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

On Fri, Dec 02, 2022 at 04:35:38PM -0800, Rick Edgecombe wrote:

From: Yu-cheng Yu <redacted>

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 <redacted>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Yu-cheng Yu <redacted>

Reviewed-by: Kees Cook <redacted>

-- 
Kees Cook