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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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