Re: [RFC PATCH v2 14/27] mm: Handle THP/HugeTLB shadow stack page fault

[Yu-cheng Yu]

On Fri, 2018-07-20 at 07:20 -0700, Dave Hansen wrote:
> 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,
> > Â		pte_t entry;
> > Â		entry = mk_pte(pages[i], vma->vm_page_prot);
> > Â		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.ÂÂWe 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)
> {
> ÂÂÂÂÂÂÂÂif (likely(vma->vm_flags & VM_WRITE))
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂpte = pte_mkwrite(pte);
> 	
> +	pte = arch_pte_mkwrite(pte, vma);
> +
> ÂÂÂÂÂÂÂÂreturn 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.ÂÂYou are probably going to need a couple
> of
> similar things.ÂÂJust remember: the bar is very high to make changes
> to
> .c files outside of arch/x86.ÂÂYou 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.

Ok, I will work on that. ÂThanks!

Yu-cheng