Re: [tip:x86/urgent] x86-64, modify_ldt: Ban 16-bit segments on 64-bit kernels

[Ingo Molnar <mingo@kernel.org>]

* H. Peter Anvin <hpa@linux.intel.com> wrote:

> On 04/11/2014 11:41 AM, Linus Torvalds wrote:
> > 
> > Ok, so you actually do this on x86-64, and it currently works? For
> > some reason I thought that 16-bit windows apps already didn't work.
> > 
> 
> Some will work, because not all 16-bit software care about the upper
> half of ESP getting randomly corrupted.
> 
> That is the "functionality bit" of the problem.  The other bit, of
> course, is that that random corruption is the address of the kernel stack.
> 
> > Because if we have working users of this, then I don't think we can do
> > the "we don't support 16-bit segments", or at least we need to make it
> > runtime configurable.
> 
> I'll let you pick what the policy should be here.  I personally 
> think that we have to be able to draw a line somewhere sometimes 
> (Microsoft themselves haven't supported running 16-bit binaries for 
> several Windows generations now), but it is your policy, not mine.

I think the mmap_min_addr model works pretty well:

 - it defaults to secure

 - allow a security policy to grant an exception to a known package, 
   built by the distro

 - end user can also grant an exception

This essentially punts any 'makes the system less secure' exceptions 
to the distro and the end user.

Thanks,

	Ingo