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
