Excerpts from David Stevens's message of June 24, 2021 1:57 pm:

KVM supports mapping VM_IO and VM_PFNMAP memory into the guest by using
follow_pte in gfn_to_pfn. However, the resolved pfns may not have
assoicated struct pages, so they should not be passed to pfn_to_page.
This series removes such calls from the x86 and arm64 secondary MMU. To
do this, this series modifies gfn_to_pfn to return a struct page in
addition to a pfn, if the hva was resolved by gup. This allows the
caller to call put_page only when necessated by gup.

This series provides a helper function that unwraps the new return type
of gfn_to_pfn to provide behavior identical to the old behavior. As I
have no hardware to test powerpc/mips changes, the function is used
there for minimally invasive changes. Additionally, as gfn_to_page and
gfn_to_pfn_cache are not integrated with mmu notifier, they cannot be
easily changed over to only use pfns.

This addresses CVE-2021-22543 on x86 and arm64.

Does this fix the problem? (untested I don't have a POC setup at hand,
but at least in concept)

I have no problem with improving the API and probably in the direction 
of your series is good. But there seems to be a lot of unfixed arch code 
and broken APIs remaining left to do after your series too. This might 
be most suitable to backport and as a base for your series that can take
more time to convert to new APIs.

Thanks,
Nick

---

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 6a6bc7af0e28..e208c279d903 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c

@@ -2104,13 +2104,21 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
 	 * Whoever called remap_pfn_range is also going to call e.g.
 	 * unmap_mapping_range before the underlying pages are freed,
 	 * causing a call to our MMU notifier.
+	 *
+	 * Certain IO or PFNMAP mappings can be backed with valid
+	 * struct pages, but be allocated without refcounting e.g.,
+	 * tail pages of non-compound higher order allocations, which
+	 * would then underflow the refcount when the caller does the
+	 * required put_page. Don't allow those pages here.
 	 */ 
-	kvm_get_pfn(pfn);
+	if (!kvm_try_get_pfn(pfn))
+		r = -EFAULT;
 
 out:
 	pte_unmap_unlock(ptep, ptl);
 	*p_pfn = pfn;
-	return 0;
+
+	return r;
 }
 
 /*

@@ -2487,6 +2495,13 @@ void kvm_set_pfn_accessed(kvm_pfn_t pfn)
 }
 EXPORT_SYMBOL_GPL(kvm_set_pfn_accessed);
 
+static int kvm_try_get_pfn(kvm_pfn_t pfn)
+{
+	if (kvm_is_reserved_pfn(pfn))
+		return 1;
+	return get_page_unless_zero(pfn_to_page(pfn));
+}
+
 void kvm_get_pfn(kvm_pfn_t pfn)
 {
 	if (!kvm_is_reserved_pfn(pfn))

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