fix(install): skip and reject links when extracting .eti archives

Security audit findings EXP2-SEC-01, SEC-IPC-04 and Codex #3 (symlink
redirection through externally extracted archives).

The ordinary install path hands every root `.eti` archive to an external
`unrar x` process and promotes the resulting staging directory to
`local/` as soon as the extractor exits 0. Nothing inspected what unrar
materialised. A symbolic link (or, on Windows, a junction) inside an
archive would survive promotion and later redirect the launch-time
settings rewrite in `apply_launch_settings_once`, the uninstall path,
or any script the game ships. The archives themselves are BLAKE3
verified against the bundled catalog before they can be installed, so a
hostile link would have to be published by the catalog operator; this
is defense in depth rather than a live remote exploit.

Two independent layers now guard promotion:

- Both `unrar` invocations (Tauri sidecar and peer-cli external
  unpacker) pass `-ol-`, which makes unrar 7.x skip symbolic-link
  entries entirely. The bundled 7.10 sidecar was checked against a
  fixture archive.
- `install_inner`/`update_inner` walk the staging tree without
  following links and refuse to promote it if any entry is a symlink or
  (on Windows) carries the reparse-point attribute. The normal rollback
  then removes staging and clears the install intent.

The audit's `-sl-` suggestion does not exist in unrar; `-sl<size>` is a
size filter. Post-extraction digest verification of every extracted
file remains out of scope for the ordinary path; Stream Install already
verifies each output entry.

Test plan: `just test` (new unix test installs with a fake unpacker
that plants a symlink and asserts install fails, `local/` is absent and
the intent is cleared; the peer-cli controlled-unrar test checks the
new argument position). Manual: install a fixture game via peer-cli.

Claude-Session: https://claude.ai/code/session_017C3Nbgwpdm3YNwZhhFLHwg

crates/lanspread-peer-cli/src/lib.rs

@@ -347,6 +347,9 @@ impl Unpacker for ExternalUnrarUnpacker {
                     std::ffi::OsString::from("x"),
                     std::ffi::OsString::from("-o+"),
                     std::ffi::OsString::from("-p-"),
+                    // Skip symbolic links inside the archive; the install
+                    // transaction also audits the extracted tree for links.
+                    std::ffi::OsString::from("-ol-"),
                     archive.as_os_str().to_owned(),
                     dest.as_os_str().to_owned(),
                 ],
@@ -649,7 +652,8 @@ mod tests {
             r#"#!/bin/sh
 set -eu
 [ "$3" = "-p-" ]
-dest=$5
+[ "$4" = "-ol-" ]
+dest=$6
 printf '%s' "$$" > "$dest/child.pid"
 printf 'started' > "$dest/started"
 while [ ! -e "$dest/release" ]; do :; done

crates/lanspread-peer/src/install/transaction.rs

@@ -462,6 +462,7 @@ async fn install_inner(
     prepare_owned_empty_dir(&staging)?;
     root_capability.sync_game_root()?;
     unpack_archives(game_root, &staging, unpacker, cancel_token).await?;
+    reject_links_in_staging(&staging)?;
     rename_path(&staging, &local)
         .wrap_err_with(|| format!("failed to promote install for {id}"))?;
     root_capability.sync_game_root()?;
@@ -490,6 +491,7 @@ async fn update_inner(
     prepare_owned_empty_dir(&staging)?;
     root_capability.sync_game_root()?;
     unpack_archives(game_root, &staging, unpacker, cancel_token).await?;
+    reject_links_in_staging(&staging)?;
     rename_path(&staging, &local).wrap_err_with(|| format!("failed to promote update for {id}"))?;
     root_capability.sync_game_root()?;
     Ok(())
@@ -543,6 +545,53 @@ async fn unpack_archives(
     Ok(())
 }
 
