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    SubjectRe: [PATCH v2 10/39] x86/mm: Introduce _PAGE_COW
    On Thu, Sep 29, 2022 at 03:29:07PM -0700, Rick Edgecombe wrote:
    > From: Yu-cheng Yu <yu-cheng.yu@intel.com>
    >
    > There is essentially no room left in the x86 hardware PTEs on some OSes
    > (not Linux). That left the hardware architects looking for a way to
    > represent a new memory type (shadow stack) within the existing bits.
    > They chose to repurpose a lightly-used state: Write=0,Dirty=1.
    >
    > The reason it's lightly used is that Dirty=1 is normally set _before_ a
    > write. A write with a Write=0 PTE would typically only generate a fault,
    > not set Dirty=1. Hardware can (rarely) both set Write=1 *and* generate the

    s/Write/Dirty/

    > fault, resulting in a Dirty=0,Write=1 PTE. Hardware which supports shadow

    s/Dirty=0,Write=1/Write=0,Dirty=1/

    > stacks will no longer exhibit this oddity.
    >
    > The kernel should avoid inadvertently creating shadow stack memory because
    > it is security sensitive. So given the above, all it needs to do is avoid
    > manually crating Write=0,Dirty=1 PTEs in software.

    Whichever way around you choose, please be consistent.

    > In places where Linux normally creates Write=0,Dirty=1, it can use the
    > software-defined _PAGE_COW in place of the hardware _PAGE_DIRTY. In other
    > words, whenever Linux needs to create Write=0,Dirty=1, it instead creates
    > Write=0,Cow=1 except for shadow stack, which is Write=0,Dirty=1. This
    > clearly separates shadow stack from other data, and results in the
    > following:
    >
    > (a) (Write=0,Cow=1,Dirty=0) A modified, copy-on-write (COW) page.
    > Previously when a typical anonymous writable mapping was made COW via
    > fork(), the kernel would mark it Write=0,Dirty=1. Now it will instead
    > use the Cow bit.
    > (b) (Write=0,Cow=1,Dirty=0) A R/O page that has been COW'ed. The user page
    > is in a R/O VMA, and get_user_pages() needs a writable copy. The page
    > fault handler creates a copy of the page and sets the new copy's PTE
    > as Write=0 and Cow=1.
    > (c) (Write=0,Cow=0,Dirty=1) A shadow stack PTE.
    > (d) (Write=0,Cow=1,Dirty=0) A shared shadow stack PTE. When a shadow stack
    > page is being shared among processes (this happens at fork()), its PTE
    > is made Dirty=0, so the next shadow stack access causes a fault, and
    > the page is duplicated and Dirty=1 is set again. This is the COW
    > equivalent for shadow stack pages, even though it's copy-on-access
    > rather than copy-on-write.
    > (e) (Write=0,Cow=0,Dirty=1) A Cow PTE created when a processor without
    > shadow stack support set Dirty=1.

    Please restureture this (and the comment) something like:


    (Write=0,Dirty=0,Cow=1):

    - copy_present_pte(): A modified copy-on-write page.
    - ...


    (Write=0,Dirty=1,Cow=0):

    - FEATURE_CET: Shadow Stack entry
    - !FEATURE_CET: see the above Cow=1 cases


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