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Subject: [Bitcoin-development] Full disclosure on the Bitcoin-Qt on Windows
problem
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Reposting from my tech blog:
http://gavintech.blogspot.com/2012/03/full-disclosure-bitcoin-qt-on-windows.html
----------------
A major bug in Bitcoin-Qt on Windows went unnoticed for over four
months. The bug was in the bitcoin-qt.pro file, in these two lines of
code:
windows:LIBS += -lws2_32 -lshlwapi
windows:DEFINES += WIN32
See the bug?
I don't-- I can't see it, because the problem was Bitcoin-Qt on
Windows should have been linked with the "mingw" multithreading
library and compiled with the -D_MT to turn on support for
multithread-safe C++ exception handling in the "mingw" library (you
can see the 3-line fix here).
There might be developers reading this who have had experience writing
multithreaded mingw-based C++ applications who will say "Duh! You guys
are idiots, everybody knows that!"
If that's you and you're interested in Bitcoin: we need people like
you to help out! Review the code, review the build process, let us
know when we're being idiots. There seem to be a lot fewer Windows
developers willing to help out with Bitcoin than Linux or Mac
developers, which is unfortunate since a majority of our downloads are
Windows users.
Non-mingw-C++-multithreaded developers are reading this and thinking
"the bug was THERE? Not in some #ifdef WINDOWS C++ code or some
slightly-out-of-date library that you were linking against?"
The other reason this bug went unnoticed for so long is because it
almost never matters. Bitcoin-Qt doesn't throw a lot of exceptions
normally, and most of the time the single-threaded-application
exception support in mingw works just fine. I'm still not sure exactly
what triggers the bug-- probably something like two threads throwing
exceptions at the same time, or one thread being interrupted while in
a try{} block and then resuming later and crashing because the
exception handling code is in a different state.
Matt Corallo deserves a lot of credit for being persistent and finding
the bug when investigating hard-to-reproduce reports of Bitcoin-Qt
crashing sometimes when run with the -server flag and handling
JSON-RPC requests. He was never able to get the code to crash when
running outside of the debugger, which is typical of bugs related to
multithreading. But he did see crashes not involving RPC requests, and
that is why we consider this a critical issue and are telling
everybody affected to upgrade immediately.
It it exploitable? Could an attacker craft bitcoin protocol messages
that triggered the bug and compromised Windows computers? Has it
already been exploited?
We don't know. We think it would be extremely difficult to craft a
usable exploit, partly because it is hard to even trigger the bug
reliably but also because Bitcoin-Qt has some security-in-depth to try
to prevent this type of bug from becoming exploitable.
If we're very careful and extremely lucky, this will be the last time
we have to tell users to shutdown and upgrade immediately because of
some potential vulnerability. But there is no such thing as perfect
security, and I expect that there will be another subtle bug either in
Bitcoin or in one of the many libraries we link against that becomes a
potential security issue.
That doesn't mean we give up and go back to paying each other with
cowrie shells; it means we assume that devices get compromised and
design around that assumption. I think that is a lesson that the
entire software industry needs to learn better.
--
--
Gavin Andresen
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