[PLUG] Trying to learn low level Linux programming for cs201...

[Jason Martin]

On Fri, Jan 9, 2009 at 9:04 PM, Tim <tim-pdxlug at sentinelchicken.org> wrote:
> Hi Fred,
>
>> Could you elaborate, please, or point to such?
>> Regards
>> Fred James
>>
>> PS:  Sorry - I should narrow the scope a bit - I meant the "unsafe" part
>> - thank you.
>
> Sure.  I guess a good file-related example would be fscanf().  If you
> try to read in strings or other variable length arguments without
> putting an explicit length limiter in there, you're essentially
> guaranteeing yourself a buffer overflow.
>
> Another example might be fprintf() which, while typically safer, is
> dangerous to include user-supplied values in the format string itself.
> For instance, doing something like:
>  fprintf(FILE_pointer, user_supplied_string);
>
> Can allow for arbitrary code execution amongst other things.  Of course
> this issue isn't limited to fprintf()...  All of the printf() family
> exhibit this.
>
> Finally, consider popen() which returns a FILE object pointing to a
> stream.  It executes '/bin/sh -c ...' with your input string.  If any
> user-supplied values slip into that string, you're in big trouble.  Even
> if this wasn't an issue, popen() and it's friend system() have a
> horrible interface in that they don't return good information about
> whether or not your external process failed.  If you want to pipe data
> to your sub process, forget trying to get anything back on it's stdout.
>
> Other examples of poor wrapper function design (temporary file creation,
> many bad string functions) move further away from FILE-specific stuff,
> but one often finds that using just some of these higher-level
> interfaces is hard.  You often have to either just write it yourself at
> the low level or give in and use all of libc's bad wrappers.
>
> In the end, these interfaces can be used safely of course, if you know
> what you're doing.  A better interface can of course be used unsafely if
> you try hard enough.  To distinguish between the two, you should ask the
> question:
>  "Is the most obvious way to do a thing also the safe way of doing it?"

The best resource on this topic I've seen is David Wheeler's free book
"Secure Programming for Linux and Unix HOWTO"
(http://www.dwheeler.com/secure-programs/).

Cheers,
Jason