Re: [PATCH v2 3/4] KVM: SEV: Drop page refcount early in VMSA reload

[Sean Christopherson <seanjc@google.com>]

On Fri, Aug 21, 2026, Ackerley Tng wrote:
> Ackerley Tng <ackerleytng@google.com> writes:
> >> This is a pre-existing issue, but can this allow a malicious guest to
> >> cause a host kernel panic?
> >>
> >> Looking at sev_gmem_make_shared() in arch/x86/kvm/svm/sev.c:
> >>
> >> arch/x86/kvm/svm/sev.c:sev_gmem_make_shared() {
> >>     ...
> >> 		rc = rmp_make_shared(pfn, use_2m_update ? PG_LEVEL_2M : PG_LEVEL_4K);
> >> 		if (WARN_ONCE(rc, "SEV: Failed to update RMP entry for PFN 0x%llx error %d\n",
> >> 			      pfn, rc))
> >> 			goto next_pfn;
> >>     ...
> >> }
> >>
> >> If an untrusted SEV-SNP guest modifies a guest_memfd page state to VMSA
> >> using RMPADJUST, and the page is subsequently freed, rmp_make_shared()
> >> will architecturally fail on the VMSA page.
> >
> > I need some help with this one.
> >
> > For the initial VMSA page, IIUC snp_safe_alloc_page() gets a
> > non-guest_memfd page. This won't ever go through sev_gmem_make_shared(),
> > so we're good. snp_leak_pages() doesn't actually leak pages, it just
> > tracks leaked pages, but __free_page() is skipped so the page is really
> > leaked.
> >
> >> Because the error is bypassed without calling snp_leak_pages(), the
> >> firmware-owned page is returned to the host page allocator.
> >
> > For guest-provided VMSA pages, those can be guest_memfd pages. The
> > refcount taken is the one guest_memfd holds. Without considering
> > conversions, just considering truncation, sev_gmem_make_shared() is
> > called from kvm_gmem_free_folio(). Sashiko is saying that
> > rmp_make_shared() will fail, and the folio is freed.
> >
> > Sounds like this would be a problem if the folio is used elsewhere...?
> 
> I looked closer, "15.36.12 Running SNP-Active Virtual Machines" in APM
> Vol2 says "After a successful VMRUN, the VMCB page, as well as any AVIC
> Backing Page and VMSA Page are marked as in-use by hardware, and any
> attempt to modify the RMP entries for these pages via instructions like
> RMPUPDATE will result in a FAIL_INUSE response. The in-use marking is
> automatically cleared by hardware after a #VMEXIT event".
> 
> So it's not true that rmp_make_shared() will always fail on a VMSA page,
> it's only true if the guest vCPU is running.

Correct.

> RMPADJUST sets up the VMSA page, and telling the host using
> SVM_VMGEXIT_AP_CREATE is how the VMSA is set up to be usable. Then, the
> other vCPU needs to be started and running, and then:
> 
> If the host truncates the VMSA page or host closes the guest_memfd file
> containing some other vCPU's VMSA page, sev_gmem_make_shared() on the
> VMSA page should fail, but the page is released anyway and we have a
> problem.
> 
> An unsolicited shared->private conversion is fine since the VMSA page is
> already private.
> 
> sev_gmem_make_shared() would fail on an unsolicited private->shared
> conversion. The RMP entry would still indicate private, and guest_memfd
> would track it as shared...? Bad.
> 
> I've been trying to write a mini reproducer like a selftest but I've
> still not gotten it to work yet. If someone knows there's no issue here
> please let me know!

I have a reproducer I created to prove and validate 74bdfb2f6d18 ("KVM: SEV:
Forcefully invalidate SNP VMSA if its backing gmem page is zapped"), I'll make
sure to run it when applying (the final version of) this series.