Re: [PATCH v8 15/27] mm: Handle shadow stack page fault

[Yu-cheng Yu]

On Tue, 2019-08-13 at 15:55 -0700, Andy Lutomirski wrote:
> On Tue, Aug 13, 2019 at 2:02 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> > 
> > When a task does fork(), its shadow stack (SHSTK) must be duplicated
> > for the child.  This patch implements a flow similar to copy-on-write
> > of an anonymous page, but for SHSTK.
> > 
> > A SHSTK PTE must be RO and dirty.  This dirty bit requirement is used
> > to effect the copying.  In copy_one_pte(), clear the dirty bit from a
> > SHSTK PTE to cause a page fault upon the next SHSTK access.  At that
> > time, fix the PTE and copy/re-use the page.
> 
> Is using VM_SHSTK and special-casing all of this really better than
> using a special mapping or other pseudo-file-backed VMA and putting
> all the magic in the vm_operations?

A special mapping is cleaner.  However, we also need to exclude normal [RO +
dirty] pages from shadow stack.

Yu-cheng