From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp1.linuxfoundation.org (smtp1.linux-foundation.org [172.17.192.35]) by mail.linuxfoundation.org (Postfix) with ESMTPS id E8EF586 for ; Tue, 18 Aug 2015 05:03:44 +0000 (UTC) X-Greylist: whitelisted by SQLgrey-1.7.6 Received: from mail-la0-f48.google.com (mail-la0-f48.google.com [209.85.215.48]) by smtp1.linuxfoundation.org (Postfix) with ESMTPS id 887A318D for ; Tue, 18 Aug 2015 05:03:42 +0000 (UTC) Received: by labd1 with SMTP id d1so92441836lab.1 for ; Mon, 17 Aug 2015 22:03:40 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:in-reply-to:references:date :message-id:subject:from:to:cc:content-type :content-transfer-encoding; bh=ptfgzu1q495XfF7T7q0V3y2vK1ZVKCW5Tvn8BI30w0g=; b=byDfLP4AcPT03mIpMrNWfIu7Te9vhcNF2rLi9e0SttkrJMaobkq3EiFw7e04bQZ6oS 8UIf0OJiUG/QvknXIApoLlL4JvIsIccjWTYmIsRSknEMnOw8wjxfvwWOyI1d3ttH6OPZ gIrU2SMigBcinsxgB8yhR8yuDBuYNm3pTqYl0jfkIgMIiMjOKRNmkMbf11bz+JWhFG2L ggT0M46M7PhaUvX9cN8vdGclriuIarrXn2T31yaUlqt00x5AA6XGbRt0py6/MMpehdVD QhO9UsVZqfczoXRhYN9yz3J6D2Vn6lKNIAa14NXI7tDh1E5GyBBHBQUmzypoTC2jQAgu Z+IQ== X-Gm-Message-State: ALoCoQlzaKc+Cov7IgExtIA7OqiU+RWOJBMbP3pum720zcla00oTPthI7BXcvu5E9nFamjfWUbXv MIME-Version: 1.0 X-Received: by 10.112.129.167 with SMTP id nx7mr4030857lbb.27.1439874220447; Mon, 17 Aug 2015 22:03:40 -0700 (PDT) Received: by 10.114.83.200 with HTTP; Mon, 17 Aug 2015 22:03:40 -0700 (PDT) In-Reply-To: References: <09C8843E-8379-404D-8357-05BDB1F749C1@me.com> <499C1F46-5EB8-4846-86B6-0B3F2E02D972@bitsofproof.com> Date: Tue, 18 Aug 2015 01:03:40 -0400 Message-ID: From: Cory Fields To: Tamas Blummer Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Spam-Status: No, score=-2.6 required=5.0 tests=BAYES_00,RCVD_IN_DNSWL_LOW autolearn=ham version=3.3.1 X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on smtp1.linux-foundation.org Cc: bitcoin-dev@lists.linuxfoundation.org Subject: Re: [bitcoin-dev] libconsensus assertion fails if used in multiple threads X-BeenThere: bitcoin-dev@lists.linuxfoundation.org X-Mailman-Version: 2.1.12 Precedence: list List-Id: Bitcoin Development Discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 18 Aug 2015 05:03:45 -0000 Back to the list (from github) in case anyone finds this via Google. The patch that I posted here a few days ago did not fix the issue for Tamas= . I spent some time tracking down this edge-case because libbitcoinconsensus needs to be as bullet-proof as possible. Thanks to Tamas for creating a bare-bones test case after some discussion. I finally managed to reproduce the issue on OSX. It's subtle and likely rare in the real-world, though obviously not impossible given the report here. For posterity, here's a rundown (braindump) of the issue. When calling EC_KEY_new_by_curve_name(), openssl internally checks to see how to setup the curve's EC_METHOD (simple, montgomery, or nist). Unfortunately, in all released OpenSSL versions (as far as I can tell master is the only branch that has fixed this issue), it's tested like so: - Try a method. If it fails, set a global error and return. - If the global error is set, try a different method. Prior to OpenSSL 1.0.0, these were tested in the order: EC_GFp_nist_method -> EC_GFp_mont_method. The secp256k1 curve fails the ec_GFp_nist_group_set_curve test and sets the global error. That error is then checked for failure, and EC_GFp_mont_method is tried (and succeeds). Obviously that global error usage is dangerous, especially since it happens for _each_ transaction verification in libbitcoinconsensus. In a multi-threaded environment, a crash is guaranteed within a few seconds. However, OpenSSL 1.0.1 reversed the order, trying EC_GFp_mont_method first, so that the global error doesn't end up being used: https://github.com/openssl/openssl/commit/17674bfdf75bffa4e225f8328b9d42cb7= 4504005 This was backported from master back to 1.0.1, but not to 1.0.0 or 0.9.8. So that change (accidentally) "solved" the problem. As you can see, it's still possible to hit the reversed order in the !defined(OPENSSL_BN_ASM_MONT) case. That's easily tested by building OpenSSL with the -no-asm config option. It's probably also the case for obscure architectures and OSs, but I haven't looked deeply into that. In that case, it's reasonable to assume that this crash would likely occur on such platforms. Also, OSX, even the latest version (10.10 as of now), still ships with OpenSSL 0.9.8. Which is how Tamas ran into it. Since Bitcoin Core and libbitcoinconsensus are switching away from OpenSSL for verification in the near future, I don't think this is much of an issue. Especially since the problem manifests as a controlled assertion failure/abort. However, I've prepared a patch for anyone who may run into the issue in the short-term: https://github.com/theuni/bitcoin/commit/adf0a691ee1c2f02e26828f976cfe5b788= 96b507 I'll open a pull-request for Bitcoin Core to discuss whether it's worth merging or not. Regards, Cory On Fri, Aug 14, 2015 at 5:10 PM, Cory Fields wrote: > Ugh, what an unfortunate oversight! > > The good news is that this issue should be solved in future versions > when we switch to the new libsecp256k1 lib for validation. > > For now, I've thrown together a quick hack to allow a user-specifiable > callback for libbitcoinconsensus. I think it's not worth messing with > the official API since it will be fixed soon, but rather hacked in as > a temporary work-around as needed. It _should_ be documented as an > issue with the current version, though. > > Please see here for a work-around to try: > https://github.com/theuni/bitcoin/commits/openssl-consensus-threads > Unfortunately it's not pretty, but it works fine here. Note that you > should give this some _serious_ testing before deploying in any real > way. It should mimic the way we do it in Core, though. > > That's on top of current master, but it should be trivial to apply to > release tags. > > Please let me know how it works out. > > Regards, > Cory > > On Fri, Aug 14, 2015 at 12:37 PM, Tamas Blummer via bitcoin-dev > wrote: >> We integrated libconsensus into bits of proof. It works well, in-line fo= r all test cases with our Java engine and is about 50% faster on a single t= hread. >> >> The performance advantage unfortunatelly reverses if libconsensus is exe= cuted on several threads simultaneously as we do with the Java engine, sinc= e an error: >> >> Assertion failed: (pkey !=3D NULL), function CECKey, file ecwrap= per.cpp, line 96. >> >> arises under that stress. >> >> I guess that the cause is that thread callbacks as advised for OpenSSL o= n https://www.openssl.org/docs/crypto/threads.html are not registered. >> Registering those however would require access to OpenSSL functions, not= exported from the lib. >> >> I=E2=80=99d be thankful for a pointer to a workaround. >> >> Tamas Blummer >> >> _______________________________________________ >> bitcoin-dev mailing list >> bitcoin-dev@lists.linuxfoundation.org >> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev >>