Validate the cached Apple-display device path before use

The cached device path was trusted for merely existing, not for being a hiddev node, and fell back to a predictable /tmp path when XDG_RUNTIME_DIR was unset. Validate the cache shape (hiddev char device) and cache only under the user-private runtime dir; asdcontrol already gates non-Apple devices downstream, so this is defense-in-depth in the layer Omarchy owns.

bin/omarchy-brightness-display-apple

@@ -4,7 +4,13 @@
 # omarchy:args=[--no-osd] [+N%|N%-|N%]
 # omarchy:examples=omarchy brightness display apple | omarchy brightness display apple +5% | omarchy brightness display apple --no-osd 50%
 
-device_cache="${XDG_RUNTIME_DIR:-/tmp}/omarchy-brightness-display-apple.device"
+# Only cache under the user-private runtime dir. With no XDG_RUNTIME_DIR we skip
+# caching (detect every run) rather than fall back to a predictable, world-writable
+# /tmp path another user could pre-create.
+device_cache=""
+if [[ -n "${XDG_RUNTIME_DIR:-}" ]]; then
+  device_cache="$XDG_RUNTIME_DIR/omarchy-brightness-display-apple.device"
+fi
 no_osd=0
 if [[ ${1:-} == "--no-osd" ]]; then
   no_osd=1
@@ -28,9 +34,14 @@ find_apple_display_device() {
   local cached=""
   local device=""
 
-  if [[ -r $device_cache ]]; then
+  if [[ -n "$device_cache" && -r $device_cache ]]; then
     read -r cached <"$device_cache" || true
-    if [[ -n $cached && -e $cached ]]; then
+    # Trust a cached value only if it still names a hiddev character device. A
+    # stale or unexpected cache (a regular file, a non-hiddev node) is ignored and
+    # we re-detect instead of handing an arbitrary path to asdcontrol. The globs
+    # are left unquoted on purpose: [[ ]] pattern-matches an unquoted right side,
+    # and quoting them would turn the match into a literal string comparison.
+    if [[ ( $cached == /dev/hiddev* || $cached == /dev/usb/hiddev* ) && -c $cached ]]; then
       printf '%s\n' "$cached"
       return 0
     fi
@@ -39,7 +50,9 @@ find_apple_display_device() {
   device="$(detect_apple_display_device)" || return 1
   [[ -n $device ]] || return 1
 
-  printf '%s\n' "$device" >"$device_cache"
+  if [[ -n "$device_cache" ]]; then
+    printf '%s\n' "$device" >"$device_cache"
+  fi
   printf '%s\n' "$device"
 }
 

