[PATCH 4/4] KVM: guest_memfd: Stop returning struct page from PFN lookup

[Ackerley Tng <ackerleytng@google.com>]

From: Sean Christopherson <seanjc@google.com>

KVM currently expects guest_memfd PFN lookups to return a refcounted
struct page, which callers hold across fault handling.

Holding a page reference across fault handling is problematic for
guest_memfd. In-place memory conversions between confidential
computing shared and private states inspect folio refcounts to ensure
exclusive ownership by guest_memfd. A concurrent guest page fault
taking a reference on the folio causes conversions to fail due to an
elevated refcount.

guest_memfd already notifies KVM of page invalidations, so callers
within KVM only need to respect the MMU invalidation protocol to safely
rely on guest_memfd for page presence.

Furthermore, removing struct page from the guest_memfd PFN lookup moves
KVM closer toward supporting memory backends that are not backed by
struct page.

Drop the folio reference immediately before returning from the
guest_memfd PFN lookup, and stop returning the struct page pointer.

For ARM, initialize the local page pointer to NULL so that the shared
cleanup path that releases fault-in pages safely no-ops for guest_memfd.

For x86, no additional changes are required in the MMU fault path
because the page fault tracking structure is zero-initialized at the
start of page fault handling, ensuring the refcounted page pointer is
already NULL.

Reported-by: Yan Zhao <yan.y.zhao@intel.com>
Closes: https://lore.kernel.org/all/anZ4W9o5pTWIEgMY@yzhao56-desk.sh.intel.com/
Signed-off-by: Sean Christopherson <seanjc@google.com>
Co-developed-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Co-developed-by: Ackerley Tng <ackerleytng@google.com>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 arch/arm64/kvm/mmu.c     | 4 ++--
 arch/arm64/kvm/nested.c  | 4 ++--
 arch/x86/kvm/mmu/mmu.c   | 2 +-
 arch/x86/kvm/svm/sev.c   | 8 ++------
 include/linux/kvm_host.h | 6 ++----
 virt/kvm/guest_memfd.c   | 9 ++-------
 6 files changed, 11 insertions(+), 22 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 6c941aaa10c63..e5d637a5ec558 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1613,7 +1613,7 @@ static int gmem_abort(const struct kvm_s2_fault_desc *s2fd)
 	enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R;
 	struct kvm_pgtable *pgt = s2fd->vcpu->arch.hw_mmu->pgt;
 	unsigned long mmu_seq;
-	struct page *page;
+	struct page *page = NULL;
 	struct kvm *kvm = s2fd->vcpu->kvm;
 	void *memcache = NULL;
 	kvm_pfn_t pfn;
@@ -1641,7 +1641,7 @@ static int gmem_abort(const struct kvm_s2_fault_desc *s2fd)
 	/* Pairs with the smp_wmb() in kvm_mmu_invalidate_end(). */
 	smp_rmb();
 
-	ret = kvm_gmem_get_pfn(kvm, s2fd->memslot, gfn, &pfn, &page, NULL);
+	ret = kvm_gmem_get_pfn(kvm, s2fd->memslot, gfn, &pfn, NULL);
 	if (ret) {
 		kvm_prepare_memory_fault_exit(s2fd->vcpu, s2fd->fault_ipa, PAGE_SIZE,
 					      write_fault, exec_fault, false);
diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
index fb54f6dad995c..43523bb17621a 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -1360,7 +1360,7 @@ static int kvm_translate_vncr(struct kvm_vcpu *vcpu, bool *is_gmem)
 	bool write_fault, writable;
 	unsigned long mmu_seq;
 	struct vncr_tlb *vt;
-	struct page *page;
+	struct page *page = NULL;
 	u64 va, pfn, gfn;
 	int ret;
 
