response to CB

doc/audit/response-202005.tex

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+\usepackage[ansinew]{inputenc}
+\usepackage{makeidx,amsmath,amssymb,exscale,multicol,epsfig,graphics}
+
+\begin{document}
+\pagestyle{headings}
+\title{Preliminary response to the \\ GNU Taler security audit in Q2/Q3 2020}
+\author{Christian Grothoff \and Florian Dold}
+
+\maketitle
+
+\section{Abstract}
+
+This is the preliminary response to the source code audit report CodeBlau
+created for GNU Taler in Q2/Q3 2020.  A final response with more details is
+expected later this year.
+
+\section{Management Summary}
+
+We thank CodeBlau for their detailed report and thorough analysis. We are
+particularly impressed that they reported issues against components that were
+not even in-scope, and also that they found an {\em interesting} new corner
+case we had not previously considered. Finally, we also find several of their
+architectural recommendations to strengthen security to be worthwhile, and
+while some were already on our long-term roadmap, we will reprioritize our
+roadmap given their recommendations.
+
+Given our extensive discussions with CodeBlau, we also have the impression
+that they really took the time to understand the system, and look forward
+to working with CodeBlau as a competent auditor for GNU Taler in the future.
+
+\section{Issues in the exchange}
+
+We agree with the issues CodeBlau discovered and both parties believe that
+they have all been addressed.
+
+\section{Issues in the auditor}
+
+We appreciate CodeBlau's extensive list of checks the Taler auditor performs,
+which was previously not documented adequately by us. We agree that the
+auditor still needs more comprehensive documentation.
+
+As for issue \#6416, we agree with the analysis and the proposed fix, even if
+the implications are not fully clear. It has not yet been implemented as we
+want to carefully review all of the SQL statements implicated in the
+resolution and ensure we fully understand the implications.
+
+\section{Issues in GNUnet}
+
+We agree with the issues CodeBlau discovered and both parties believe that
+they have all been addressed.
+
+\section{General remarks on the code}
+
+We understand that writing the code in another programming language may make
+certain checks for the auditor less work. However, other programming languages
+also have disadvantages (from the complexity of the languages to the
+complexity of the compilers to tool support).  We believe creating a parallel
+implementation in other languages would provide advantages, especially with
+respect to avoiding ``the implementation is the specification''-style issues.
+However, given limited resources will not make this a priority.
+
+We disagree that all modern software development has embraced the idea that
+memory errors are to be handled in ways other than terminating or restarting
+the process. Many programming languages (Erlang, Java) hardly offer any other
+means of handling out-of-memory situations than to terminate the process. We
+also insist that Taler {\em does} handle out-of-memory as it does have code
+that terminates the process (we do {\em not} simply ignore the return value
+from {\tt malloc()} or other allocation functions!). We simply consider that
+terminating the process (which is run by a hypervisor that will restart the
+service) is the correct way to handle out-of-memory situations.  We also have
+limits in place that should prevent attackers from causing large amounts of
+memory to be consumed, and also have code to automatically preemptively
+restart the process to guard against memory exhaustion from memory
+fragmentation.  Finally, a common problem with abrupt termination may be
+corrupted files.  However, the code mostly only reads from files and limits
+writing to the Postgres database. Hence, there is no possibility of corrupt
+files being left behind even in the case of abnormal termination.
+
+
+\section{More specs and documentation code}
+
+We agree with the recommendation that the documentation should be improved,
+and will try to improve it along the lines recommended by CodeBlau.
+
+\section{Protocol change: API for uniformuly distributed seeds}
+
+We agree with the suggestion, have made the necessary changes, and both
+parties believe that the suggestion has been implemented.
+
+\section{Reduce code complexity}
+
+\subsection{Reduce global variables}
+
+While we do not disagree with the general goal to have few global variables,
+we also believe that there are cases where global variables make sense.
+
+We have already tried to minimize the scope of variables. The remaining few
+global variables are largely ``read-only'' configuration data. The report does
+not point out specific instances that would be particularly beneficial to
+eliminate. As we continue to work on the code, we will of course evaluate
+whether the removal of a particular global variable would make the code
+cleaner.
+
+Also, we want to point out that all global variables we introduce
+in the exchange are indicated with a prefix {\tt TEH\_} in the code, so they
+are easy to identify as such.
