Re: [PATCH v5 10/39] x86/mm: Introduce _PAGE_COW

On Thu, Jan 19, 2023 at 01:22:48PM -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.

Implement only the infrastructure for _PAGE_COW. Changes to start
creating _PAGE_COW PTEs will follow once other pieces are in place.

Tested-by: Pengfei Xu <redacted>
Tested-by: John Allen <john.allen@amd.com>
Co-developed-by: Yu-cheng Yu <redacted>
Signed-off-by: Yu-cheng Yu <redacted>

Reviewed-by: Kees Cook <redacted>

-- 
Kees Cook