如何安全地通过ssh返回bash数组？

Question

在bash外壳中，可以轻松地用引用数组declare -p，然后在eval稍后编辑以使其恢复正常。对于通过SSH将数组（作为脚本的一部分）传递到远程计算机，这似乎可以接受。

问题是，我不希望对方获得相同级别的信任。如果远程计算机受到威胁，则感染可能会通过未经eval声明的语句传播到本地计算机。

当前，要在机器之间传递数组，我使用的是这样的方法：

#!/bin/bash

# Define the modules we expect to find installed on the remote machine
expected_modules=(foo-module bar 'baz 2.0')

# SSH into the remote machine, send the arrays back and forth with "declare -p"
unparsed_missing_modules=$(ssh remote-machine /bin/bash << EOF
    check_for_module() {
        # Placeholder so that this can be tested locally
        case \$1 in
            foo*) true;;
            *) false;;
        esac
    }

    $(declare -p expected_modules)
    missing_modules=()
    for module in "\${expected_modules[@]}"; do
        if ! check_for_module "\$module"; then
            missing_modules+=( "\$module" )
        fi
    done

    declare -p missing_modules
EOF
)

# Unpack the result (this is what I want to find an alternative to)
eval "$unparsed_missing_modules"

# Do something with the result after unpacking into an array
for module in "${missing_modules[@]}"; do
    echo "Warning: Remote machine is missing $module" >&2
done

当ssh会话的输出直接传递到时，此脚本中的主要不安全因素已接近尾声eval。我该如何清理输入内容bash？

Answer 1

通用，安全的答案是：对数组的条目进行NUL分隔，在stdout上传递文字NUL分隔的数据，然后使用while read循环来解释它。

观察：

get_remote_array() {
  local args
  local hostname=$1; shift
  printf -v args '%q ' "$@"
  ssh "$hostname" "bash -s $args" <<'EOF'
# in real-world use, print something more useful than the arguments we were started with
# ...but for here, this demonstrates the point:
printf '%s\0' "$@" 
EOF
}

array=( )
while IFS= read -r -d ''; do
  array+=( "$REPLY" )
done < <(get_remote_array "localhost" \
            $'I\ncontain\nnewlines' \
            'I want to $(touch /tmp/security-fail)' \
            "'"'I REALLY want to $(touch /tmp/security-fail), even in single quotes'"'")

echo "---- Shell-escaped content"
printf '%q\n' "${array[@]}"

echo "---- Unescaped content"
printf '<<%s>>\n' "${array[@]}"

此演示在两个方向上传递了潜在的恶意数据，并演示了它在不受损害的往返过程中仍然可以生存。