Re: [PATCH v5 11/39] x86/mm: Update pte_modify for _PAGE_COW

Kees Cook
2023-01-20T00:57:33Z

On Thu, Jan 19, 2023 at 01:22:49PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@intel.com>
>
> The Write=0,Dirty=1 PTE has been used to indicate copy-on-write pages.
> However, newer x86 processors also regard a Write=0,Dirty=1 PTE as a
> shadow stack page. In order to separate the two, the software-defined
> _PAGE_DIRTY is changed to _PAGE_COW for the copy-on-write case, and
> pte_*() are updated to do this.
>
> pte_modify() takes a "raw" pgprot_t which was not necessarily created
> with any of the existing PTE bit helpers. That means that it can return a
> pte_t with Write=0,Dirty=1, a shadow stack PTE, when it did not intend to
> create one.
>
> However pte_modify() changes a PTE to 'newprot', but it doesn't use the
> pte_*(). Modify it to also move _PAGE_DIRTY to _PAGE_COW. Do this by
> using the pte_mkdirty() helper. Since pte_mkdirty() also sets the soft
> dirty bit, extract a helper that optionally doesn't set
> _PAGE_SOFT_DIRTY. This helper will allow future logic for deciding when to
> move _PAGE_DIRTY to _PAGE_COW can live in one place.
>
> Apply the same changes to pmd_modify().
>
> Tested-by: Pengfei Xu <pengfei.xu@intel.com>
> Tested-by: John Allen <john.allen@amd.com>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>

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

--
Kees Cook