BGMXPCHelper now runs as an unprivileged user, instead of root.

BGMXPCHelper's install script now creates a user and group for
BGMXPCHelper to run as. This reduces the risk of BGMXPCHelper being used
for privilege escalation.

BGMApp/BGMXPCHelper/com.bearisdriving.BGM.XPCHelper.plist.template

@@ -27,15 +27,14 @@
     <string>Adaptive</string>
 
     <!--
-    {{# TODO: Have post_install.sh create this user and group. Then uncomment this so BGMXPCHelper doesn't run  #}}
+    {{# post_install.sh creates this user and group. #}}
-    {{#       as root.                                                                                          #}}
-    {{#                                                                                                         #}}
-    {{# <key>UserName</key>                                                                                     #}}
-    {{# <string>_BGMXPCHelper</string>                                                                          #}}
-    {{# <key>GroupName</key>                                                                                    #}}
-    {{# <string>_BGMXPCHelper</string>                                                                          #}}
     -->
 
+    <key>UserName</key>
+    <string>{{BGMXPCHELPER_USER_NAME}}</string>
+    <key>GroupName</key>
+    <string>{{BGMXPCHELPER_GROUP_NAME}}</string>
+
     <!-- 
     {{# Uncomment this to log output from BGMXPCHelper. (It logs to syslog by default, so you'd have to change  #}}
     {{# that if you want logs to go to these files.)                                                            #}}

BGMApp/BGMXPCHelper/post_install.sh

@@ -24,8 +24,8 @@
 #
 # Installs BGMXPCHelper's launchd plist file and "bootstraps" (registers/enables) it with launchd.
 #
-# When you install BGMXPCHelper with xcodebuild (Or Xcode itself. I can't figure out if that's
+# When you install BGMXPCHelper with xcodebuild (or Xcode itself, if that's possible) this script
-# possible) this script runs as the final build phase.
+# runs as the final build phase.
 #
 
 # Check the environment variables we need from Xcode.
@@ -58,10 +58,60 @@ if [[ $(bash "${RESOURCES_PATH}/safe_install_dir.sh" "${INSTALL_DIR}") != 1 ]];
          "insecure. See safe_install_dir.sh for details." >&2
 fi
 
+HELPER_USER=_BGMXPCHelper
+
 LAUNCHD_PLIST_INSTALL_PATH=/Library/LaunchDaemons
 LAUNCHD_PLIST_FILENAME=com.bearisdriving.BGM.XPCHelper.plist
 LAUNCHD_PLIST=${LAUNCHD_PLIST_INSTALL_PATH}/${LAUNCHD_PLIST_FILENAME}
 
+# Create an unprivileged user for BGMXPCHelper to run as.
+
+if [[ "$(dscl . -search /Users RecordName ${HELPER_USER})" == "" ]]; then
+    # Find an unused UID and GID. UIDs from 0 to 500 are reserved by OSX.
+    HELPER_UID=$(dscl . -list /Users UniqueID | \
+        awk '{ a[$2] } END { for (i=501; i in a; i++); print i }')
+    HELPER_GID=$(dscl . -list /Groups PrimaryGroupID | \
+        awk '{ a[$2] } END { for (i='${HELPER_UID}'; i in a; i++); print i }')
+
+    # Check the UID and GID.
+    NUMERIC_ID_REGEX='^[1-9][0-9]*$'
+
+    ([[ "${HELPER_UID}" =~ ${NUMERIC_ID_REGEX} ]] && \
+        [[ ${HELPER_UID} -gt 500 ]] && \
+        [[ "$(dscl . -search /Users UniqueID ${HELPER_UID})" == "" ]]) || \
+        (echo "Internal error. Failed to generate a user ID. HELPER_UID=${HELPER_UID}" >&2; exit 1)
+
+    ([[ "${HELPER_GID}" =~ ${NUMERIC_ID_REGEX} ]] && \
+        [[ ${HELPER_GID} -gt 500 ]] && \
+        [[ "$(dscl . -search /Groups PrimaryGroupID ${HELPER_GID})" == "" ]]) || \
+        (echo "Internal error. Failed to generate a group ID. HELPER_GID=${HELPER_GID}" >&2; exit 1)
+
+    # Create the group.
+    sudo dscl . -create /Groups/${HELPER_USER} PrimaryGroupID ${HELPER_GID}
+    sudo dscl . -create /Groups/${HELPER_USER} RealName "Background Music XPC Helper Group"
+    sudo dscl . -create /Groups/${HELPER_USER} Password '*'
+
+    # Create the user.
+    sudo dscl . -create /Users/${HELPER_USER} UniqueID ${HELPER_UID}
+    sudo dscl . -create /Users/${HELPER_USER} PrimaryGroupID ${HELPER_GID}
+    sudo dscl . -create /Users/${HELPER_USER} RealName "Background Music XPC Helper"
+    sudo dscl . -create /Users/${HELPER_USER} Password '*'
+    sudo dscl . -create /Users/${HELPER_USER} UserShell /usr/bin/false
+    sudo dscl . -create /Users/${HELPER_USER} NFSHomeDirectory /var/empty
+    sudo dscl . -delete /Users/${HELPER_USER} AuthenticationAuthority
+
+    # Add the user to the group
+    sudo dscl . -append /Groups/${HELPER_USER} GroupMembership ${HELPER_USER}
+
+    # Check the user and group were created.
+    [[ "$(dscl . -search /Users RecordName ${HELPER_USER})" != "" ]] || \
+        (echo "Internal error. Failed to ${HELPER_USER} user." >&2; exit 1)
+    [[ "$(dscl . -search /Groups RecordName ${HELPER_USER})" != "" ]] || \
+        (echo "Internal error. Failed to ${HELPER_USER} group." >&2; exit 1)
+
+    echo "Created ${HELPER_USER} user."
+fi
+
 # Copy the plist template into place.
 sudo cp "${RESOURCES_PATH}/${LAUNCHD_PLIST_FILENAME}.template" "${LAUNCHD_PLIST}"
 
