Re: [PATCH RFC v2 34/50] KVM: guest_memfd: Skip checking outstanding references in PFN mode

[Fred Griffoul]

> +		if (!kvm_gmem_is_pfn_mode(gi) &&
> +		    kvm_gmem_has_outstanding_references(inode, start, nr_pages,
> 							err_index)) {

In folio mode this check is also what guarantees that a private page
belongs to the guest alone. In PFN mode there is nothing like it: if
the provider also exposes the frame somewhere else, say a ring shared
with a device, the VMM can still make it private.

And it is not only this guest that suffers then. Whoever else uses the
frame breaks: an RMP fault on SEV-SNP, a poisoned page or a machine
check on TDX, a granule protection fault on CCA, or, with
SW_PROTECTED, silent access to "private" memory.

Only the provider knows about such frames. With an attributes output
from get_pfn() (see my reply to 31/50), it could flag them as aliased,
and guest_memfd would keep them shared: refuse the conversion, treat a
private fault as a hole, and refuse populate.

Fred