Re: [RFC PATCH 08/21] KVM: TDX: Increase/decrease folio ref for huge pages

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

On Tue, 2025-06-17 at 09:38 +0800, Yan Zhao wrote:
> > We talked about doing something like having tdx_hold_page_on_error() in
> > guestmemfd with a proper name. The separation of concerns will be better if
> > we
> > can just tell guestmemfd, the page has an issue. Then guestmemfd can decide
> > how
> > to handle it (refcount or whatever).
> Instead of using tdx_hold_page_on_error(), the advantage of informing
> guest_memfd that TDX is holding a page at 4KB granularity is that, even if
> there
> is a bug in KVM (such as forgetting to notify TDX to remove a mapping in
> handle_removed_pt()), guest_memfd would be aware that the page remains mapped
> in
> the TDX module. This allows guest_memfd to determine how to handle the
> problematic page (whether through refcount adjustments or other methods)
> before
> truncating it.

I don't think a potential bug in KVM is a good enough reason. If we are
concerned can we think about a warning instead?

We had talked enhancing kasan to know when a page is mapped into S-EPT in the
past. So rather than design around potential bugs we could focus on having a
simpler implementation with the infrastructure to catch and fix the bugs.

> 
> > > 
> > > This would allow guest_memfd to maintain an internal reference count for
> > > each
> > > private GFN. TDX would call guest_memfd_add_page_ref_count() for mapping
> > > and
> > > guest_memfd_dec_page_ref_count() after a successful unmapping. Before
> > > truncating
> > > a private page from the filemap, guest_memfd could increase the real folio
> > > reference count based on its internal reference count for the private GFN.
> > 
> > What does this get us exactly? This is the argument to have less error prone
> > code that can survive forgetting to refcount on error? I don't see that it
> > is an
> > especially special case.
> Yes, for a less error prone code.
> 
> If this approach is considered too complex for an initial implementation,
> using
> tdx_hold_page_on_error() is also a viable option.

I'm saying I don't think it's not a good enough reason. Why is it different then
other use-after free bugs? I feel like I'm missing something.