fix: secure sockets via parent directory when filesystem cannot chmod them

[caner-akca]

Issue

herdr server exits with InvalidInput when the Herdr config directory is on a filesystem that cannot change permissions on Unix-socket inodes, such as virtiofs. The bind succeeds; startup fails afterwards, while applying 0600 to the socket's pathname:

fchmodat(AT_FDCWD, "~/.config/herdr/herdr.sock", 0600) = -1 EINVAL (Invalid argument)

HERDR_SOCKET_PATH is not a complete workaround, because named-session and handoff sockets live in the config tree.

How did we fix it?

Stop applying permissions to the socket's pathname. The mode is now applied to the socket descriptor before bind(), so the affected filesystems are never asked to chmod a socket inode at all — on Linux the mode carries into the node bind() creates.

Platforms whose kernel cannot set a socket's mode before bind report Unsupported; those fall back to the previous bind-then-restrict behavior, unchanged.

The API, client-protocol, handoff, and remote-bridge listeners now share one private binder instead of four copies of bind + restrict_socket_permissions.

Scope

Deliberately minimal. This changes how the 0600 mode is applied and nothing else:

• No parent directory is opened, inspected, or modified.
• Socket paths, socket modes, and directory modes are unchanged on every platform.
• No new environment requirements: HERDR_SOCKET_PATH pointing into a shared directory keeps working exactly as before.
• No test-infrastructure changes.

Verification

• New coverage pins the contract on every Unix: the socket is 0600, the parent directory is never modified, the Unsupported path restricts after bind, and any other permission error stays fatal.
• Confirmed on macOS (where the kernel rejects fchmod on a socket, so the fallback path is the one taken) that startup, socket modes, and directory modes match master.
• just check passes.

refs #3212

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📝 Walkthrough

Walkthrough

Private Unix socket binding now requests mode 0o600 during listener creation and applies the same mode after an unsupported creation-mode response. The IPC layer delegates private binding to the platform and adds Unix listener conversion for descriptor-passing protocols. API, handoff, headless, and SSH bridge paths use centralized binding functions. Obsolete permission helpers and constants were removed. Tests verify private socket modes and fallback behavior.

Suggested reviewers: ogulcancelik, akbash-bot

Merge Risk: 🟡 Moderate · up to 9c3ba

On platforms that cannot set socket permissions during creation, listeners are briefly published before owner-only permissions are applied, which can expose local IPC endpoints or allow a race before restriction. Because this shared fallback covers several server and bridge sockets, the PR needs explicit security acceptance or mitigation before merge.

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Title check	⚠️ Warning	The title identifies the socket-permission problem but incorrectly states that the fix uses the parent directory. The implementation applies mode 0600 before bind and falls back to post-bind restricti…	Update the title to describe pre-bind socket permission handling and the fallback behavior, for example: "fix: set Unix socket permissions before bind".
Docstring Coverage	⚠️ Warning	Docstring coverage is 52.31% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 65 functions across 17 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description clearly explains the filesystem failure, the pre-bind permission fix, fallback behavior, affected listeners, scope, and verification.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Title check

Explanation

The title identifies the socket-permission problem but incorrectly states that the fix uses the parent directory. The implementation applies mode 0600 before bind and falls back to post-bind restriction when required.

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[greptile-apps]

Greptile Summary

This PR centralizes private Unix listener creation and attempts to apply mode 0600 during listener creation, retaining a bind-then-restrict fallback when creation-time modes are unsupported.

• Migrates API, client-protocol, handoff, and remote-bridge listeners to the shared binder.
• Removes the socket after a fallback restriction failure, but the cleanup does not verify pathname identity.
• Retains previously reported publication and pathname-replacement risks in the Unsupported fallback.

Confidence Score: 1/5

The PR does not appear safe to merge because the Unsupported fallback still permits unauthorized queued connections and replacement-inode chmod, while its new failure cleanup can delete an unrelated replacement path.

The fallback binds a publicly reachable pathname before restricting it, applies permissions to whatever inode the pathname currently resolves to, and unconditionally removes that pathname if restriction fails.

Files Needing Attention: src/platform/unix_common.rs

Important Files Changed

Filename	Overview
src/platform/unix_common.rs	Adds the shared Unix binder; its fallback retains previously reported exposure and pathname races, and its new failure cleanup can remove a replacement inode.
src/ipc.rs	Routes private Unix listeners through the platform binder and retains the existing identity-checked socket cleanup helper.
src/api/server.rs	Replaces API socket bind-then-restrict logic with the shared private-listener binder.
src/server/headless.rs	Migrates headless client listeners to the shared private-listener binder.
src/server/handoff.rs	Migrates the Unix handoff listener to the shared private-listener binder.
src/remote/attach.rs	Migrates remote-bridge listeners to the shared private-listener binder.
src/server/socket_paths.rs	Removes per-caller pathname permission restriction in favor of centralized binding.
src/platform/mod.rs	Exposes the new Unix private-listener implementation through the platform boundary.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[Bind with mode 0600] --> B{Supported?}
  B -->|Yes| C[Return private listener]
  B -->|No| D[Bind without mode]
  D --> E[Restrict socket pathname]
  E -->|Success| C
  E -->|Failure| F[Drop listener]
  F --> G[Remove current pathname]
  G --> H[Return permission error]

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[caner-akca]

Addressed the review findings in the latest head: private socket creation now retains one verified parent-directory handle across pre-bind hardening, bind, and fallback; external directories remain unmodified; parent ownership, mode, and socket identity are re-verified before returning the listener. The PR description contains the current design and verification summary.