@@ -1411,7 +1411,7 @@ static int kvm_translate_vncr(struct kvm_vcpu *vcpu, bool *is_gmem)
 		if (is_error_noslot_pfn(pfn) || (write_fault && !writable))
 			return -EFAULT;
 	} else {
-		ret = kvm_gmem_get_pfn(vcpu->kvm, memslot, gfn, &pfn, &page, NULL);
+		ret = kvm_gmem_get_pfn(vcpu->kvm, memslot, gfn, &pfn, NULL);
 		if (ret) {
 			kvm_prepare_memory_fault_exit(vcpu, vt->wr.pa, PAGE_SIZE,
 					      write_fault, false, false);
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index c519e8e8d646f..129d403308051 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4604,7 +4604,7 @@ static int kvm_mmu_faultin_pfn_gmem(struct kvm_vcpu *vcpu,
 	}
 
 	r = kvm_gmem_get_pfn(vcpu->kvm, fault->slot, fault->gfn, &fault->pfn,
-			     &fault->refcounted_page, &max_order);
+			     &max_order);
 	if (r) {
 		kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
 		return r;
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 3fc582fa343cc..0a324e5b556b1 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -4016,7 +4016,6 @@ static void __sev_snp_reload_vmsa(struct kvm_vcpu *vcpu, gpa_t gpa)
 	struct kvm *kvm = vcpu->kvm;
 	gfn_t gfn = gpa_to_gfn(gpa);
 	unsigned long mmu_seq;
-	struct page *page;
 	kvm_pfn_t pfn;
 
 	lockdep_assert_held(&svm->sev_es.snp_vmsa_mutex);
@@ -4060,9 +4059,8 @@ static void __sev_snp_reload_vmsa(struct kvm_vcpu *vcpu, gpa_t gpa)
 	 * The new VMSA will be private memory guest memory, so retrieve the
 	 * PFN from the gmem backend.
 	 */
-	if (kvm_gmem_get_pfn(vcpu->kvm, slot, gfn, &pfn, &page, NULL))
+	if (kvm_gmem_get_pfn(vcpu->kvm, slot, gfn, &pfn, NULL))
 		return;
-	kvm_release_page_clean(page);
 
 	read_lock(&kvm->mmu_lock);
 	/*
@@ -5002,7 +5000,6 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
 	struct kvm_memory_slot *slot;
 	struct kvm *kvm = vcpu->kvm;
 	int order, rmp_level, ret;
-	struct page *page;
 	bool assigned;
 	kvm_pfn_t pfn;
 	gfn_t gfn;
@@ -5029,13 +5026,12 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
 		return;
 	}
 
-	ret = kvm_gmem_get_pfn(kvm, slot, gfn, &pfn, &page, &order);
+	ret = kvm_gmem_get_pfn(kvm, slot, gfn, &pfn, &order);
 	if (ret) {
 		pr_warn_ratelimited("SEV: Unexpected RMP fault, no backing page for private GPA 0x%llx\n",
 				    gpa);
 		return;
 	}
-	kvm_release_page_unused(page);
 
 	ret = snp_lookup_rmpentry(pfn, &assigned, &rmp_level);
 	if (ret || !assigned) {
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 03bfc92864b6e..502465119ca0c 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2586,13 +2586,11 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
 
 #ifdef CONFIG_KVM_GUEST_MEMFD
 int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
-		     gfn_t gfn, kvm_pfn_t *pfn, struct page **page,
-		     int *max_order);
+		     gfn_t gfn, kvm_pfn_t *pfn, int *max_order);
 #else
 static inline int kvm_gmem_get_pfn(struct kvm *kvm,
 				   struct kvm_memory_slot *slot, gfn_t gfn,
-				   kvm_pfn_t *pfn, struct page **page,
-				   int *max_order)
+				   kvm_pfn_t *pfn, int *max_order)
 {
 	KVM_BUG_ON(1, kvm);
 	return -EIO;
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index b596486d184ca..589762140c3ef 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -751,8 +751,7 @@ static struct folio *__kvm_gmem_get_pfn(struct file *file,
 }
 
 int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
-		     gfn_t gfn, kvm_pfn_t *pfn, struct page **page,
-		     int *max_order)
+		     gfn_t gfn, kvm_pfn_t *pfn, int *max_order)
 {
 	pgoff_t index = kvm_gmem_get_index(slot, gfn);
 	struct folio *folio;
@@ -780,11 +779,7 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 #endif
 
 	folio_unlock(folio);
-
-	if (!r)
-		*page = folio_file_page(folio, index);
-	else
-		folio_put(folio);
+	folio_put(folio);
 
 	return r;
 }

-- 
2.55.0.699.gb54405d56f-goog