Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and pmdp_set_wrprotect for _PAGE_DIRTY_SW

[Jann Horn <jannh@google.com>]

On Thu, Aug 30, 2018 at 4:43 PM Yu-cheng Yu <yu-cheng.yu@intel.com> 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@intel.com>
> ---
>  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?