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[caner-akca]

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🧹 Nitpick comments (1)

src/platform/unix_common.rs (1)

156-168: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚖️ Poor tradeoff

Consider sharing one SocketFileIdentity definition.

SocketFileIdentity and socket_file_identity now exist here and in src/ipc.rs, src/api/server.rs, src/server/headless.rs, and src/remote/attach.rs with identical bodies. Five copies of an identity contract can drift, and identity comparison decides whether a socket is removed. Export one definition (for example from src/ipc.rs) and re-use it.

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📒 Files selected for processing (10)

• src/api/server.rs
• src/config.rs
• src/ipc.rs
• src/platform/mod.rs
• src/platform/unix_common.rs
• src/remote/attach.rs
• src/server/handoff.rs
• src/server/headless.rs
• src/server/socket_paths.rs
• tests/live_handoff.rs

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[caner-akca]

Reworked the head down to the minimal fix for #3212 and force-pushed a single commit.

The previous approach used interprocess's creation-time socket mode and, when that returned Unsupported, moved the access boundary onto a hardened parent directory. That fallback is not a rare path: macOS (and any kernel that rejects fchmod on a socket) takes it on every listener, so the branch silently made the parent directory the only boundary there. Consequences that surfaced while reviewing it:

• HERDR_SOCKET_PATH pointing into any directory that is not owner-only failed startup on macOS, including /var/tmp — the workaround documented in herdr server fails with EINVAL on virtiofs-mounted home directory #3212. master works there.
• The socket file itself went from 0600 to 0755 on macOS.
• Herdr chmod'ed the user's config directory to 0700 on every start.
• It required a new macOS-only bridge socket directory and 0700 runtime directories across eight test files to keep CI green — which is also why CI passing did not catch any of the above.

The current head keeps only the part that actually fixes the bug: apply the mode to the socket descriptor before bind(), so the socket pathname is never chmod()ed and virtiofs is never asked to. Platforms that cannot do that fall back to the previous bind-then-restrict behavior, byte-for-byte unchanged. No parent directory is opened, inspected, or modified anywhere.

That resolves the earlier findings on this PR by removing the code they were about — there is no longer any parent-directory tightening, ownership check, canonicalization, or replacement race to reason about. The remaining InvalidInput comments are answered inline.

Diff is now +188/-78 across 8 source files, down from +602/-135 across 16. just check passes.

[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@src/platform/unix_common.rs`:
- Around line 44-48: Update the Unsupported branch in bind_local_listener so
that if restrict_socket_permissions(path) fails after bind succeeds, the bound
listener is dropped and the socket path is removed before propagating the
original error. Preserve the successful return path and ensure cleanup also
accounts for reclaim_name(false).

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[caner-akca]

Two commits have landed since the last bot review (e88ee77d): the fix for CodeRabbit's post-bind cleanup finding, and a rebase onto current master. Current head is 9c3ba747.

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@caner-akca Full review requested for the current head 9c3ba747, including the post-bind cleanup fix and the rebase changes.

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Actionable comments posted: 1

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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
minimal, and validate.

Inline comments:
In `@src/platform/unix_common.rs`:
- Around line 46-47: Update the fallback flow around ListenerOptionsExt::mode,
bind, and restrict so the socket is never published before its access boundary
is secure: verify the parent directory is private before binding, or fail closed
when mode returns Unsupported. Add a regression test using a permissive umask
that attempts a connection before restriction and confirms it cannot succeed.

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• src/api/server.rs
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• src/platform/unix_common.rs
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• src/server/socket_paths.rs

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[caner-akca]

Closing this in favour of a fresh PR against current master.

Two reasons:

1. The branch is stale in a way that can't be rebased cleanly. master has moved 24+ commits, and the client-render refactor rewrites most of headless.rs. Rebasing produces conflicts across ~6,500 lines, which is a worse review artifact than a clean re-application.

2. The design in this PR is wrong, and I can now show that with measurements rather than argument. The parent-directory hardening here assumes a 0700 config directory protects the socket inside it. I reproduced #3212 on the reporter's actual stack — Apple container 1.3.1 on macOS 26.2, real virtiofs (rw,relatime) — and that assumption does not hold:

virtiofs, 0700 parent, socket 0755   second local user -> CONNECTED   *** not protected ***
local fs, 0700 parent, socket 0775   second local user -> blocked (PermissionError)

virtiofs enforces no Unix permissions between guest users at all, and it silently discards the requested socket mode. So the directory boundary this PR adds would ship a guarantee that isn't real on the one filesystem it was written for.

It also regressed macOS. Because fchmod on an unbound AF_UNIX socket returns EINVAL there, macOS always takes the fallback branch, so the hardening ran on every start: HERDR_SOCKET_PATH into a non-owner-only directory started failing with PermissionDenied ... not owner-only where master works, the socket degraded from 0600 to 0755, and the user's config directory was chmod'ed on every launch.

The replacement keeps only the part that actually fixes the reported crash: apply the mode to the socket descriptor before bind(2), so the pathname is never chmoded. That's ~25 lines completing the Unix half of the bind_private_local_listener function that already exists in ipc.rs, plus routing the remaining listeners through it. No parent-directory logic, no new state, no protocol surface, and no test-harness churn.

[caner-akca]

Replacement is #3588.