Run fleet desktop with user SELinux context when applicable

[sgress454]

For #29793

Details

This PR changes the way that Orbit launches processes such as the desktop app on Linux, in order to ensure that on SELinux-enabled systems the correct user context is set when running the command.

Previously, sudo -u was used to launch commands on Linux. This PR switches to use runuser instead, which is recommended in situations where the root user wants to execute a command as a user with reduced privileges (see the blog post by one of the creators of runuser). This avoids certain errors that can come from interacting with PAM modules as the system user.

Additionally, if we detect that SELinux is set up on a system, we now use runcon to force the command to run using the logged-in user's SELinux context. It's possible that on some systems they may have configuration where sudo will switch to the user's SELinux context automatically, but this is not guaranteed. Using runuser + runcon is our best bet for ensuring that the desktop app (and anything that it spawns) runs under the correct context.

This PR also does some refactoring so that the three run methods for Linux (run, runWithOutput and runWithStdin) all use the same base code to create the command with the correct args and env vars, and differ only in how they handle the i/o.

Checklist for submitter

If some of the following don't apply, delete the relevant line.

• Changes file added for user-visible changes in changes/, orbit/changes/ or ee/fleetd-chrome/changes.
  See Changes files for more information.
• For Orbit and Fleet Desktop changes:
  • Make sure fleetd is compatible with the latest released version of Fleet (see Must rule).
  • Orbit runs on macOS, Linux and Windows. Check if the orbit feature/bugfix should only apply to one platform (runtime.GOOS).
  • Manual QA must be performed in the three main OSs, macOS, Windows and Linux. (n/a, code is linux only)
  • Auto-update manual QA, from released version of component to new version (see tools/tuf/test). n/a

Testing

• Ubuntu with SELinux on
• Ubuntu with SELinux off
• Fedora with SELinux on
• Fedora with SELinux off
• Debian with SELinux on
• Debian with SELinux off
• runWithOutput still works (tested with go run ./tools/dialog/main.go --dialog=zenity)
• runWithStdin still works (this isn't currently used by Linux)

Summary by CodeRabbit

• Bug Fixes

  • Improved security and user context handling when launching the fleet desktop application on Linux systems.

• Refactor

  • Enhanced process launch mechanism to use proper SELinux context and user session, ensuring processes start under the correct user and security environment.

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📝 Walkthrough

Walkthrough

The changes update how the fleet desktop application is launched on Linux. Instead of using sudo, the code now uses runuser and, when needed, runcon to set the correct user and SELinux context. Environment variables are handled more clearly, and a new function retrieves the user's SELinux context.

Changes

File(s)	Change summary
orbit/pkg/execuser/execuser_linux.go	Refactored command execution to use runuser and runcon for correct user and SELinux context. Added baserun helper. Improved env handling.
orbit/pkg/user/user_linux.go	Added GetUserContext function to fetch the SELinux context of a user.

Sequence Diagram(s)

sequenceDiagram
    participant Launcher as Fleet Desktop Launcher
    participant User as Target User
    participant SELinux as SELinux Context
    participant OS as Linux OS

    Launcher->>User: Get user info and environment
    Launcher->>SELinux: Get SELinux context (if enabled)
    alt SELinux context present
        Launcher->>OS: Launch process with runcon + runuser (correct context and user)
    else SELinux context absent
        Launcher->>OS: Launch process with runuser (correct user)
    end
    OS-->>Launcher: Process started under correct user and context

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• Commit unit tests in branch sgress454/use-runuser-to-launch-desktop

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 80 lines in your changes missing coverage. Please review.

Project coverage is 64.01%. Comparing base (e898814) to head (c8d9506).
Report is 84 commits behind head on main.

Files with missing lines	Patch %	Lines
orbit/pkg/execuser/execuser_linux.go	0.00%	48 Missing ⚠️
orbit/pkg/user/user_linux.go	0.00%	32 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #30882      +/-   ##
==========================================
- Coverage   64.01%   64.01%   -0.01%     
==========================================
  Files        1905     1896       -9     
  Lines      187625   187405     -220     
  Branches     5369     5287      -82     
==========================================
- Hits       120108   119966     -142     
+ Misses      58073    57993      -80     
- Partials     9444     9446       +2     

Flag	Coverage Δ
backend	64.86% <0.00%> (-0.01%)	⬇️

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[sgress454]

@lucasmrod this is ready for code review, I still need to test it on a few OSs to check I didn't break anything.

[lucasmrod]

LGTM! Left some nit comments

[sgress454]

@lucasmrod addressed your comments, thanks!