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diff --git a/doc/audit/response-202012.tex b/doc/audit/response-202012.tex new file mode 100644 index 000000000..97d8a0ce9 --- /dev/null +++ b/doc/audit/response-202012.tex @@ -0,0 +1,272 @@ +\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 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 exchange process is compromised, we do not sign keys +into existence that did 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 +into the database, in particular information about offline signatures +(including key revocations) and wire fees. This simplifies the deployment and +the interaction with the offline key. 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} |