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diff --git a/doc/audit/response-202012.tex b/doc/audit/response-202012.tex deleted file mode 100644 index 90bd59544..000000000 --- a/doc/audit/response-202012.tex +++ /dev/null @@ -1,273 +0,0 @@ -\documentclass[11pt]{article} -\oddsidemargin=0in \evensidemargin=0in -\textwidth=6.2in \textheight=8.7in -%\topmargin=-0.2in - -\usepackage[ansinew]{inputenc} -\usepackage{makeidx,amsmath,amssymb,exscale,multicol,epsfig,graphics} - -\begin{document} -\pagestyle{headings} -\title{Final response to the \\ GNU Taler security audit in Q2/Q3 2020} -\author{Christian Grothoff \and Florian Dold} - -\maketitle - -\section{Abstract} - -This is the response to the source code audit report CodeBlau -created for GNU Taler in Q2/Q3 2020. - -\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. However, the proposed fix -of making the primary key include the denomination would create other problems, -such as the exchange sometimes not having the denomination key (link, refund) -and the code in various places relying on the assumption of the coin's -public key being unique. Furthermore, allowing coin key re-use may validate -a terrible practice. We thus decided it is better to ``fail early'', and -modified the code to check that the coin public key is ``unique'' during -deposit, refresh and recoup and ensured that the exchange returns a proof -of non-uniqueness in case of a violation. The test suite was extended to -cover the corner case. - -{\bf Update:} We have now also addressed the (``soft'') exchange online -signing key revocation issue (\#6161) reported originally by CodeBlau. -The auditor now checks for key revocations before recording deposit -confirmations. The impact is very minor, as this will merely prevent -an adversary controlling an exchange online signing key from submitting -false claims to the auditor. - - - -\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 to implement. However, our choice of C -is based on the advantages that make it superior to contemporary languages for -our use case: relatively low complexity of the language (compared to C++); -availability of mature compilers, static and dynamic analysis tools; -predictable performance; access to stable and battle-tested libraries; and -future-proofness due to portability to older systems as well as new platforms. - -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 uniformly 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 pruning} - -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. - -{\bf Update:} We have now implemented full privilege separation for access to the online -cryptographic signing keys. Details about the design are documented in the -section ``Exchange crypto helper design'' at \url{https://docs.taler.net/} of -Chapter 12. - -{\bf Update:} In doing so, we also added a new type of signing key, the -``security module'' signing key. This is used by the newly separated ``security -module`` processes to sign the public keys that they guard the private keys -for. The security module signatures are verified by the new -``taler-exchange-offline`` tool to ensure that even if the {\tt -taler-exchange-httpd} process is compromised, the offline signature tool would -refuse to sign new public keys that do not originate from the security -module(s). The security module public keys can be given in the configuration, -or are learned TOFU-style. - - -\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. - -{\bf Update:} On the exchange side, we have now moved additional information -from the file system into the database, in particular information about offline signatures -(including key revocations) and wire fees. This simplifies the deployment and -the interaction with offline key signing mechanism. The remaining disk accesses are for -quite fundamental configuration data (which ports to bind to, configuration to -access the database, etc.), and of course the program logic itself. - - -\subsection{Avoid dlopen} - -Taler actually uses {\tt ltdlopen()} from GNU libtool, which provides -compiler flags to convert 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} - -CodeBlau's suggestion to use an append-only transaction logging service in -addition to the PostgreSQL database is a reasonable suggestion for a -production-grade deployment of GNU Taler, as it would allow partial disaster -recovery even in the presence of an attacker that has gained write access to -the exchange's database. - -We are currently still investigating whether the transaction logging should be -implemented directly by the exchange service, or via the database's extensible -replication mechanism. Any implementation of such an append-only logging -mechanism must be carefully designed to ensure it does not negatively impact -the exchange's availability and does not interfere with serializability of -database transactions. As such we believe that transaction logging can only be -provided on a best-effort basis. Fortunately, even a best-effort append-only -transaction log would serve to limit the financial damage incurred by the -exchange in an active database compromise scenario. - -\end{document} |