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

[Dave Hansen]

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.