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

On Wed, 2019-08-14 at 09:48 -0700, Dave Hansen wrote:

On 8/14/19 9:27 AM, Yu-cheng Yu wrote:

quoted

On Tue, 2019-08-13 at 15:55 -0700, Andy Lutomirski wrote:

quoted

On Tue, Aug 13, 2019 at 2:02 PM Yu-cheng Yu [off-list ref] wrote:

quoted

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