TLS Fallback Signaling Cipher Suite Value by aeris · Pull Request #165 · ruby/openssl

aeris · 2017-10-29T17:40:25Z

Support for fallback SCSV RFC 7507.

Expected behaviour is to refuse connection if the client signals a protocol with
the fallback flag but the server supports a better one (downgrade attack detection).

rhenium · 2017-10-30T01:28:45Z

ext/openssl/ossl_ssl.c

+    long modes;
+
+    GetSSLCTX(self, ctx);
+    if(!ctx){


ctx is never NULL.

rhenium · 2017-10-30T01:31:05Z

ext/openssl/ossl_ssl.c

+
+    modes = SSL_CTX_get_mode(ctx);
+    modes |= SSL_MODE_SEND_FALLBACK_SCSV;
+    SSL_CTX_set_mode(ctx, modes);


SSL_CTX_set_mode() preserves the previously set flags, so just SSL_CTX_set_mode(ctx, SSL_MODE_SEND_FALLBACK_SCSV) is fine.

rhenium · 2017-10-30T01:33:30Z

test/utils.rb

+              rescue OpenSSL::SSL::SSLError => e
+                retry if ignore_listener_error
+                raise unless e.message =~ /inappropriate fallback/
+              rescue IOError, Errno::EBADF, Errno::EINVAL,


Are changes in start_server really needed? I'd like to have all SCSV specific code in TestSSL#test_fallback_scsv.

An exception occurs on server part too in case of SCSV error.
Without this catch, unit test detect only the server part and not the tested client part as expected.
I didn't find a proper way to do this :'(

You can get a raw TCP socket pair by socketpair defined in TestSSL when the exception raised by SSLSocket#accept has to be tested.

Ah, I misinterpreted your comment. Errors raised in server can be suppressed by passing 'ignore_listener_error: true' to start_server.

I fix the test with socketpair, to also test server side error.

rhenium · 2017-10-30T01:43:24Z

ext/openssl/ossl_ssl.c

+static VALUE
+ossl_sslctx_enable_fallback_scsv(VALUE self)
+{
+#ifdef SSL_MODE_SEND_FALLBACK_SCSV


The method should not be defined at all if SSL_MODE_SEND_FALLBACK_SCSV is not usable.

rhenium · 2017-10-30T01:45:15Z

lib/openssl/ssl.rb

+      def initialize(version = nil, fallback_scsv: false)
        self.options |= OpenSSL::SSL::OP_ALL
        self.ssl_version = version if version
+        self.enable_fallback_scsv if fallback_scsv


I think #initialize should take all or nothing -- it's just confusing to have only fallback_scsv option.

OpenSSL::SSL::SSLContext is generally used with the short form SSLContext.new version, without configuration after that. Having SCSV set on initialize allows to keep short form with SCSV protection SSLContext.new version, fallback_scsv: true without configuration too.

As in the RDoc comment above, use of the form "SSL::SSLContext.new(ssl_method_name)" (and SSL::SSLContext#ssl_version= which is called from it) is not recommended anymore. This is because the version-specific SSL methods has been deprecated in the OpenSSL C API.

In addition to the protocol version which is now preferably set by SSL::SSLContext#{min,max}_version=, clients would usually need to set other options such as verify_mode, too. IMHO SSL::SSLContext has too much options to enumerate in kwargs...

Ping on this?

rhenium · 2017-10-30T01:55:33Z

test/test_ssl.rb


+  def test_fallback_scsv
+    ctx_proc = proc { |ctx|
+      ctx.ciphers = "kRSA"


Avoid kRSA since it will not work with TLS 1.3-capable OpenSSL. It's not finalized yet, but I try to keep the tests pass with the master branch of OpenSSL.

rhenium · 2017-10-30T09:58:29Z

Just out of curiosity (I'm fine with either one), is there any particular reason to choose #enable_fallback_scsv over #{send_,}fallback_scsv=true|false?

aeris · 2017-10-30T10:51:06Z

fallback_scsv=false is a little non-sense. In practice, you activate SCSV only when you fallback to lower version after a failed connection. "Disabling" SCSV has no meaning.

rhenium · 2017-10-30T12:15:47Z

Fair enough, I agree nobody would bother with setting that explicitly.

rhenium · 2017-11-03T17:08:08Z

test/test_ssl.rb

    end
  end

+  def test_fallback_scsv


This test case is failing with LibreSSL or older patch versions of OpenSSL 1.0.1 because they lack support for TLS_FALLBACK_SCSV. pend if the method does not exist.

https://travis-ci.org/ruby/openssl/jobs/294781400#L660

rhenium · 2017-11-03T17:09:37Z

test/test_ssl.rb

+    }
+
+    assert t.join
+  end


Sockets created by socketpair have to be closed manually to prevent FD leak (which could be an issue when running tests repeatedly for debugging).

