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SubjectRe: [RFC PATCH v2 14/27] mm: Handle THP/HugeTLB shadow stack page fault
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On 07/10/2018 03:26 PM, Yu-cheng Yu wrote:
> @@ -1193,6 +1195,8 @@ static int do_huge_pmd_wp_page_fallback(struct vm_fault *vmf, pmd_t orig_pmd,
> pte_t entry;
> entry = mk_pte(pages[i], vma->vm_page_prot);
> entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> + if (is_shstk_mapping(vma->vm_flags))
> + entry = pte_mkdirty_shstk(entry);

Peter Z was pointing out that we should get rid of all this generic code
manipulation. We might not easily be able to do it *all*, but we can do
better than what we've got here.

Basically, if you have code outside of arch/x86 in your patch set that
refers to shadow stacks, you should consider it a bug (for now),
especially if you have to hack .c files.

For instance, in the code above, you could move the is_shstk_mapping() into:

static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
{
if (likely(vma->vm_flags & VM_WRITE))
pte = pte_mkwrite(pte);

+ pte = arch_pte_mkwrite(pte, vma);
+
return pte;
}

... and add an arch callback that does:

static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
{
if (!is_shstk_mapping(vma->vm_flags))
return pte;

WARN_ON(... pte bits incompatible with shadow stacks?);

/* Lots of comments of course */
entry = pte_mkdirty_shstk(entry);
}

This is just one example. You are probably going to need a couple of
similar things. Just remember: the bar is very high to make changes to
.c files outside of arch/x86. You can do a _bit_ more in non-x86
headers, but you have the most freedom to patch what you want as long as
it's in arch/x86.

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