Fix #41: users.json.bak PermissionError when /app is read-only

[Sloth-on-meth]

Fixes #41 — dooropener - ERROR - Error updating user: [Errno 13] Permission denied: '/app/users.json.bak' on V1.14.1

This PR contains only the fix for that issue. It is independent of the other open PRs and merges cleanly with them.

Problem (issue #41)

Since 1.14.1, creating, editing or deleting a user fails with PermissionError: [Errno 13] Permission denied: '/app/users.json.bak' when users.json is bind-mounted as a single file (the documented compose setup, and the reporter's Unraid setup).

UsersStore._save_atomic() can't rename onto a single-file mount, so it falls back to overwriting in place. Since 99ec04c that fallback first copies the file to users.json.bak in the same directory. /app is root-owned and the app runs as PUID/PGID, so creating a new file there is denied. (This is the same reason the temp file already falls back to /tmp.) config.ini edits still worked because they never created a sibling file.

Fix

• The backup is created with mkstemp in the temp file's directory (the one location already known to be writable), not at self.path + ".bak".
• The backup and the restore use shutil.copyfile (contents only). copy2 would copy users.json's permission bits onto the backup, leaving a world-readable copy of the PINs in the shared temp dir when users.json is 0644; it would also rewrite users.json's own mode on restore.
• The backup is removed only after a successful write or restore. If restoring fails too, it is kept (it is then the only intact copy) and the raised UsersStoreError names its path. Previously a finally deleted it either way.
• If the backup can't be made, the save aborts rather than overwriting with no safety net, so the 1.14.1 data-loss protection is preserved.

The workaround in the issue (mounting a directory and setting USERS_STORE_PATH) is no longer needed.

Verification

• Reproduced with real permissions: a root-owned app/ dir containing a users.json, run as uid 65534. On main this fails with exactly PermissionError: ... users.json.bak; with the fix create/update/delete/touch all succeed, no stray files remain, and a 0644 users.json keeps its mode.
• tests/test_users_store_readonly_dir.py emulates that setup (the suite runs as root, so access is denied explicitly):
  • edits succeed with nothing created beside the target (fails on main);
  • a failed overwrite restores the original;
  • the backup survives a failed restore and its path is in the error;
  • the backup is never group/world readable even if users.json is.
• Full suite passes, ruff clean, CI green.

Related

#51 fixes the same pattern in the new atomic_io helper used for config.ini. Once it merges, users_store could adopt that helper; kept separate so this regression can ship on its own.

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📝 Walkthrough

Walkthrough

The save fallback now creates backups beside the temporary file. It removes backups after successful saves or restoration, and retains a backup if restoration fails. Regression tests cover restricted directory writes and simulated overwrite and restoration errors.

Changes

UsersStore save handling

Layer / File(s)	Summary
Fallback backup placement users_store.py, tests/test_users_store_readonly_dir.py	The cross-device fallback creates an existing-store backup beside the temporary file. It removes partial backups if copying fails. Regression tests simulate restricted writes and exercise create, update, delete, and touch operations.
Overwrite failure recovery users_store.py, tests/test_users_store_readonly_dir.py	If overwriting fails, the store is restored from the backup. Successful restoration removes the backup; failed restoration retains it. Tests check restoration after ENOSPC and backup retention after EIO.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Merge Risk: 🟡 Moderate · up to ab8d4

Saving users when the app directory is read-only now works, but the temporary backup of users.json can be readable by other local users in shared temporary storage. Keep the backup's permissions restrictive before merging. If both the save and the recovery fail, the error should also say where the surviving backup is.

Security Architecture Review

Security architecture risk: 🟡 Moderate · up to ab8d4

The fix improves recovery, but backups can inherit permissive permissions when moved into temporary storage. Other local users could read stored PINs where that storage is shared. Actual exposure depends on file permissions and isolation; normal saves remove the backup.

Retained concerns

• Medium · security · inferred: Fallback backups inherit users.json permission bits but may now reside outside its directory protection. When temporary storage is accessible to other local principals and the copied mode permits reading, the entire PIN snapshot becomes accessible; failed restoration leaves that copy in place.

Security review details

Security Blast Radius

• inferred — One exposed backup contains the complete user snapshot for its UsersStore, not merely the edited user's record. A local reader who can also reach the application could reuse still-valid active PINs to operate its configured door entity. This trace does not establish disclosure of the Home Assistant token, administrator credentials or access to other deployments.

Security Findings and Attack Paths

• inferred — The retained sensitive-data finding applies when a fallback backup receives readable source permission bits in temporary storage accessible to an unauthorized local principal. Ordinary saves expose it briefly; failed restoration, interruption or unsuccessful cleanup can extend its lifetime. Relocation newly bypasses source-directory protection where that protection existed. Exposure is not shown to increase for principals already able to read the original store.

Trust Boundaries and Controls

• observed — Random backup names and mkstemp's initial restrictive mode do not guarantee confidentiality after copy2 copies source permissions. Normal overwrite and successful restoration remove the backup. Admin authentication and CSRF protect mutation routes, while PIN matching and active blocks protect door access; neither prevents local filesystem disclosure.

Resilience and Maintainability Implications

• inferred — The fallback remains an in-place write without serialization across shared store mutations. Concurrent saves can interfere with identity updates, and backup or restoration copying lacks a durable crash-recovery protocol. These limitations predate the PR and are not separate introduced concerns. Unique backup names reduce fixed-name collisions, and retaining the intact copy improves recoverability after restoration failure.

Hardening Proposals

• proposed — Keep backup contents owner-only throughout creation and retention rather than inheriting permissive source metadata. Use private writable scratch storage where practical, test permissions with permissive source files, and provide a secure way to locate and restore retained backups without deleting the only intact copy.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 8.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 12 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Issue #41 requires user edits to work when users.json is a single-file bind mount. UsersStore._save_atomic() now creates the fallback backup in the temporary file’s directory instead of beside `us…
Out of Scope Changes check	✅ Passed	The reviewed changes are limited to the #41 save fallback and regression tests for the bind-mounted users.json failure and its data-safety behavior. The backup cleanup and restore handling directly …
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the fix for the users.json.bak permission error that the changes address.

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Actionable comments posted: 2

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🤖 Prompt to fix review comments

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Inline comments:
Review comments at @users_store.py:
- Line 112: Update the backup creation flow containing shutil.copy2 to copy
users.json contents into the pre-created backup without copying the source
file’s permission bits, preserving the restrictive permissions established when
the backup was created.
- Around line 124-128: Update the restoration error handling around shutil.copy2
so that if restoring from backup_path fails, the raised error identifies
backup_path for recovery. Preserve the backup file when restoration fails.

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• tests/test_users_store_readonly_dir.py
• users_store.py

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