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

[Yu-cheng Yu]

On Wed, 2018-07-11 at 11:29 +0200, Peter Zijlstra wrote:
> On Tue, Jul 10, 2018 at 03:26:28PM -0700, Yu-cheng Yu wrote:
> > 
> > There are three possible shadow stack PTE settings:
> > 
> > Â Normal SHSTK PTE: (R/O + DIRTY_HW)
> > Â SHSTK PTE COW'ed: (R/O + DIRTY_HW)
> > Â SHSTK PTE shared as R/O data: (R/O + DIRTY_SW)
> I count _2_ distinct states there.
> 
> > 
> > Update can_follow_write_pte/pmd for the shadow stack.
> So the below disallows can_follow_write when shstk && _PAGE_DIRTY_SW,
> but this here Changelog doesn't explain why. Doesn't even get close.

Can we add the following to the log:

When a SHSTK PTE is shared, it is (R/O + DIRTY_SW); otherwise it is
(R/O + DIRTY_HW).

When we (FOLL_WRITE | FOLL_FORCE) on a SHSTK PTE, the following
must be true:

 - It has been COW'ed at least once (FOLL_COW is set);
 - It still is not shared, i.e. PTE is (R/O + DIRTY_HW);

> 
> Also, the code is a right mess :/ Can't we try harder to not let this
> shadow stack stuff escape arch code.

We either check here if the VMA is SHSTK mapping or move the logic
to pte_dirty(). ÂThe latter would be less obvious. ÂOr can we
create a can_follow_write_shstk_pte()?

Yu-cheng