Harden filesystem safety after a security review

Defense in depth around the only operations that touch the filesystem,
so no request can escape /data/patchdl or leave the process able to
write to a system path:

- Validate the HTTP title_id with path_segment_safe at the top of the
  title-action route, and again inside remove_title_dir and
  cleanup_installed_download. The delete primitives are now self-
  protecting instead of relying only on the "title exists in the scan"
  guard, so a future refactor cannot reintroduce a /data-wiping
  traversal (a title_id of ".." would otherwise resolve the dir to
  /data).
- Only swap the process root vnode when the current root was captured
  and can be restored, in both the scan and the debug dump. Otherwise
  the process could be left rooted at the system root, sending later
  absolute-path writes to the wrong place.

Reviewed and confirmed safe with no change needed: the installer only
delegates to Sony's signed AppInstUtil service (it never writes or
redirects to system paths itself), app.db is opened read-only and
immutable, every write targets /data/patchdl, the patch picker never
selects an update that needs a newer firmware, and the /api/pkg file
server already blocks path traversal.

src/patchdl_scan.c

@@ -415,9 +415,14 @@ patchdl_scan(patchdl_title_t **titles_out, size_t *count_out) {
     root_vnode = kernel_get_root_vnode();
     if (root_vnode) {
         saved_root = kernel_get_proc_rootdir(pid);
+        /* Only swap if we captured the current root, so we can always restore
+           it. Leaving the process rooted at the system root would make later
+           absolute-path writes land in the wrong place. */
+        if (saved_root) {
             kernel_set_proc_rootdir(pid, root_vnode);
             using_vswap = 1;
         }
+    }
 
     for (int i = 0; SCAN_DIRS[i]; i++)
         scan_base(SCAN_DIRS[i], arr, &cnt, MAX_TITLES, mounts, nmounts);
@@ -486,9 +491,12 @@ patchdl_scan_debug_json(void) {
     root_vnode = kernel_get_root_vnode();
     if (root_vnode) {
         saved_root = kernel_get_proc_rootdir(pid);
+        /* Only swap if we can restore it afterwards (see patchdl_scan). */
+        if (saved_root) {
             kernel_set_proc_rootdir(pid, root_vnode);
             using_vswap = 1;
         }
+    }
 
     for (int i = 0; SCAN_DIRS[i]; i++) {
         DIR *d = opendir(SCAN_DIRS[i]);

src/patchdl_websrv.c

@@ -644,6 +644,8 @@ static void
 cleanup_installed_download(const char *title_id, const char *patch_url) {
     char        dir[256], path[320];
     const char *base = strrchr(patch_url, '/');
+    if (!path_segment_safe(title_id))
+        return;
     base = base ? base + 1 : "patch.pkg";
     snprintf(dir, sizeof(dir), "/data/patchdl/%s", title_id);
     snprintf(path, sizeof(path), "%s/%s", dir, base);
@@ -800,6 +802,12 @@ remove_title_dir(const char *title_id) {
     DIR           *d;
     struct dirent *e;
 
+    /* Self-protecting: never build a delete path from an unsafe segment, even
+       if a future caller forgets the upstream check. A title_id of ".." would
+       otherwise resolve dir to /data and wipe it. */
+    if (!path_segment_safe(title_id))
+        return;
+
     snprintf(dir, sizeof(dir), "%s/%s", PATCHDL_DL_DIR, title_id);
     if ((d = opendir(dir))) {
         while ((e = readdir(d))) {
@@ -1006,6 +1014,14 @@ handle_title_action(struct MHD_Connection *conn, const char *url) {
                            action, sizeof(action)))
         return queue_text(conn, MHD_HTTP_NOT_FOUND, "not found");
 
+    /* Defense in depth: title_id becomes part of /data/patchdl/<id> paths that
+       get created, written, and recursively deleted. Reject anything that
+       isn't a plain id BEFORE it can reach mkdir/unlink/rmdir, so no request
+       can ever escape the download directory (a title_id of ".." would resolve
+       the dir to /data). Real PS5 title ids only use [A-Za-z0-9_-.]. */
+    if (!path_segment_safe(title_id))
+        return queue_text(conn, MHD_HTTP_FORBIDDEN, "forbidden");
+
     if (!get_title_action_info(title_id, &src, patch_url, sizeof(patch_url),
                                patch_title_id, sizeof(patch_title_id),
                                patch_storage_title_id,