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

[Dave Hansen]

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?