fix: reduce mode of SQLite storage file to 0o600

[tonyandrewmeyer]

sqlite3 creates new database files with 0o600 permissions. Although Charms should generally not store private data in the backend storage, this does happen sometimes, and it would be nicer to ensure that only the user (that is, root) can access the underlying storage file.

This chmod's the file (to -rw-------) after every open even if it already exists. It seems simpler to do this and get the additional access fix rather than only do it on initial creation.

Fixes #1051.

[benhoyt]

Looks good, thanks!

[jameinel]

I wouldn't block this, but I would say that if we really want to care about the security of this file, we might as well take the step of creating it secure in the first place to avoid ever having a file handle that an attacker could exploit.

[tonyandrewmeyer]

Moving back to draft until I verify the new approach with a real (non-unit, manual) test.

[tonyandrewmeyer]

Manually verified the simple cases on micro-k8s (no existing state file, existing state file).

[benhoyt]

Looks good, and thanks for the tests. Per discussion, we'll fail hard in both cases. As Tony said on Mattermost:

It does seem like if we are going to add this (minimal) layer of security then we ought to ensure that it either happens or fails, so that people don't assume that it's happening when it's not.