Re: chmod 111

[Nick Warne <nick@linicks.net>]

On Friday 17 March 2006 20:11, Linus Torvalds wrote:
> On Fri, 17 Mar 2006, Phillip Susi wrote:
> > Linus Torvalds wrote:
> > > In particular, it's fairly easy to create a shared library that
> > > replaces a system library (LD_LIBRARY_PATH) and then just dumps out the
> > > binary image.
> >
> > What prevents you from injecting a shared library and manipulating a suid
> > executable?
>
> Suid executables do not accept LD_LIBRARY_PATH.
>
> > Does the environment get cleared when you exec a suid program?
>
> No, but the startup environment can tell if it's suid, and refuse to load
> anything but the fixed environment, so it's _effectively_ cleared for a
> subset of the environment strings.
>
> Suid executables also get some special handling by the kernel (it will,
> for example, refuse to dump core for them - another way to get readable
> information from an executable). Similarly, you can't ptrace a suid
> executable (a _third_ way of getting information from a execute-only
> binary in general).
>
> So suid executables are a separate issue: they actually _do_ have security
> protection (regardless of whether they are marked "readable" or not).
>
> And finally don't get me wrong - you _can_ build up security around the
> executable bits, but it has to be a lot more involved than just assuming
> that being unreadable means that nobody can see what a binary does. So for
> example, you _can_ create a system where you only have a certain subset of
> binaries that will be run (no debuggers), and where user-supplied binaries
> simply won't execute (mounting any user-writable area no-exec, and make
> sure that none of the executable loaders like /lib/ld.so will load a
> non-exec image).
>
> But in general, I'd say that is only applicable in some embedded
> environments (you could have a special chroot'ed jail environment where it
> could be very hard to read the binaries that you expose in the jail
> environment, for example). It's not useful in something that gives shell
> access and allows the user to create his own executable program files.

Well, I thought after making my post and read the first few replies that I 
made 'dork of the week' post.

But now I am glad I did post.  I have learnt a lot.

Thanks everybody.  A real insight to the sub-system.

Nick 


-- 
"Person who say it cannot be done should not interrupt person doing it."
-Chinese Proverb