Why this blog post?

I won’t say this is the way to install macOS Catalina on VirtualBox, but it’s certainly a way, and it’s difficult to find information about a way to do it. There are a lot of posts indicating that 10.15 or 10.15.1 worked but 10.15.2 and beyond don’t. Or that there are difficulties with VirtualBox and APFS. There are also, if you look for blog posts about macOS guests, many blog posts about Apple-unsanctioned “hackintosh” setups on Windows or Linux hosts.

So there may be a way easier way to set up Catalina as a guest on a Mac host, but at least this is something (happy to link to better tutorials if people post them).

Install macOS High Sierra

I did a modified version of How to Install macOS High Sierra in VirtualBox on Windows 10 to get High Sierra installed on VirtualBox. These are the key parts:

Creating the .iso

hdiutil create -o /tmp/HighSierra.cdr -size 7316m -layout SPUD -fs HFS+J
hdiutil attach /tmp/HighSierra.cdr.dmg -noverify -nobrowse -mountpoint /Volumes/install_build
asr restore -source /Applications/Install\ macOS\ High\ Sierra.app/Contents/SharedSupport/BaseSystem.dmg -target /Volumes/install_build -noprompt -noverify -erase
hdiutil detach /Volumes/OS\ X\ Base\ System
hdiutil convert /tmp/HighSierra.cdr.dmg -format UDTO -o /tmp/HighSierra.iso
mv /tmp/HighSierra.iso.cdr ~/Desktop/HighSierra.iso

Creating the VM

Call the VM macOS (the tutorial says to use High Sierra, but we’ll be upgrading this later). Use two processors and 128 MB of video memory. Add the .iso to the optical drive part of storage.

Configuring the VM

Run these commands:
VBoxManage.exe modifyvm "macOS" --cpuidset 00000001 000306a9 04100800 7fbae3ff bfebfbff
VBoxManage setextradata "macOS" "VBoxInternal/Devices/efi/0/Config/DmiSystemProduct" "MacBookPro11,3"
VBoxManage setextradata "macOS" "VBoxInternal/Devices/efi/0/Config/DmiSystemVersion" "1.0"
VBoxManage setextradata "macOS" "VBoxInternal/Devices/efi/0/Config/DmiBoardProduct" "Mac-2BD1B31983FE1663"
VBoxManage setextradata "macOS" "VBoxInternal/Devices/smc/0/Config/DeviceKey" "ourhardworkbythesewordsguardedpleasedontsteal(c)AppleComputerInc"
VBoxManage setextradata "macOS" "VBoxInternal/Devices/smc/0/Config/GetKeyFromRealSMC" 1

Installing High Sierra

The tutorial mentions having to deal with the EFI Internal Shell. You shouldn’t have to. Installing High Sierra at this point with HFS+ (not APFS) should work.

Upgrading to Catalina

Once you’ve booted up to Setup Assistant and answered all the questions, you should be logged in and able to go to the Mac App Store to download and install the macOS Catalina upgrade.

Booting to recovery mode

I didn’t find Cmd-R to be a very reliable way to boot to recovery mode. And Rich Trouton’s trick to booting to recovery mode using VMWare doesn’t really apply to VirtualBox.

Another blog post (How to boot into recovery mode on mac without holding Command + R Key on keyboard ?) did have the solution, though. Just run
sudo nvram "recovery-boot-mode=unused"
and then reboot. That will do a one-time boot to recovery mode. Then when you reboot again, it should boot back to regular mode.

Enabling FileVault… sort of

If you want to enable FileVault, you’ll have to do so from the command-line (it’ll be greyed out in the GUI), but FileVault will basically be unusable (unless you just want to test in recovery mode):
sudo fdesetup enable
Don’t do this unless you already have your VM snapshotted or backed up. It will likely make your VM unbootable.

Changing the serial number

By default, the macOS VirtualBox guest serial number is 0, but you can change it (make sure the VM is powered off first). h/t to Utsav Dusad on Superuser.com:
VBoxManage setextradata "macOS" VBoxInternal/Devices/efi/0/Config/DmiSystemSerial "ACTUALSERIALNUMBER"