Support decryption of SSL key

This commit makes it possible to store an encrypted SSL key on disk
and have Puma decrypt it at runtime by supplying a
`key_password_command`. Supplying a `key_password_command` will cause
Puma to:

1. Execute the external program.
2. Read the password from stdout and remove the trailing newline.
2. Configure the OpenSSL callbacks to use the password.

Other Web servers, such as NGINX and Apache, have a similar feature.
NGINX only allows supplying a password file via the `ssl_password`
parameter (https://www.nginx.com/blog/secure-distribution-ssl-private-keys-nginx/),
while Apache has a `SSLPassPhraseDialog` option (https://httpd.apache.org/docs/2.2/mod/mod_ssl.html#sslpassphrasedialog)
that allows an admin to execute an external program.

Closes puma#3132

examples/puma/encrypted_puma_keypair.pem

@@ -0,0 +1,30 @@
+-----BEGIN RSA PRIVATE KEY-----
+Proc-Type: 4,ENCRYPTED
+DEK-Info: AES-256-CBC,90F30BF02BE2889D8DA5B8168E8BC346
+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-----END RSA PRIVATE KEY-----

examples/puma/key_password_command.sh

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+echo "hello world"

ext/puma_http11/mini_ssl.c

@@ -185,6 +185,18 @@ static int engine_verify_callback(int preverify_ok, X509_STORE_CTX* ctx) {
   return preverify_ok;
 }
 
+static int password_callback(char *buf, int size, int rwflag, void *userdata) {
+    const char *password = (const char *) userdata;
+    size_t len = strlen(password);
+
+    if (len > (size_t) size) {
+      return 0;
+    }
+
+    memcpy(buf, password, len);
+    return (int) len;
+}
+
 static VALUE
 sslctx_alloc(VALUE klass) {
   SSL_CTX *ctx;
@@ -212,10 +224,12 @@ sslctx_initialize(VALUE self, VALUE mini_ssl_ctx) {
   SSL_CTX* ctx;
   int ssl_options;
   VALUE key, cert, ca, verify_mode, ssl_cipher_filter, no_tlsv1, no_tlsv1_1,
-    verification_flags, session_id_bytes, cert_pem, key_pem;
+    verification_flags, session_id_bytes, cert_pem, key_pem, key_password_command, decryption_key;
   BIO *bio;
   X509 *x509;
   EVP_PKEY *pkey;
+  pem_password_cb *password_cb = NULL;
+  const char *password = NULL;
 #ifdef HAVE_SSL_CTX_SET_MIN_PROTO_VERSION
   int min;
 #endif
@@ -235,6 +249,8 @@ sslctx_initialize(VALUE self, VALUE mini_ssl_ctx) {
 
   key = rb_funcall(mini_ssl_ctx, rb_intern_const("key"), 0);
 
+  key_password_command = rb_funcall(mini_ssl_ctx, rb_intern_const("key_password_command"), 0);
+
   cert = rb_funcall(mini_ssl_ctx, rb_intern_const("cert"), 0);
 
   ca = rb_funcall(mini_ssl_ctx, rb_intern_const("ca"), 0);
@@ -261,6 +277,18 @@ sslctx_initialize(VALUE self, VALUE mini_ssl_ctx) {
     }
   }
 
+  if (!NIL_P(key_password_command)) {
+      decryption_key = rb_funcall(mini_ssl_ctx, rb_intern_const("key_password"), 0);
+
+      if (!NIL_P(decryption_key)) {
+          StringValue(decryption_key);
+          password_cb = password_callback;
+          password = RSTRING_PTR(decryption_key);
+          SSL_CTX_set_default_passwd_cb(ctx, password_cb);
+          SSL_CTX_set_default_passwd_cb_userdata(ctx, (void *) password);
+      }
+  }
+
   if (!NIL_P(key)) {
     StringValue(key);
 
@@ -285,7 +313,7 @@ sslctx_initialize(VALUE self, VALUE mini_ssl_ctx) {
   if (!NIL_P(key_pem)) {
     bio = BIO_new(BIO_s_mem());
     BIO_puts(bio, RSTRING_PTR(key_pem));
-    pkey = PEM_read_bio_PrivateKey(bio, NULL, NULL, NULL);
+    pkey = PEM_read_bio_PrivateKey(bio, NULL, password_cb, (void *) password);
 
     if (SSL_CTX_use_PrivateKey(ctx, pkey) != 1) {
       BIO_free(bio);

lib/puma/dsl.rb

@@ -89,6 +89,7 @@ def self.ssl_bind_str(host, port, opts)
 
         cert_flags = (cert = opts[:cert]) ? "cert=#{Puma::Util.escape(cert)}" : nil
         key_flags = (key = opts[:key]) ? "&key=#{Puma::Util.escape(key)}" : nil
+        password_flags = (password_command = opts[:key_password_command]) ? "&key_password_command=#{Puma::Util.escape(password_command)}" : nil
 
         reuse_flag =
           if (reuse = opts[:reuse])
@@ -114,7 +115,7 @@ def self.ssl_bind_str(host, port, opts)
             nil
           end
 