+/// Refuses to promote an extracted tree that contains symbolic links or, on
+/// Windows, any reparse point (junctions, mount points).
+///
+/// Extraction is delegated to an external `unrar` process whose output is
+/// otherwise trusted verbatim. The archives themselves are BLAKE3-verified
+/// against the bundled catalog, so a link would have to originate from the
+/// catalog publisher; this audit is defense in depth so that a link can never
+/// redirect later launch-time rewrites (`apply_launch_settings_once`) or an
+/// uninstall outside the game directory. `unrar` is additionally invoked
+/// with `-ol-` so links are skipped at extraction time.
+fn reject_links_in_staging(staging: &Path) -> eyre::Result<()> {
+    let staging = staging.to_path_buf();
+    scoped_blocking(move || {
+        for entry in walkdir::WalkDir::new(&staging).follow_links(false) {
+            let entry = entry.wrap_err_with(|| {
+                format!("failed to audit extracted tree {}", staging.display())
+            })?;
+            let metadata = fs::symlink_metadata(entry.path()).wrap_err_with(|| {
+                format!(
+                    "failed to inspect extracted entry {}",
+                    entry.path().display()
+                )
+            })?;
+            if is_link_or_reparse_point(&metadata) {
+                eyre::bail!(
+                    "extracted archive contains a link at {}; refusing to install it",
+                    entry.path().display()
+                );
+            }
+        }
+        Ok(())
+    })
+}
+
+#[cfg(windows)]
+fn is_link_or_reparse_point(metadata: &fs::Metadata) -> bool {
+    use std::os::windows::fs::MetadataExt as _;
+    const FILE_ATTRIBUTE_REPARSE_POINT: u32 = 0x400;
+    metadata.file_type().is_symlink()
+        || metadata.file_attributes() & FILE_ATTRIBUTE_REPARSE_POINT != 0
+}
+
+#[cfg(not(windows))]
+fn is_link_or_reparse_point(metadata: &fs::Metadata) -> bool {
+    metadata.file_type().is_symlink()
+}
+
 pub(crate) fn root_eti_archives(game_root: &Path) -> eyre::Result<Vec<PathBuf>> {
     scoped_blocking(|| {
         let mut archives = Vec::new();
@@ -1149,6 +1198,7 @@ mod tests {
     struct FakeUnpacker {
         fail: bool,
         create_commit_conflict: bool,
+        create_symlink: bool,
         archives: Mutex<Vec<PathBuf>>,
     }
 
@@ -1156,16 +1206,22 @@ mod tests {
         fn failing() -> Self {
             Self {
                 fail: true,
-                create_commit_conflict: false,
-                archives: Mutex::new(Vec::new()),
+                ..Self::default()
             }
         }
 
         fn commit_conflict() -> Self {
             Self {
-                fail: false,
                 create_commit_conflict: true,
-                archives: Mutex::new(Vec::new()),
+                ..Self::default()
+            }
+        }
+
+        #[cfg(unix)]
+        fn symlinking() -> Self {
+            Self {
+                create_symlink: true,
+                ..Self::default()
             }
         }
     }
@@ -1187,6 +1243,11 @@ mod tests {
                 }
                 scoped_blocking(|| {
                     fs::write(dest.join("payload.txt"), b"installed")?;
+                    #[cfg(unix)]
+                    if self.create_symlink {
+                        fs::create_dir_all(dest.join("nested"))?;
+                        std::os::unix::fs::symlink("/", dest.join("nested").join("escape"))?;
+                    }
                     if self.create_commit_conflict {
                         let game_root = dest
                             .parent()
@@ -1695,6 +1756,31 @@ mod tests {
         assert_intent_missing(state.path(), &root, "game");
     }
 
+    #[cfg(unix)]
+    #[tokio::test]
+    async fn install_refuses_extracted_trees_that_contain_links() {
+        let temp = TempDir::new("lanspread-install");
+        let state = test_state();
+        let root = temp.game_root();
+        write_file(&root.join("game.eti"), b"archive");
+        write_file(&root.join("version.ini"), b"20250101");
+
+        let err = install(
+            &root,
+            state.path(),
+            "game",
+            Arc::new(FakeUnpacker::symlinking()),
+            CancellationToken::new(),
+        )
+        .await
+        .expect_err("install should refuse a link in the extracted tree");
+
+        assert!(err.to_string().contains("contains a link"), "{err:#}");
+        assert!(!root.join("local").exists());
+        assert!(!root.join(".local.installing").exists());
+        assert_intent_missing(state.path(), &root, "game");
+    }
+
     #[tokio::test]
     async fn update_commit_conflict_preserves_every_ambiguous_directory() {
         let temp = TempDir::new("lanspread-install");

crates/lanspread-tauri-deno-ts/src-tauri/src/lib.rs

@@ -3201,6 +3201,10 @@ async fn run_unrar_sidecar(
         [
             std::ffi::OsString::from("x"),
             std::ffi::OsString::from("-p-"),
+            // Skip symbolic links inside the archive entirely. The install
+            // transaction additionally refuses to promote a tree containing
+            // links, so a link can never redirect later file operations.
+            std::ffi::OsString::from("-ol-"),
             paths.archive.as_os_str().to_owned(),
             std::ffi::OsString::from("-y"),
             std::ffi::OsString::from("-o"),