Re: [PATCH v4 10/39] x86/mm: Introduce _PAGE_COW

[Kees Cook]

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@xxxxxxxxx>
> Tested-by: John Allen <john.allen@xxxxxxx>
> Co-developed-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-- 
Kees Cook