Re: [PATCH v2 10/39] x86/mm: Introduce _PAGE_COW

Peter Zijlstra
2022-10-14T09:42:12Z

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