This module adds an option `security.hideProcessInformation` that, when
enabled, restricts access to process information such as command-line
arguments to the process owner. The module adds a static group "proc"
whose members are exempt from process information hiding.
Ideally, this feature would be implemented by simply adding the
appropriate mount options to `fileSystems."/proc".fsOptions`, but this
was found to not work in vmtests. To ensure that process information
hiding is enforced, we use a systemd service unit that remounts `/proc`
after `systemd-remount-fs.service` has completed.
To verify the correctness of the feature, simple tests were added to
nixos/tests/misc: the test ensures that unprivileged users cannot see
process information owned by another user, while members of "proc" CAN.
Thanks to @abbradar for feedback and suggestions.
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