On Thu, 2018-08-30 at 17:49 +0200, Jann Horn wrote:
> On Thu, Aug 30, 2018 at 4:43 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> wrote:
> >
> >
> > When Shadow Stack is enabled, the read-only and PAGE_DIRTY_HW PTE
> > setting is reserved only for the Shadow Stack. To track dirty of
> > non-Shadow Stack read-only PTEs, we use PAGE_DIRTY_SW.
> >
> > Update ptep_set_wrprotect() and pmdp_set_wrprotect().
> >
> > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> > ---
> > arch/x86/include/asm/pgtable.h | 42
> > ++++++++++++++++++++++++++++++++++
> > 1 file changed, 42 insertions(+)
> >
> > diff --git a/arch/x86/include/asm/pgtable.h
> > b/arch/x86/include/asm/pgtable.h
> > index 4d50de77ea96..556ef258eeff 100644
> > --- a/arch/x86/include/asm/pgtable.h
> > +++ b/arch/x86/include/asm/pgtable.h
> > @@ -1203,7 +1203,28 @@ static inline pte_t
> > ptep_get_and_clear_full(struct mm_struct *mm,
> > static inline void ptep_set_wrprotect(struct mm_struct *mm,
> > unsigned long addr, pte_t
> > *ptep)
> > {
> > + pte_t pte;
> > +
> > clear_bit(_PAGE_BIT_RW, (unsigned long *)&ptep->pte);
> > + pte = *ptep;
> > +
> > + /*
> > + * Some processors can start a write, but ending up seeing
> > + * a read-only PTE by the time they get to the Dirty bit.
> > + * In this case, they will set the Dirty bit, leaving a
> > + * read-only, Dirty PTE which looks like a Shadow Stack
> > PTE.
> > + *
> > + * However, this behavior has been improved and will not
> > occur
> > + * on processors supporting Shadow Stacks. Without this
> > + * guarantee, a transition to a non-present PTE and flush
> > the
> > + * TLB would be needed.
> > + *
> > + * When change a writable PTE to read-only and if the PTE
> > has
> > + * _PAGE_DIRTY_HW set, we move that bit to _PAGE_DIRTY_SW
> > so
> > + * that the PTE is not a valid Shadow Stack PTE.
> > + */
> > + pte = pte_move_flags(pte, _PAGE_DIRTY_HW, _PAGE_DIRTY_SW);
> > + set_pte_at(mm, addr, ptep, pte);
> > }
> I don't understand why it's okay that you first atomically clear the
> RW bit, then atomically switch from DIRTY_HW to DIRTY_SW. Doesn't
> that
> mean that between the two atomic writes, another core can
> incorrectly
> see a shadow stack?
Yes, we had that concern earlier and checked.
On processors supporting Shadow Stacks, that will not happen.
Yu-cheng
