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

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