Re: [PATCH v4 11/39] x86/mm: Update pte_modify for _PAGE_COW

[Kees Cook]

On Fri, Dec 02, 2022 at 04:35:38PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> 
> 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. Apply the same
> changes to pmd_modify().
> 
> Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
> Tested-by: John Allen <john.allen@xxxxxxx>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-- 
Kees Cook