+
+\subsection{Callbacks, type p(r)unning}
+
+We understand that higher order functions in C can be confusing, but this
+is also a common pattern to enable code re-use and asynchronous execution
+which is essential for network applications. We do not believe that we
+use callbacks {\em excessively}.  Rewriting the code in another language
+may indeed make this part easier to understand, alas would have other
+disadvantages as pointed out previously.
+
+\subsection{Initializing structs with memset}
+
+Using {\tt memset()} first prevents compiler (or valgrind) warnings about
+using uninitialized memory, possibly hiding bugs. We also do use struct
+initialization in many cases.
+
+The GNUnet-wrappers are generally designed to be ``safer'' or ``stricter''
+variants of the corresponding libc functions, and not merely ``the same''.
+Hence we do not believe that renaming {\tt GNUNET\_malloc} is indicated.
+
+The argument that {\tt memset()}ing first makes the code inherently more
+obvious also seems fallacious, as it would commonly result in dead stores,
+which can confuse developers and produce false-positive warnings from static
+analysis tools.
+
+\subsection{NULL pointer handling}
+
+The problem with the ``goto fail'' style error handling is that it rarely
+results in specific error handling where diagnostics are created that are
+specific to the error. Using this style of programming encourages developers
+to create simplistic error handling, which can result in inappropriate error
+handling logic and also makes it harder to attribute errors to the specific
+cause.
+
+However, we have no prohibition on using this style of error handling either:
+if it is appropriate, develpers should make a case-by-case decision as to how
+to best handle a specific error.
+
+We have made some first changes to how {\tt GNUNET\_free()} works in response
+to the report, and will discuss further changes with the GNUnet development
+team.
+
+\subsection{Hidden security assumptions}
+
+We disagree that the assumptions stated are ``hidden'', as (1) the Taler code
+has its own checks to warrant that the requirements of the {\tt
+  GNUNET\_malloc()} API are satisfied (so enforcement is not limited to the
+abstraction layer), and (2) the maximum allocation size limit is quite clearly
+specified in the GNUnet documentation.  Also, the GNUnet-functions are not
+merely an abstraction layer for portability, but they provided extended
+semantics that we rely upon. So it is not like it is possible to swap this
+layer and expect anything to continue to work.
+
+When we use the libjansson library, it is understood that it does not use
+the GNUnet operations, and the code is careful about this distinction.
+
+\subsection{Get rid of boolean function arguments}
+
+We agree that this can make the code more readable, and have in some places
+already changed the code in this way.
+
+\section{Structural Recommendation}
+
+\subsection{Least privilege}
+
+It is wrong to say that GNU Taler has ``no work done'' on privilege separation.
+For example, the {\tt taler-exchange-dbinit} tool is the only tool that requires
+CREATE, ALTER and DROP rights on database tables, thus enusring that the ``main''
+process does not need these rights.
+
+We also already had the {\tt taler-exchange-keyup} tool responsible for
+initializing keys. In response to the audit, we already changed the GNUnet API
+to make sure that tools do not create keys as a side-effect of trying to read
+non-existent key files.
+
+We agree with the recommendation on further privilege separation for access
+to cryptographic keys, and intend to implement this in the near future.
+
+\subsection{File system access}
+
+The auditor helpers actually only read from the file system, only the LaTeX
+invocation to compile the final report to PDF inherently needs write
+access. We do not predict that we will retool LaTeX.  Also, the file system
+access is completely uncritical, as the auditor by design runs on a system
+that is separate from the production exchange system.
+
+Because that system will not have {\em any} crypto keys (not even the one of
+the auditor!), CodeBlau is wrong to assume that reading from or writing to the
+file system represents a security threat.
+
+We have started to better document the operational requirements on running the
+auditor.
+
+\subsection{Avoid dlopen}
+
+Taler actually uses {\tt ltdlopen()} from GNU libtool, which provides
+compiler flags to conver the dynamic linkage into static linkage.  For
+development, dynamic linkage has many advantages.
+
+We plan to test and document how to build GNU Taler with only static
+linkage, and will recommend this style of deployment for the Taler
+exchange for production.
+
+\subsection{Reduce reliance on PostgreSQL}
+
+Using other mechanisms beyond the database as a ``Plan B'' would create
+serious availability and cost concerns, as now either mechanism may create
+serialization issues and require database rollbacks.  Also, any such
+append-only logging mechanism would itself have a similar complexity as the
+primary database.  Thus, we do not believe that the drastic complexity
+increase from the combined solution represents a valid security trade-off.
+
+\end{document}