Re: [RFC PATCH v2 16/27] mm: Modify can_follow_write_pte/pmd for shadow stack

[Yu-cheng Yu]

On Wed, 2018-07-18 at 17:06 -0700, Dave Hansen wrote:
> > 
> > > 
> > > > 
> > > > -static inline bool can_follow_write_pte(pte_t pte, unsigned
> > > > int flags)
> > > > +static inline bool can_follow_write(pte_t pte, unsigned int
> > > > flags,
> > > > +				ÂÂÂÂstruct vm_area_struct
> > > > *vma)
> > > > Â{
> > > > -	return pte_write(pte) ||
> > > > -		((flags & FOLL_FORCE) && (flags & FOLL_COW)
> > > > && pte_dirty(pte));
> > > > +	if (!is_shstk_mapping(vma->vm_flags)) {
> > > > +		if (pte_write(pte))
> > > > +			return true;
> > > Let me see if I can say this another way.
> > > 
> > > The bigger issue is that these patches change the semantics of
> > > pte_write().ÂÂBefore these patches, it meant that you *MUST*
> > > have this
> > > bit set to write to the page controlled by the PTE.ÂÂNow, it
> > > means: you
> > > can write if this bit is set *OR* the shadowstack bit
> > > combination is set.
> > Here, we only figure out (1) if the page is pointed by a writable
> > PTE; or
> > (2) if the page is pointed by a RO PTE (data or SHSTK) and it has
> > been
> > copied and it still exists. ÂWe are not trying to
> > determine if the
> > SHSTK PTE is writable (we know it is not).
> Please think about the big picture.ÂÂI'm not just talking about this
> patch, but about every use of pte_write() in the kernel.
> 
> > 
> > > 
> > > That's the fundamental problem.ÂÂWe need some code in the kernel
> > > that
> > > logically represents the concept of "is this PTE a shadowstack
> > > PTE or a
> > > PTE with the write bit set", and we will call that pte_write(),
> > > or maybe
> > > pte_writable().
> > > 
> > > You *have* to somehow rectify this situation.ÂÂWe can absolutely
> > > no
> > > leave pte_write() in its current, ambiguous state where it has
> > > no real
> > > meaning or where it is used to mean _both_ things depending on
> > > context.
> > True, the processor can always write to a page through a shadow
> > stack
> > PTE, but it must do that with a CALL instruction. ÂCan we define
> > aÂ
> > write operation as: MOV r1, *(r2). ÂThen we don't have any doubt
> > on
> > pte_write() any more.
> No, we can't just move the target. :)
> 
> You can define it this way, but then you also need to go to every
> spot
> in the kernel that calls pte_write() (and _PAGE_RW in fact) and
> audit it
> to ensure it means "mov ..." and not push.

Which pte_write() do you think is right?

bool is_shstk_pte(pte) {
	return (_PAGE_RW not set) &&
(_PAGE_DIRTY_HW set);
}

int pte_write_1(pte) {
	return (_PAGE_RW set) && !is_shstk_pte(pte);
}

int pte_write_2(pte) {
	return (_PAGE_RW set) || is_shstk_pte(pte);
}

Yu-cheng