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Re: [PATCH] Secure programming additions to HACKING

From: Karl Fogel <kfogel_at_newton.ch.collab.net>
Date: 2002-04-23 00:50:08 CEST

Before applying this, it would be nice to identify some security
boundaries already present in Subversion, and note their constraints
in the code. I think that would give us a better idea of how to write
security guidelines in HACKING.

It would be best to avoid saying things that everyone already knows
(for loose values of "everyone", of course), such as don't use strcat
and strcpy, don't accept arbitrary length input into fixed-length
arrays, etc. :-) Text that simply repeats C orthodoxy dilutes the
effect of the HACKING file, because a reader may assume that since she
already knew everything she just read in the security section, she
probably doesn't need to read the rest of the file either. (I'm
exaggerating the dilution effect, of course; it's really a continuum.)

Let's try to keep the HACKING file as Subversion-specific as possible,
so that it remains an effective resource.

(This isn't to say that the spirit of Alex's patch is bad, just that
much of the new material is generic C advice, or repeats things
already said elsewhere in HACKING. The stuff about enforcing security
boundaries is great, *if* we can make it specific to Subversion.)

-Karl

Alex Holst <a@area51.dk> writes:
> Here's a tiny patch so that I may understand how you see things. Comments?
>
>
> Sprinkle some comments about secure coding over HACKING.
>
> * (Secure coding guidelines): new section.
> * (Document everything): describe how to document security boundries as
> source code comments.
> * Insert a missing page break.
>
> Index: ./HACKING
> ===================================================================
> --- ./HACKING
> +++ ./.svn/tmp/HACKING.54676.00001.tmp Mon Apr 22 23:19:41 2002
> @@ -16,6 +16,7 @@
> * What to read
> * Directory layout
> * Coding style
> + * Secure coding guidelines
> * Document everything
> * Using page breaks
> * Other conventions
> @@ -140,7 +141,16 @@
> * hashes and arrays: apr_hash.h, apr_tables.h
>
>
> +Subversion also tries to deliver reliable and secure software. This can
> +only be achieved by educating the developers on secure programming in
> +the C programming language. Please see 'notes/assurance.txt' for the
> +full rationale behind this. Specifically, you should make it a point to
> +carefully read David Wheeler's Secure Programming (as mentioned in
> +'notes/assurance.txt'). If at any point you have questions about the
> +security implications of a change, you are urged to ask for review on
> +the developer mailinglist.
>
> +
> Directory layout
> ================
>
> @@ -240,6 +250,53 @@
>
>
>
> +Secure coding guidelines
> +========================
> +
> +Just like almost any other programming language, C has undesireable
> +features which enables an attacker to make your program fail in
> +predictable ways, often to the attacker's benefit. The goal of these
> +guidelines is to make you aware of the pitfalls of C as they apply to
> +the Subversion project. You are encouraged to keep these pitfalls in
> +mind when reviewing code of your peers, as even the most skilled and
> +paranoid programmers make occasional mistakes.
> +
> +Input validation is the act of defining legal input and rejecting
> +everything else. The code must perform input validation on all untrusted
> +input.
> +
> +
> +Security boundaries
> +
> +A security boundary in the Subversion server code must be identified as
> +such as this enables auditors to quickly determine the quality of the
> +boundary. Security boundaries exist where the running code has access
> +to information the user does not or where the code runs with priviliges
> +above those of the user making the request. Typical examples of such is
> +code that does access control or an application with the SUID bit set.
> +
> +Functions which make calls to a security boundary must include
> +validation checks of the arguments passed. Functions which themselves
> +are security boundaries should audit the input received and alarm when
> +invoked with improper values.
> +
> +
> +String operations
> +
> +Use sane string functions. When operating on strings, you must always
> +use the functions provided by APR. You should never use C library calls
> +such as strcat or strcpy as they do not provide bounds checking, and may
> +also blow up in a number of other circumstances.
> +
> +
> +Password storage
> +
> +Help users keep their passwords secret: When the client reads or writes
> +password locally, it should ensure that the file is mode 0600. If the
> +file is readable by other users, the client should exit with a message
> +that tells the user to change the filemode due to the risk of exposure.
> +
> +
> Document everything
> ===================
>
> @@ -250,6 +307,12 @@
> which the function could return an error. Put the parameter names in
> upper case in the doc string, even when they are not upper case in the
> actual declaration, so that they stand out to human readers.
> +
> +Security boundaries should be identified in a comment prefixed by the
> +word SECURITY in uppercase, followed by a description of the boundary,
> +including ranges of legal values for each parameter. Specifications of
> +legal input could be 'digits only, a maximum of 10', 'a-zA-Z0-9, maximum
> +30 characters', and so on.
>
> For public or semi-public API functions, the doc string should go
> above the function in the .h file where it is declared; otherwise, it
>
> --
> I prefer the dark of the night, after midnight and before four-thirty,
> when it's more bare, more hollow. http://a.area51.dk/
>
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Received on Tue Apr 23 00:47:44 2002

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