Re: [RFC PATCH v2 04/51] KVM: guest_memfd: Introduce KVM_GMEM_CONVERT_SHARED/PRIVATE ioctls

[Ira Weiny]

Xu Yilun wrote:
> On Thu, Jul 17, 2025 at 09:56:01AM -0700, Ackerley Tng wrote:
> > Xu Yilun <yilun.xu@xxxxxxxxxxxxxxx> writes:
> > 
> > > On Wed, Jul 16, 2025 at 03:22:06PM -0700, Ackerley Tng wrote:
> > >> Yan Zhao <yan.y.zhao@xxxxxxxxx> writes:
> > >> 
> > >> > On Tue, Jun 24, 2025 at 07:10:38AM -0700, Vishal Annapurve wrote:
> > >> >> On Tue, Jun 24, 2025 at 6:08 AM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> > >> >> >
> > >> >> > On Tue, Jun 24, 2025 at 06:23:54PM +1000, Alexey Kardashevskiy wrote:
> > >> >> >
> > >> >> > > Now, I am rebasing my RFC on top of this patchset and it fails in
> > >> >> > > kvm_gmem_has_safe_refcount() as IOMMU holds references to all these
> > >> >> > > folios in my RFC.
> > >> >> > >
> > >> >> > > So what is the expected sequence here? The userspace unmaps a DMA
> > >> >> > > page and maps it back right away, all from the userspace? The end
> > >> >> > > result will be the exactly same which seems useless. And IOMMU TLB
> > >> >> 
> > >> >>  As Jason described, ideally IOMMU just like KVM, should just:
> > >> >> 1) Directly rely on guest_memfd for pinning -> no page refcounts taken
> > >> >> by IOMMU stack
> > >> > In TDX connect, TDX module and TDs do not trust VMM. So, it's the TDs to inform
> > >> > TDX module about which pages are used by it for DMAs purposes.
> > >> > So, if a page is regarded as pinned by TDs for DMA, the TDX module will fail the
> > >> > unmap of the pages from S-EPT.
> > >> >
> > >> > If IOMMU side does not increase refcount, IMHO, some way to indicate that
> > >> > certain PFNs are used by TDs for DMA is still required, so guest_memfd can
> > >> > reject the request before attempting the actual unmap.
> > >> > Otherwise, the unmap of TD-DMA-pinned pages will fail.
> > >> >
> > >> > Upon this kind of unmapping failure, it also doesn't help for host to retry
> > >> > unmapping without unpinning from TD.
> > >> >
> > >> >
> > >> 
> > >> Yan, Yilun, would it work if, on conversion,
> > >> 
> > >> 1. guest_memfd notifies IOMMU that a conversion is about to happen for a
> > >>    PFN range
> > >
> > > It is the Guest fw call to release the pinning.
> > 
> > I see, thanks for explaining.
> > 
> > > By the time VMM get the
> > > conversion requirement, the page is already physically unpinned. So I
> > > agree with Jason the pinning doesn't have to reach to iommu from SW POV.
> > >
> > 
> > If by the time KVM gets the conversion request, the page is unpinned,
> > then we're all good, right?
> 
> Yes, unless guest doesn't unpin the page first by mistake.

Or maliciously?  :-(

My initial response to this was that this is a bug and we don't need to be
concerned with it.  However, can't this be a DOS from one TD to crash the
system if the host uses the private page for something else and the
machine #MC's?

Ira

> Guest would
> invoke a fw call tdg.mem.page.release to unpin the page before
> KVM_HC_MAP_GPA_RANGE.
> 
> > 
> > When guest_memfd gets the conversion request, as part of conversion
> > handling it will request to zap the page from stage-2 page tables. TDX
> > module would see that the page is unpinned and the unmapping will
> > proceed fine. Is that understanding correct?
> 
> Yes, again unless guess doesn't unpin.
> 
> > 
> > >> 2. IOMMU forwards the notification to TDX code in the kernel
> > >> 3. TDX code in kernel tells TDX module to stop thinking of any PFNs in
> > >>    the range as pinned for DMA?
> > >
> > > TDX host can't stop the pinning. Actually this mechanism is to prevent
> > > host from unpin/unmap the DMA out of Guest expectation.
> > >
> > 
> > On this note, I'd also like to check something else. Putting TDX connect
> > and IOMMUs aside, if the host unmaps a guest private page today without
> > the guest requesting it, the unmapping will work and the guest will be
> > broken, right?
> 
> Correct. The unmapping will work, the guest can't continue anymore.
> 
> Thanks,
> Yilun