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

[Yu-cheng Yu]

On Wed, 2018-07-11 at 11:12 +0200, Peter Zijlstra wrote:
> 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,
> > > Â	error = -ENOMEM;
> > > Â	if (!vma)
> > > Â		goto out;
> > > +
> > > +	/*
> > > +	Â* Do not allow changing shadow stack memory.
> > > +	Â*/
> > > +	if (vma->vm_flags & VM_SHSTK) {
> > > +		error = -EINVAL;
> > > +		goto out;
> > > +	}
> > > +
> > I think this is a _bit_ draconian.ÂÂWhy shouldn't we be able to use
> > protection keys with a shadow stack?ÂÂOr, 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?

Agree. ÂI will remove the patch.