Re: [RFC PATCH v2 13/27] mm: Handle shadow stack page fault

[Yu-cheng Yu <yu-cheng.yu@intel.com>]

On Wed, 2018-07-11 at 11:06 +0200, Peter Zijlstra wrote:
> On Tue, Jul 10, 2018 at 04:06:25PM -0700, Dave Hansen wrote:
> > 
> > On 07/10/2018 03:26 PM, Yu-cheng Yu wrote:
> > > 
> > > +	if (is_shstk_mapping(vma->vm_flags))
> > > +		entry = pte_mkdirty_shstk(entry);
> > > +	else
> > > +		entry = pte_mkdirty(entry);
> > > +
> > > +	entry = maybe_mkwrite(entry, vma);
> > >  	if (ptep_set_access_flags(vma, vmf->address, vmf->pte,
> > > entry, 1))
> > >  		update_mmu_cache(vma, vmf->address, vmf->pte);
> > >  	pte_unmap_unlock(vmf->pte, vmf->ptl);
> > > @@ -2526,7 +2532,11 @@ static int wp_page_copy(struct vm_fault
> > > *vmf)
> > >  		}
> > >  		flush_cache_page(vma, vmf->address,
> > > pte_pfn(vmf->orig_pte));
> > >  		entry = mk_pte(new_page, vma->vm_page_prot);
> > > -		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> > > +		if (is_shstk_mapping(vma->vm_flags))
> > > +			entry = pte_mkdirty_shstk(entry);
> > > +		else
> > > +			entry = pte_mkdirty(entry);
> > > +		entry = maybe_mkwrite(entry, vma);
> > Do we want to lift this hunk of code and put it elsewhere?  Maybe:
> > 
> > 	entry = pte_set_vma_features(entry, vma);
> > 
> > and then:
> > 
> > pte_t pte_set_vma_features(pte_t entry, struct vm_area_struct)
> > {
> > 		/*
> > 		 * Shadow stack PTEs are always dirty and always
> > 		 * writable.  They have a different encoding for
> > 		 * this than normal PTEs, though.
> > 		 */
> > 		if (is_shstk_mapping(vma->vm_flags))
> > 			entry = pte_mkdirty_shstk(entry);
> > 		else
> > 			entry = pte_mkdirty(entry);
> > 
> > 		entry = maybe_mkwrite(entry, vma);
> > 
> > 	return entry;
> > }
> Yes, that wants a helper like that. Not sold on the name, but
> whatever.
> 
> Is there any way we can hide all the shadow stack magic in arch
> code?

We use is_shstk_mapping() only to determine PAGE_DIRTY_SW or
PAGE_DIRTY_HW should be set in a PTE.  One way to remove this shadow
stack code from generic code is changing pte_mkdirty(pte) to
pte_mkdirty(pte, vma), and in the arch code we handle shadow stack.
Is this acceptable?

Thanks,
Yu-cheng