rhenium · 2017-11-03T17:10:32Z

test/test_ssl.rb


+  def test_fallback_scsv
+    ctx_proc = proc { |ctx|
+        ctx.ssl_version = :TLSv1_2


Please use #max_version= and #min_version= in new tests. In this case, specifying just max_version would be enough. (This applies to the other instances too.)

rhenium · 2017-11-03T17:12:58Z

test/test_ssl.rb

+    sock1, sock2 = socketpair
+    ctx1 = OpenSSL::SSL::SSLContext.new
+    ctx1.min_version = OpenSSL::SSL::TLS1_1_VERSION
+    ctx1.max_version = OpenSSL::SSL::TLS1_2_VERSION


The server doesn't need to set neither min_version nor max_version since we know all protocol versions are enabled by default.

rhenium · 2017-11-03T17:13:19Z

test/test_ssl.rb

+    ctx1 = OpenSSL::SSL::SSLContext.new
+    ctx1.min_version = OpenSSL::SSL::TLS1_1_VERSION
+    ctx1.max_version = OpenSSL::SSL::TLS1_2_VERSION
+    s1 = OpenSSL::SSL::SSLSocket.new sock1, ctx1


Please parenthesize arguments for consistency with other tests in this file.

rhenium · 2017-11-03T17:18:56Z

ext/openssl/ossl_ssl.c

+ * Activate TLS_FALLBACK_SCSV for this context.
+ * See RFC 7507.
+ */
+#ifdef SSL_MODE_SEND_FALLBACK_SCSV


I think ossl_sslctx_enable_fallback_scsv() should be placed after ossl_sslctx_set_security_level() so the ordering match with the rb_define_method() lines. (We haven't been always consistent, but ...)

By the way, RDoc seems to fail to find the doc. Moving #ifdef SSL_MODE_SEND_FALLBACK_SCSV above the block comment fixes it for me. This might be considered a bug of RDoc's C parser, though.

rhenium · 2017-11-03T17:44:53Z

test/test_ssl.rb

+
+    start_server do |port|
+      ctx = OpenSSL::SSL::SSLContext.new
+      ctx.ssl_version = :TLSv1_2


Please use #max_version= for client SSL contexts, too.

Ok, fixed.
But for client, it's strange to have to use min/max, because AFAIK, SSLContext doesn't handle fallback by itself and support only a single version (the max_version).

Yes, in a real use case, the client will surely enable only one protocol version. The point is, in this case, that #ssl_version= should not be used because it's an old and deprecated method. New applications should write as:

ctx.min_version = ctx.max_version = ...

In this case, why not reimplement old ssl_version with min/max_version under the hood ?
min/max is cool for server part, but for client part, better to have a single version, no ?

Actually It's been rewritten to call #min_version= and #max_version=. But it's a different story whether we should recommend using that or not.

openssl/lib/openssl/ssl.rb

Line 186 in d834e86

# call-seq:

It's a decision of the OpenSSL C API that the SSL/TLS protocol version specific SSL methods, such as TLSv1_2_method() which corresponds to :TLSv1_2, are now deprecated. Please see TLSv1_2_method(3).

https://www.openssl.org/docs/manmaster/man3/TLSv1_2_method.html
(The web site seems down now)

rhenium · 2017-11-03T17:48:43Z

This looks really nice to me. Thanks for your patience!

Lastly, can you squash the commits into one commit?

Support for fallback SCSV [RFC 7507](https://tools.ietf.org/html/rfc7507). Expected behaviour is to refuse connection if the client signals a protocol with the fallback flag but the server supports a better one (downgrade attack detection).

aeris · 2017-11-03T18:00:50Z

Done :)

rhenium requested changes Oct 30, 2017

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rhenium reviewed Nov 3, 2017

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aeris force-pushed the scsv branch from 1df82e7 to da9be3b Compare November 3, 2017 17:48

rhenium approved these changes Nov 3, 2017

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aeris force-pushed the scsv branch from da9be3b to 7c4028a Compare November 3, 2017 18:00

rhenium merged commit 819d7e5 into ruby:master Nov 3, 2017

aeris deleted the scsv branch November 3, 2017 18:51

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aeris commented Oct 29, 2017

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aeris Oct 30, 2017 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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aeris Oct 30, 2017 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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rhenium commented Oct 30, 2017

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aeris commented Oct 30, 2017

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rhenium commented Oct 30, 2017

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rhenium commented Nov 3, 2017

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aeris commented Nov 3, 2017

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aeris Oct 30, 2017 •

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aeris Oct 30, 2017 •

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