When instance types are not specified in the machine pool, the installer checks which instance types (from a list) are available in a given az. If the ec2:DescribeInstanceType permission is not present, the check will fail gracefully and default to using the m6i instance type. This instance type is not available in all regions (e.g. ap-southeast-4 and eu-south-2), so those installs will fail.

OCPBUGS-45218 describes a similar issue with edge nodes.

ec2:DescribeInstanceTypeOfferings is not a controversial permission and should be required by default for all installs to avoid this type of issue.

    Affects all versions, but we will just fix in main (4.19)

    Always

See OCPBUGS-45218 for one example.

Another example (unverified)
    1. Use permissions without ec2:DescribeInstanceTypeOfferings
    2. Install config: set region to eu-south-2 or ap-southeast-4. Do not set instance types
    3. Installer should default to m6i instance type (can be confirmed from machine manifests).
    4.  Install will fail as m6i instances are not available in those regions: https://docs.aws.amazon.com/ec2/latest/instancetypes/ec2-instance-regions.html     

    Install fails due to unavailable m6i instance

    Installer should select different instance type, m5