Re: [PATCH v19 11/25] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW

Kees Cook
2021-02-04T20:21:04Z

On Wed, Feb 03, 2021 at 02:55:33PM -0800, Yu-cheng Yu wrote:
> When Shadow Stack is introduced, [R/O + _PAGE_DIRTY] PTE is reserved for
> shadow stack. Copy-on-write PTEs have [R/O + _PAGE_COW].
>
> When a PTE goes from [R/W + _PAGE_DIRTY] to [R/O + _PAGE_COW], it could
> become a transient shadow stack PTE in two cases:
>
> The first case is that some processors can start a write but end up seeing
> a read-only PTE by the time they get to the Dirty bit, creating a transient
> shadow stack PTE. However, this will not occur on processors supporting
> Shadow Stack, and a TLB flush is not necessary.
>
> The second case is that when _PAGE_DIRTY is replaced with _PAGE_COW non-
> atomically, a transient shadow stack PTE can be created as a result.
> Thus, prevent that with cmpxchg.
>
> Dave Hansen, Jann Horn, Andy Lutomirski, and Peter Zijlstra provided many
> insights to the issue. Jann Horn provided the cmpxchg solution.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

--
Kees Cook