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

Edgecombe, Rick P
2022-10-14T15:53:01Z

On Fri, 2022-10-14 at 11:41 +0200, Peter Zijlstra wrote:
> 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/

Oops, yes.

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

Ok, I'll scrub the series for the order.

>
> > 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

Yes, I incorporated feedback from your earlier comment. Sorry for bad
communication.