JIRA doesn't have field level security. I don't know what plugins are available in Cloud, but there may be a plugin. If not the only way is to place the fields you want limited on a transition, going back to the same status, screen with a condition only the project role (or group) you want to edit the field can execute. Then only put the fields on the view screen. If they need to update the field on a regular basis you'll need to put the transition on all statuses.
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