Re: [RFC PATCH v2 15/27] mm/mprotect: Prevent mprotect from changing shadow stack

[Peter Zijlstra]

On Tue, Jul 10, 2018 at 04:10:08PM -0700, Dave Hansen wrote:
> On 07/10/2018 03:26 PM, Yu-cheng Yu wrote:
> > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> 
> This still needs a changelog, even if you think it's simple.
> > --- a/mm/mprotect.c
> > +++ b/mm/mprotect.c
> > @@ -446,6 +446,15 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
> >  	error = -ENOMEM;
> >  	if (!vma)
> >  		goto out;
> > +
> > +	/*
> > +	 * Do not allow changing shadow stack memory.
> > +	 */
> > +	if (vma->vm_flags & VM_SHSTK) {
> > +		error = -EINVAL;
> > +		goto out;
> > +	}
> > +
> 
> I think this is a _bit_ draconian.  Why shouldn't we be able to use
> protection keys with a shadow stack?  Or, set it to PROT_NONE?

Right, and then there's also madvise() and some of the other accessors.

Why do we need to disallow this? AFAICT the worst that can happen is
that a process wrecks itself, so what?