@@ -81,6 +131,9 @@ EXECUTABLE_PATH_ESCAPED="${EXECUTABLE_PATH//\//\\/}"
 sudo sed -i.tmp "s/{{PATH_TO_BGMXPCHELPER}}/${INSTALL_DIR_ESCAPED}/g" "${LAUNCHD_PLIST}"
 # EXECUTABLE_PATH is set by Xcode, currently to "BGMXPCHelper.xpc/Contents/MacOS/BGMXPCHelper".
 sudo sed -i.tmp "s/{{BGMXPCHELPER_EXECUTABLE_PATH}}/${EXECUTABLE_PATH_ESCAPED}/g" "${LAUNCHD_PLIST}"
+sudo sed -i.tmp "s/{{BGMXPCHELPER_USER_NAME}}/${HELPER_USER}/g" "${LAUNCHD_PLIST}"
+sudo sed -i.tmp "s/{{BGMXPCHELPER_GROUP_NAME}}/${HELPER_USER}/g" "${LAUNCHD_PLIST}"
+
 # Remove template-only comments.
 sudo sed -i.tmp 's/{{#.*#}}//g' "${LAUNCHD_PLIST}"
 

README.md

@@ -57,7 +57,18 @@ apps playing audio.
 
 - Delete `Background Music.app` from `/Applications`.
 - Delete `Background Music Device.driver` from `/Library/Audio/Plug-Ins/HAL`.
+
+#### Optional
+
 - Delete `BGMXPCHelper.xpc` from `/usr/local/libexec` or possibly `/Library/Application Support/Background Music`.
+- Unregister BGMXPCHelper, delete its user and group, and delete its launchd.plist:
+
+  ```shell
+  sudo launchctl bootout system /Library/LaunchDaemons/com.bearisdriving.BGM.XPCHelper.plist
+  sudo rm /Library/LaunchDaemons/com.bearisdriving.BGM.XPCHelper.plist
+  sudo dscl . -delete /Users/_BGMXPCHelper
+  sudo dscl . -delete /Groups/_BGMXPCHelper
+  ```
 - Pause apps that are playing audio, if you can.
 - Restart `coreaudiod`:
   [//]: # ( <sup>(Open `/Applications/Utilities/Terminal.app` and paste the following at the prompt.)</sup> )

build_and_install.sh

@@ -155,9 +155,9 @@ show_spinner
 
 # BGMXPCHelper
 
-INSTALL_DIR=$(BGMApp/BGMXPCHelper/safe_install_dir.sh)
+XPC_HELPER_INSTALL_DIR=$(BGMApp/BGMXPCHelper/safe_install_dir.sh)
 
-echo "Installing $(bold_face BGMXPCHelper.xpc) to $(bold_face ${INSTALL_DIR})." \
+echo "Installing $(bold_face BGMXPCHelper.xpc) to $(bold_face ${XPC_HELPER_INSTALL_DIR})." \
      | tee -a ${LOG_FILE}
 
 (set +e;
@@ -166,7 +166,7 @@ sudo xcodebuild -project BGMApp/BGMApp.xcodeproj \
                 -configuration ${CONFIGURATION} \
                 RUN_CLANG_STATIC_ANALYZER=0 \
                 DSTROOT="/" \
-                INSTALL_PATH="${INSTALL_DIR}" \
+                INSTALL_PATH="${XPC_HELPER_INSTALL_DIR}" \
                 install >> ${LOG_FILE} 2>&1) &
 
 show_spinner