bug#72245: [PATCH] Fix integer overflow when reading XPM

[Stefan Kangas <stefankangas@gmail.com>]

Po Lu <luangruo@yahoo.com> writes:

> I'm saying that there is nothing to be done.  This change is needless,
> and the report should be closed, whatever opinions the security theater
> might hold on the matter.

I wasn't the one that started a subthread about security.  You did.

The primary consideration here is correctness.  Undefined behaviour is
generally undesirable, and is a source of both bugs and security issues
in the wild.  This is not "security theater", but a fact.  No amount of
handwaving or throwing expletives around will make it go away.

That said, since you are asking, we are indeed discussing security
sensitive code, that is executed without prompting, for example, when
users receive emails or browse the web.  We are also discussing image
processing, an area that is notorious for the bugs and security issues
that tend to lurk in its many complexities.  On the CWE-190 page that I
linked, there are several examples of integer overflow in image
processing that has lead to very real exploits.  This is not some
academic issue.

Whether or not anyone has demonstrated that Emacs can be exploited using
this vector frankly misses the point.  Let's start with making Emacs
behave correctly and predictably in the face of invalid input.  This
really is the bare minimum.  Then we can discuss whether or not we have
more work to do, security implications, and all the rest of it.

XPM being a relatively simple format, I'm sure that this code can be
fully audited.  I invite you to do so, and I'm hoping that this will
reveal that your faith in this code is well-founded.  Meanwhile, I
reported an unrelated crash in XPM image processing in Bug#72255.

Since we don't have an alternative patch, I will install the one I
proposed in the next couple of days.  Thanks.