party OSes need to implement a full secure boot chain for this to be as secure as macOS (Asahi Linux does not yet, but it's been planned from day one and will at some point).

The Asahi Linux install process is, essentially, this:

• Create a new APFS container
• Install a "stub" macOS (downloaded directly from Apple's CDN), of our own chosen version, containing everything but the root filesystem (this includes recoveryOS and all components needed to run it, as well as the iBoot bootloader and OS-paired firmware that cannot be replaced in a third-party OS install)
• Install Asahi Linux (EFI/ext4 partitions). This is inert at this point.
• Authorize the fresh stub macOS to be bootable on this machine. If you are installing from macOS, this will go through the phone home process behind the scenes and authorize it in Full security mode. If you are installing Asahi Linux directly from a recoveryOS Terminal, it will instead prompt you for login credentials and jump straight to Reduced security mode (which is as low as you can go from an unpaired recoveryOS belonging to the wrong macOS install), avoiding the phone-home step.
• Mark the new OS as the default boot option and ask the user to boot into recoveryOS (hold down the power button). This will, implicitly, boot into the fresh recoveryOS instance that was just installed, which is the only environment that can authorize a change to Permissive security for the new OS. The Asahi Linux installer uses a trick to make this more ergonomic for users, so it looks like you are selecting the Asahi Linux install and it goes straight into the setup script instead of you having to pull up a Terminal and launch it manually, but you still have to be in the right recoveryOS (which is what you get by holding down the power button). Note that the "boot picker" that lets you choose an OS is already a running recoveryOS!
• Change the security mode for this OS to Permissive security, prompting the user for credentials in the process
• Finally, install the Asahi Linux bootloader (which locally signs it and authorizes it to boot on this machine - this is a local process, no phoning home), again prompting for credentials.

At that point you reboot and you're in the new OS.

So how does this affect the security of other OSes on the machine? It doesn't. Your existing macOS install remains at Full Security, with all the privileges that entails (like being able to run iOS apps and watch DRM-protected content). The OS image itself is still signed by Apple and verified at runtime, and cannot be compromised from Linux. User data can be if you have not enabled FileVault (though in practice we don't have support for APFS with transparent encryption anyway, so an attacker would have to develop that first!). If you have, then the most the third-party OS can do is just destroy/corrupt your data, not extract or compromise it.

Enabling FileVault also has one additional benefit in that it forces recoveryOS to prompt you for credentials upfront, which means you cannot use that environment to break into your machine or bypass password login requirements.

So, these machines are designed so you can do this without compromising the security of your main OS. Whether you can successfully sell that to your company's security team, that's a different question :-)

You can find a more detailed description of the Apple Silicon security architecture on the Introduction to Apple Silicon page on the Asahi Linux wiki.