Re: [patch] remove MNT_NOEXEC check for PROT_EXEC mmaps

[Kyle Moffett <mrmacman_g4@mac.com>]

On Sep 24, 2006, at 15:14:02, David Wagner wrote:
> Stas Sergeev  wrote:
> > Ulrich Drepper wrote:
> > > The consensus has been to add the same checks to mprotect.  They  
> > > were
> > > not left out intentionally.
> >
> > But how about the anonymous mmap with PROT_EXEC set?
>
> I'm curious about this, too.  ld-linux.so is a purely unprivileged
> program.  It isn't setuid root.  Can you write a variant of ld- 
> linux.so
> that reads an executable into memory off of a partition mounted  
> noexec and
> then begins executing that code?  (perhaps by using anonymous mmap  
> with
> PROT_EXEC or some other mechanism) It sure seems like the answer would
> be yes.  If so, I'm having a hard time understanding what guarantees
> noexec gives you.  Isn't the noexec flag just a speedbump that raises
> the bar a little but doesn't really prevent anything?

I seem to recall somewhere that it was possible to prevent anonymous  
memory from being mapped PROT_EXEC during or after being mapped  
PROT_WRITE; and that in fact your average SELinux-enabled system had  
such protections for everything but the Java binary and a few other  
odd programs.  If you can't ever execute any data blobs except those  
that came directly from a properly-secured SELinux-enabled filesystem  
it makes exploiting a server significantly harder.

Cheers,
Kyle Moffett

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