Add privileged mount helper + polkit policy

POSIX sh helper (pkexec target) that mounts/unmounts CIFS shares with strict validation of the unprivileged caller's arguments: mountpoint confined to $HOME or /run/media/<user>, //host/share source, credfile under the caller's runtime dir at mode 0600, whitelisted SMB version/options/modes. Polkit action authorizes it with auth_admin_keep. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
2026-07-15 14:28:49 +02:00
parent fab2d65dd2
commit 6448ea4f22
2 changed files with 138 additions and 0 deletions
@@ -0,0 +1,118 @@
+#!/bin/sh
+#
+# synology-nas-helper — privileged mount/unmount helper for the Synology NAS
+# plasmoid. It is invoked as root via pkexec (see the accompanying
+# sk.houmeres.synologynas.policy). The CALLER IS UNPRIVILEGED, so every argument
+# is treated as hostile and strictly validated before any mount happens.
+#
+# Usage (all arguments positional):
+#   synology-nas-helper mount   <source> <mountpoint> <credfile> <vers> <opts> <fmode> <dmode>
+#   synology-nas-helper unmount <mountpoint>
+#
+# Guarantees enforced below:
+#   * mountpoint must resolve to a path under the CALLING user's $HOME or
+#     /run/media/<user> (rejects .. traversal via realpath -m).
+#   * source must look like //host/share.
+#   * credfile must live under /run/user/<caller-uid>/, be a regular file,
+#     owned by the caller, mode 0600.
+#   * vers, permission modes and extra options are charset/whitelist checked.
+#
+set -eu
+
+die() { echo "synology-nas-helper: $1" >&2; exit 1; }
+
+# --- identify the calling (unprivileged) user ---------------------------------
+[ -n "${PKEXEC_UID:-}" ] || die "must be run through pkexec (PKEXEC_UID unset)"
+case "$PKEXEC_UID" in
+    ''|*[!0-9]*) die "invalid PKEXEC_UID" ;;
+esac
+
+pwline=$(getent passwd "$PKEXEC_UID") || die "unknown caller uid $PKEXEC_UID"
+CALLER_USER=$(printf '%s\n' "$pwline" | cut -d: -f1)
+CALLER_GID=$(printf '%s\n' "$pwline" | cut -d: -f4)
+CALLER_HOME=$(printf '%s\n' "$pwline" | cut -d: -f6)
+[ -n "$CALLER_HOME" ] || die "caller has no home directory"
+
+# --- shared validators --------------------------------------------------------
+validate_mountpoint() {
+    # Canonicalise (resolves .. lexically, no need for the path to exist yet).
+    canon=$(realpath -m -- "$1") || die "cannot canonicalise mountpoint"
+    case "$canon" in
+        "$CALLER_HOME"/*) : ;;
+        "/run/media/$CALLER_USER"/*) : ;;
+        *) die "mountpoint must be under \$HOME or /run/media/$CALLER_USER" ;;
+    esac
+    printf '%s' "$canon"
+}
+
+# --- subcommands --------------------------------------------------------------
+do_mount() {
+    [ $# -eq 7 ] || die "mount: expected 7 arguments"
+    source=$1; mountpoint=$2; credfile=$3; vers=$4; extra=$5; fmode=$6; dmode=$7
+
+    case "$source" in
+        //?*/?*) : ;;
+        *) die "source must be //host/share" ;;
+    esac
+
+    mp=$(validate_mountpoint "$mountpoint")
+
+    # Credentials file: under the caller's runtime dir, their file, mode 0600.
+    cf=$(realpath -m -- "$credfile") || die "cannot canonicalise credfile"
+    case "$cf" in
+        "/run/user/$PKEXEC_UID"/*) : ;;
+        *) die "credfile must be under /run/user/$PKEXEC_UID" ;;
+    esac
+    [ -f "$cf" ] || die "credfile not found"
+    [ "$(stat -c %u "$cf")" = "$PKEXEC_UID" ] || die "credfile not owned by caller"
+    [ "$(stat -c %a "$cf")" = "600" ] || die "credfile must be mode 0600"
+
+    case "$vers" in
+        default|1.0|2.0|2.1|3.0|3.02|3.1.1) : ;;
+        *) die "unsupported SMB version: $vers" ;;
+    esac
+    case "$fmode" in 0[0-7][0-7][0-7]) : ;; *) die "bad file mode" ;; esac
+    case "$dmode" in 0[0-7][0-7][0-7]) : ;; *) die "bad dir mode" ;; esac
+    case "$extra" in
+        '') : ;;
+        *[!A-Za-z0-9=,_.:-]*) die "extra mount options contain illegal characters" ;;
+    esac
+
+    if findmnt -rn -o TARGET --mountpoint "$mp" >/dev/null 2>&1; then
+        die "already mounted: $mp"
+    fi
+
+    # Create the mount point and hand the freshly-created dirs to the caller.
+    mkdir -p "$mp" || die "cannot create mount point"
+    chown "$PKEXEC_UID:$CALLER_GID" "$mp" 2>/dev/null || true
+    p=$(dirname "$mp")
+    while [ "$p" != "$CALLER_HOME" ] && [ "$p" != "/" ] && [ "$p" != "/run/media" ]; do
+        chown "$PKEXEC_UID:$CALLER_GID" "$p" 2>/dev/null || true
+        p=$(dirname "$p")
+    done
+
+    opts="credentials=$cf,uid=$PKEXEC_UID,gid=$CALLER_GID,forceuid,forcegid,file_mode=$fmode,dir_mode=$dmode,vers=$vers"
+    [ -n "$extra" ] && opts="$opts,$extra"
+
+    exec mount.cifs "$source" "$mp" -o "$opts"
+}
+
+do_unmount() {
+    [ $# -eq 1 ] || die "unmount: expected 1 argument"
+    mp=$(validate_mountpoint "$1")
+    findmnt -rn -t cifs -o TARGET --mountpoint "$mp" >/dev/null 2>&1 \
+        || die "not a cifs mount: $mp"
+    umount "$mp" || die "umount failed"
+    # Tidy up empty leaf + host dirs (never recursive, never touches files).
+    rmdir "$mp" 2>/dev/null || true
+    rmdir "$(dirname "$mp")" 2>/dev/null || true
+}
+
+[ $# -ge 1 ] || die "no subcommand"
+cmd=$1
+shift
+case "$cmd" in
+    mount)   do_mount "$@" ;;
+    unmount) do_unmount "$@" ;;
+    *) die "unknown subcommand: $cmd" ;;
+esac