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

[Yu-cheng Yu]

On Wed, 2019-08-14 at 09:48 -0700, Dave Hansen wrote:
> On 8/14/19 9:27 AM, Yu-cheng Yu wrote:
> > 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.
> 
> I don't understand what you are saying.
> 
> Are you saying that we need this VM_SHSTK flag in order to exclude
> RO+HW-Dirty pages from being created in non-shadow-stack VMAs?

We use VM_SHSTK for page fault handling (the special-casing).  If we have a
special mapping, all these become cleaner (but more code).  However, we still
need most of the PTE macros (e.g. ptep_set_wrprotect, PAGE_DIRTY_SW, etc.).

Yu-cheng