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

[Dave Hansen]

>>> -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.  I'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.