test/shell.d/brightness-display-apple-cache-test.sh

@@ -0,0 +1,120 @@
+#!/bin/bash
+
+set -euo pipefail
+
+source "$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)/base-test.sh"
+
+TMPDIR=$(mktemp -d)
+trap 'rm -rf "$TMPDIR"' EXIT
+
+# Stubs on PATH: drop sudo so asdcontrol runs directly, record every asdcontrol
+# invocation, make detection deterministic by having --detect report no device,
+# and no-op the OSD. On a host without any /dev/*hiddev* node the wrapper's
+# detect_apple_display_device returns before it ever runs asdcontrol, so the
+# reject cases assert on the negative: a refused cache value is never handed to
+# `asdcontrol <dev> -- <step>`. Blind-trust validation would hand it over and be
+# caught here.
+stub_dir="$TMPDIR/stubs"
+mkdir -p "$stub_dir"
+
+asd_log="$TMPDIR/asdcontrol.log"
+
+cat >"$stub_dir/sudo" <<'STUB'
+#!/bin/bash
+exec "$@"
+STUB
+chmod +x "$stub_dir/sudo"
+
+cat >"$stub_dir/asdcontrol" <<STUB
+#!/bin/bash
+printf '%s\n' "\$*" >>"$asd_log"
+# --detect reports nothing, so detection never yields a device.
+if [[ \$1 == "--detect" ]]; then
+  exit 0
+fi
+# A brightness read (a lone device arg) returns a plausible value; a set
+# (<device> -- <step>) just succeeds.
+if [[ \$# -eq 1 ]]; then
+  printf '%s: BRIGHTNESS=30000\n' "\$1"
+fi
+exit 0
+STUB
+chmod +x "$stub_dir/asdcontrol"
+
+cat >"$stub_dir/omarchy-osd" <<'STUB'
+#!/bin/bash
+exit 0
+STUB
+chmod +x "$stub_dir/omarchy-osd"
+
+run_wrapper() {
+  # $1: value for XDG_RUNTIME_DIR ("" means unset); remaining args go to the wrapper.
+  local xdg="$1"
+  shift
+  : >"$asd_log"
+  if [[ -n $xdg ]]; then
+    XDG_RUNTIME_DIR="$xdg" PATH="$stub_dir:$ROOT/bin:$PATH" \
+      omarchy-brightness-display-apple "$@" 2>&1 || true
+  else
+    env -u XDG_RUNTIME_DIR PATH="$stub_dir:$ROOT/bin:$PATH" \
+      omarchy-brightness-display-apple "$@" 2>&1 || true
+  fi
+}
+
+# --- A cache value that is not a hiddev character device is rejected ----------
+xdg_dir="$TMPDIR/xdg"
+mkdir -p "$xdg_dir"
+cache_file="$xdg_dir/omarchy-brightness-display-apple.device"
+
+regular_file="$TMPDIR/not-a-device"
+: >"$regular_file"
+
+for poison in "/dev/null" "$regular_file" "/tmp/omarchy-evil"; do
+  printf '%s\n' "$poison" >"$cache_file"
+  output=$(run_wrapper "$xdg_dir" "+5%")
+  if grep -qF -- "$poison -- +5%" "$asd_log"; then
+    fail "wrapper handed a non-hiddev cache value to asdcontrol: $poison" "$output"
+  fi
+done
+pass "wrapper rejects a cached path that is not a hiddev character device"
+
+# NOTE: the complementary arm (a cache value that DOES match /dev/hiddev* but is
+# not a character device) cannot be built without root -- only real device nodes
+# live under /dev. It is covered by the -c test and exercised below only when a
+# real hiddev node happens to be present.
+
+# --- A legitimate cached hiddev node is trusted (only where HW is present) ----
+real_hiddev=""
+for candidate in /dev/usb/hiddev* /dev/hiddev*; do
+  if [[ -c $candidate ]]; then
+    real_hiddev="$candidate"
+    break
+  fi
+done
+if [[ -n $real_hiddev ]]; then
+  printf '%s\n' "$real_hiddev" >"$cache_file"
+  run_wrapper "$xdg_dir" "+5%" >/dev/null
+  grep -qF -- "$real_hiddev -- +5%" "$asd_log" ||
+    fail "wrapper did not trust a valid cached hiddev node: $real_hiddev"
+  pass "wrapper trusts a cached hiddev character device without re-detecting"
+else
+  pass "no /dev/hiddev* character device present; skipping the valid-cache case"
+fi
+
+# --- With no XDG_RUNTIME_DIR, the predictable /tmp cache is not consulted ------
+# Guard on the real path not pre-existing so we never clobber a live cache, and
+# remove what we create. Old code read /tmp and would hand /dev/null to
+# asdcontrol; new code has no cache path at all when XDG_RUNTIME_DIR is unset.
+tmp_cache="/tmp/omarchy-brightness-display-apple.device"
+if [[ -e $tmp_cache ]]; then
+  pass "$tmp_cache already exists on this host; skipping the /tmp-fallback case"
+else
+  printf '%s\n' "/dev/null" >"$tmp_cache"
+  output=$(run_wrapper "" "+5%")
+  used=1
+  grep -qF -- "/dev/null -- +5%" "$asd_log" || used=0
+  rm -f "$tmp_cache"
+  (( used == 0 )) ||
+    fail "wrapper consulted the world-writable /tmp cache with no XDG_RUNTIME_DIR" "$output"
+  pass "wrapper ignores the /tmp cache path when XDG_RUNTIME_DIR is unset"
+fi