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    SubjectRe: [PATCH v4 10/39] x86/mm: Introduce _PAGE_COW
    On Fri, Dec 02, 2022 at 04:35:37PM -0800, Rick Edgecombe wrote:
    > Some OSes have a greater dependence on software available bits in PTEs than
    > 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. So in order to support
    > shadow stack memory, Linux should avoid creating memory with this PTE bit
    > combination unless it intends for it to be shadow stack.
    >
    > The reason it's lightly used is that Dirty=1 is normally set by HW
    > _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 Dirty=1 *and*
    > generate the fault, resulting in a Write=0,Dirty=1 PTE. Hardware which
    > supports shadow stacks will no longer exhibit this oddity.
    >
    > So that leaves Write=0,Dirty=1 PTEs created in software. To achieve this,
    > 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.
    > Further differentiated by VMA flags, these PTE bit combinations would be
    > set as follows for various types of memory:
    >
    > (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. This
    > happens in copy_present_pte().
    > - A R/O page that has been COW'ed. The user page is in a R/O VMA,
    > and get_user_pages(FOLL_FORCE) 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.
    > - 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.
    >
    > (Write=0,Cow=0,Dirty=1):
    > - A shadow stack PTE.
    > - A Cow PTE created when a processor without shadow stack support set
    > Dirty=1.
    >
    > There are six bits left available to software in the 64-bit PTE after
    > consuming a bit for _PAGE_COW. No space is consumed in 32-bit kernels
    > because shadow stacks are not enabled there.
    >
    > This is a prepratory patch. Changes to actually start marking _PAGE_COW
    > will follow once other pieces are in place.
    >
    > Tested-by: Pengfei Xu <pengfei.xu@intel.com>
    > Tested-by: John Allen <john.allen@amd.com>
    > Co-developed-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
    > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>

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

    --
    Kees Cook

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