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

> /*
> * Clear the pte entry and flush it first, before updating the
> * pte with the new entry. This will avoid a race condition
> @@ -3201,6 +3211,14 @@ static int do_anonymous_page(struct vm_fault *vmf)
> mem_cgroup_commit_charge(page, memcg, false, false);
> lru_cache_add_active_or_unevictable(page, vma);
> setpte:
> + /*
> + * If this is within a shadow stack mapping, mark
> + * the PTE dirty. We don't use pte_mkdirty(),
> + * because the PTE must have _PAGE_DIRTY_HW set.
> + */
> + if (is_shstk_mapping(vma->vm_flags))
> + entry = pte_mkdirty_shstk(entry);
> +
> set_pte_at(vma->vm_mm, vmf->address, vmf->pte, entry);

I'm not sure this is the right spot to do this.

The other code does pte_mkdirty_shstk() near where we do the
pte_mkwrite(). Why not here? I think you might have picked this
because it's a common path used by both allocated pages and zero pages.
But, we can't have the zero pages in shadow stack areas since they can't
be read-only. I think you need to move this up. Can you even
consolidate it with the other two pte_mkdirt_shstk() call sites?

> /* No need to invalidate - it was non-present before */
> @@ -3983,6 +4001,14 @@ static int handle_pte_fault(struct vm_fault *vmf)
> entry = vmf->orig_pte;
> if (unlikely(!pte_same(*vmf->pte, entry)))
> goto unlock;
> +
> + /*
> + * Shadow stack PTEs are copy-on-access, so do_wp_page()
> + * handling on them no matter if we have write fault or not.
> + */

I'd say this differently:

Shadow stack PTEs can not be read-only and because of that can
not have traditional copy-on-write semantics. This essentially
performs a copy-on-write operation, but on *any* access, not
just actual writes.

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