-        "ssl://#{host}:#{port}?#{cert_flags}#{key_flags}#{ssl_cipher_filter}" \
+        "ssl://#{host}:#{port}?#{cert_flags}#{key_flags}#{password_flags}#{ssl_cipher_filter}" \
           "#{reuse_flag}&verify_mode=#{verify}#{tls_str}#{ca_additions}#{v_flags}#{backlog_str}#{low_latency_str}"
       end
     end

lib/puma/minissl.rb

@@ -5,6 +5,7 @@
 rescue LoadError
 end
 
+require 'open3'
 # need for Puma::MiniSSL::OPENSSL constants used in `HAS_TLS1_3`
 # use require, see https://github.com/puma/puma/pull/2381
 require 'puma/puma_http11'
@@ -277,6 +278,7 @@ def check
       else
         # non-jruby Context properties
         attr_reader :key
+        attr_reader :key_password_command
         attr_reader :cert
         attr_reader :ca
         attr_reader :cert_pem
@@ -291,6 +293,10 @@ def key=(key)
           @key = key
         end
 
+        def key_password_command=(key_password_command)
+          @key_password_command = key_password_command
+        end
+
         def cert=(cert)
           check_file cert, 'Cert'
           @cert = cert
@@ -316,6 +322,17 @@ def check
           raise "Cert not configured" if @cert.nil? && @cert_pem.nil?
         end
 
+        # Executes the command to return the password needed to decrypt the key.
+        def key_password
+          raise "Key password command not configured" if @key_password_command.nil?
+
+          stdout_str, stderr_str, status = Open3.capture3(key_password_command)
+
+          return stdout_str.chomp if status.success?
+
+          raise "Key password failed with code #{status.exitstatus}: #{stderr_str}"
+        end
+
         # Controls session reuse.  Allowed values are as follows:
         # * 'off' - matches the behavior of Puma 5.6 and earlier.  This is included
         #   in case reuse 'on' is made the default in future Puma versions.

lib/puma/minissl/context_builder.rb

@@ -38,6 +38,7 @@ def context
 
           ctx.key = params['key'] if params['key']
           ctx.key_pem = params['key_pem'] if params['key_pem']
+          ctx.key_password_command = params['key_password_command'] if params['key_password_command']
 
           if params['cert'].nil? && params['cert_pem'].nil?
             log_writer.error "Please specify the SSL cert via 'cert=' or 'cert_pem='"

test/test_integration_ssl.rb

@@ -174,6 +174,70 @@ def test_ssl_run_with_localhost_authority
 
 activate_control_app 'tcp://#{HOST}:#{control_tcp_port}', { auth_token: '#{TOKEN}' }
 
+app do |env|
+  [200, {}, [env['rack.url_scheme']]]
+end
+RUBY
+
+    with_server(config) do |http|
+      body = nil
+      http.start do
+        req = Net::HTTP::Get.new '/', {}
+        http.request(req) { |resp| body = resp.body }
+      end
+      assert_equal 'https', body
+    end
+  end
+
+  def test_ssl_run_with_encrypted_key
+    skip_if :jruby
+
+    config = <<RUBY
+  key_path  = '#{File.expand_path '../examples/puma/encrypted_puma_keypair.pem', __dir__}'
+  cert_path = '#{File.expand_path '../examples/puma/cert_puma.pem', __dir__}'
+  key_command = '#{File.expand_path '../examples/puma/key_password_command.sh', __dir__}'
+
+  ssl_bind '#{HOST}', '#{bind_port}', {
+    cert: cert_path,
+    key: key_path,
+    verify_mode: 'none',
+    key_password_command: key_command
+  }
+
+activate_control_app 'tcp://#{HOST}:#{control_tcp_port}', { auth_token: '#{TOKEN}' }
+
+app do |env|
+  [200, {}, [env['rack.url_scheme']]]
+end
+RUBY
+
+    with_server(config) do |http|
+      body = nil
+      http.start do
+        req = Net::HTTP::Get.new '/', {}
+        http.request(req) { |resp| body = resp.body }
+      end
+      assert_equal 'https', body
+    end
+  end
+
+  def test_ssl_run_with_encrypted_pem
+    skip_if :jruby
+
+    config = <<RUBY
+  key_path  = '#{File.expand_path '../examples/puma/encrypted_puma_keypair.pem', __dir__}'
+  cert_path = '#{File.expand_path '../examples/puma/cert_puma.pem', __dir__}'
+  key_command = '#{File.expand_path '../examples/puma/key_password_command.sh', __dir__}'
+
+  ssl_bind '#{HOST}', '#{bind_port}', {
+    cert_pem: File.read(cert_path),
+    key_pem: File.read(key_path),
+    verify_mode: 'none',
+    key_password_command: key_command
+  }
+
+activate_control_app 'tcp://#{HOST}:#{control_tcp_port}', { auth_token: '#{TOKEN}' }
+
 app do |env|
   [200, {}, [env['rack.url_scheme']]]
 end

test/test_minissl.rb

@@ -83,5 +83,12 @@ def test_raises_with_invalid_cert_pem
       exception = assert_raises(ArgumentError) { ctx.cert_pem = nil }
       assert_equal("'cert_pem' is not a String", exception.message)
     end
+
+    def test_raises_with_invalid_key_password_command
+      ctx = Puma::MiniSSL::Context.new
+      ctx.key_password_command = '/unreadable/decrypt_command'
+
+      assert_raises(Errno::ENOENT) { ctx.key_password }
+    end
   end
 end if ::Puma::HAS_SSL