Re: [Announcement] "Exec Shield", new Linux security feature

[Jesse Pollard]

On Sunday 04 May 2003 02:03, Calin A. Culianu wrote:
> On Sat, 3 May 2003 [e-mail] wrote:
> > On Sat, 03 May 2003 13:19:52 -0000, [e-mail]  said:
> > > An interesting question arises: is the number of useful interpreter
> > > functions (system, popen, exec*) sufficiently low that they could be
> > > removed from libc.so entirely and only staticly linked, so processes
> > > that didn't use them wouldn't even have them in their address space,
> > > and ones that did would have them at less predictible addresses?
> > >
> > > Right now, I'm thinking only of functions that end up calling execve();
> > > are there any other sufficiently powerful interpreters hiding in common
> > > system libraries?  regexec()?
> >
> > This does absolutely nothing to stop an exploit from providing its own
> > inline version of execve().  There's nothing in libc that a process can't
> > do itself, inline.
> >
> > A better bet is using an LSM module that prohibits exec() calls from any
> > unauthorized combinations of running program/user/etc.
>
> Is that practical?  I can see how with some daemons it would definitely be
> useful to prohibit exec calls (maybe things like BIND don't need to exec
> anything).. but some daemons do need to exec.  An SMTPD may need to exec()
> some helper processes (postfix for instance has a whole slew of helper
> programs it uses).. and things like sshd need to exec a shell, etc..

What you actually would do is have the LSM module turn exec into MAY_EXEC_IF
where the IF is done to match security context information to the binary being
execed. ssh shouldn't exec (well most of the time anyway) a root shell. 
Instead it should switch security context, (including a security id and 
uid's/gids) first, then exec if the new security context is valid for what is 
being execed. (and the new securid is not necessarily selected from kernel 
mode - it may depend on network connection + user id/gids requested + 
security level permitted)

More complicated, but the full security context being switched to cannot be
selected by the application itself.

> It's still a good idea though, since some daemons don't need to exec,
> ever.  I guess this is one extra layer of protection.  As Ingo said in his
> announcement, the more layers of protection you have, the better.. and the
> more difficult a cracker's job is.

true.