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Date: Mon, 31 Dec 2018 20:16:42 +0100
From: Vincent Lefevre <vincent@...c17.net>
To: Jeffrey Walton <noloader@...il.com>
Cc: oss-security@...ts.openwall.com, gmp-bugs@...lib.org
Subject: Re: Asserts considered harmful (or GMP spills its sensitive
 information)

On 2018-12-31 13:03:27 -0500, Jeffrey Walton wrote:
> The GMP library uses asserts to crash a program at runtime when
> presented with data it did not expect. The library also ignores user
> requests to remove asserts using Posix's -DNDEBUG. Posix asserts are a
> deugging aide intended for developement, and using them in production
> software ranges from questionable to insecure.

That's much better than letting the program run erratically, with
possible memory corruption and/or sensitive information leakage
to unauthorized users. You'd better fix bugs in your program.

> Many programs can safely use assert to crash a program at runtime.
> However, the prequisite is, the program cannot handle sensitive
> information like user passwords, user keys or sensitive documents.
> 
> High integrity software, like GMP and Nettle, cannot safely use an
> assert to crash a program. To understand why the data flow must be
> examined. First, when an assert fires, a SIGABRT is eventually sent to
> the program on Unix and Linux
> (http://pubs.opengroup.org/onlinepubs/009695399/functions/assert.html).
> 
> Second, the SIGABRT terminates the process and can write a core file.

That's the default behavior, but you can trap SIGABRT if you want.
Of course, there is no guarantee because the memory may already be
in an inconsistent state.

> This is the first point of unwanted data egress. Sensitive information
> like user passwords and keys can be written to the filesystem
> unprotected.

This can occur with any program, even not using asserts, e.g. due to
a segmentation fault (which may happen as a consequence of not using
asserts, with possibly worse consequences).

If you don't want a core file, then you can instruct the kernel not
to write a core file. See getrlimit.

> Third, the dump is sometimes sent to an error reporting service like
> Apple Crash Report, Android Crash Report, Ubuntu Apport, and Windows
> Error Reporting. This is the second point of unwanted data egress.
> Sensitive information can be sent to the error reporting service. The
> platform provider like Apple, Google, Microsoft and Ubuntu gain access
> to the sensitive information, in addition to the developer.

If you don't like them, do not use these services. Not using asserts
can also yield a crash, which will have the same consequences.

-- 
Vincent Lefèvre <vincent@...c17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)

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