[PATCH v2 0/4] Stop returning struct page from guest_memfd PFN lookup

[PATCH v2 0/4] Stop returning struct page from guest_memfd PFN lookup

[Ackerley Tng]

KVM currently expects kvm_gmem_get_pfn() to return a refcounted struct
page. Callers (such as x86 TDP MMU, arm64 Stage-2 fault handler, and SEV-SNP
VMSA / RMP handlers) hold this refcount across page fault handling.

Holding a page refcount 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 users of guest_memfd
within KVM only need to respect the MMU invalidation protocol to safely rely on
guest_memfd to ensure page presence.

This series first prepares the SEV-SNP handlers by treating unassigned RMP
entries as benign races on PSMASH failure (which can occur on concurrent
truncation) and dropping page references early in the RMP fault and VMSA reload
paths. It then updates kvm_gmem_get_pfn() to drop the folio reference internally
and stop returning a struct page pointer across x86 and arm64.

Removing struct page from kvm_gmem_get_pfn() also moves KVM closer toward
supporting memory backends that are not backed by struct page.

I really want in-place conversions to merge in time for 7.4 and so I went
ahead to try this, building off Sean's sample code [1].

[1] https://lore.kernel.org/all/an5RJYTwlYeym--O@google.com/

I also split the patch up so it's easier to review :)

Changes from v1:

+ sev_handle_rmp_fault() does need to adopt the MMU invalidation protocol,
  please see reason in patch description.

v1: https://patch.msgid.link/20260818-gmem-no-return-page-v1-0-4f8d939efdbc@google.com

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
Ackerley Tng (2):
      KVM: SEV: Treat unassigned RMP entry as benign race on PSMASH failure
      KVM: SEV: Drop page refcount early in VMSA reload

Sean Christopherson (2):
      KVM: SEV: Drop page refcount early during RMP fault handling
      KVM: guest_memfd: Stop returning struct page from PFN lookup

 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   | 45 +++++++++++++++++++++++++--------------------
 include/linux/kvm_host.h |  6 ++----
 virt/kvm/guest_memfd.c   |  9 ++-------
 6 files changed, 34 insertions(+), 36 deletions(-)
---
base-commit: 1b731e5ded480bd1e5546aed35584238661ce72e
change-id: 20260818-gmem-no-return-page-614927a29f97

Best regards,
--  
Ackerley Tng <ackerleytng@google.com>

[PATCH v2 1/4] KVM: SEV: Treat unassigned RMP entry as benign race on PSMASH failure

[Ackerley Tng]

When handling an RMP fault, KVM attempts to split a 2MB page via PSMASH.
If PSMASH fails, the only expected return value is FAIL_BADADDR, which does
not distinguish the reason for the bad address. Hence, another RMP entry
lookup is required to determine whether the failure was benign.

Specifically, KVM re-checks the RMP entry to determine if another CPU raced
and already smashed the entry into 4KB pages.

A concurrent operation (such as guest_memfd truncation or hole punching)
can also race and transition the page to shared, removing the page from the
RMP table and causing PSMASH to fail. This can happen even if the page is
still referenced by KVM, because guest_memfd reclaim transitions the RMP
entry to shared when the folio is removed from the page cache.

Treat an unassigned RMP entry as an expected race when re-checking after a
failed PSMASH, and skip logging an error warning.

Fixes: c63cf135cc99 ("KVM: SEV: Add support to handle RMP nested page faults")
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 arch/x86/kvm/svm/sev.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index fcb41dfde4c02..b2738362a928b 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -5074,10 +5074,11 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
 		/*
 		 * Look it up again. If it's 4K now then the PSMASH may have
 		 * raced with another process and the issue has already resolved
-		 * itself.
+		 * itself. If it's not assigned, then this must have raced with
+		 * another process that made this page shared.
 		 */
 		if (!snp_lookup_rmpentry(pfn, &assigned, &rmp_level) &&
-		    assigned && rmp_level == PG_LEVEL_4K)
+		    ((assigned && rmp_level == PG_LEVEL_4K) || !assigned))
 			goto out;
 
 		pr_warn_ratelimited("SEV: Unable to split RMP entry for GPA 0x%llx PFN 0x%llx ret %d\n",

-- 
2.55.0.699.gb54405d56f-goog

[PATCH v2 2/4] KVM: SEV: Drop page refcount early during RMP fault handling

[Ackerley Tng]

From: Sean Christopherson <seanjc@google.com>

When handling an RMP fault, KVM retrieves the PFN for a private GPA from
guest_memfd.

Drop the page reference immediately after retrieving the PFN instead of
holding it across the entire handler so that the later patch can follow up
with completely not returning refcounted pages from kvm_gmem_get_pfn().

On a first look, existing RMP table handling (psmash and checking for
errors) might seem like it works fine, since truncation of the page from
guest_memfd would have called rmp_make_shared() and removed the PFN from
the RMP table. However, that is insufficient since a freed page may already
be used in a different SNP VM.

Hence, adopt the MMU invalidation protocol to guard committing anything
based on the PFN.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 arch/x86/kvm/svm/sev.c | 39 ++++++++++++++++++++++++---------------
 1 file changed, 24 insertions(+), 15 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index b2738362a928b..b34b11d7f8fad 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -5003,6 +5003,7 @@ 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;
+	unsigned long mmu_seq;
 	struct page *page;
 	bool assigned;
 	kvm_pfn_t pfn;
@@ -5030,18 +5031,22 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
 		return;
 	}
 
+	mmu_seq = kvm->mmu_invalidate_seq;
+	smp_rmb();
+
 	ret = kvm_gmem_get_pfn(kvm, slot, gfn, &pfn, &page, &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) {
 		pr_warn_ratelimited("SEV: Unexpected RMP fault, no assigned RMP entry found for GPA 0x%llx PFN 0x%llx error %d\n",
 				    gpa, pfn, ret);
-		goto out_no_trace;
+		return;
 	}
 
 	/*
@@ -5069,27 +5074,31 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
 	if (rmp_level == PG_LEVEL_4K)
 		goto out;
 
-	ret = snp_rmptable_psmash(pfn);
-	if (ret) {
-		/*
-		 * Look it up again. If it's 4K now then the PSMASH may have
-		 * raced with another process and the issue has already resolved
-		 * itself. If it's not assigned, then this must have raced with
-		 * another process that made this page shared.
-		 */
-		if (!snp_lookup_rmpentry(pfn, &assigned, &rmp_level) &&
-		    ((assigned && rmp_level == PG_LEVEL_4K) || !assigned))
+	scoped_guard(read_lock, &kvm->mmu_lock) {
+		if (mmu_invalidate_retry_gfn(kvm, mmu_seq, gfn))
 			goto out;
 
-		pr_warn_ratelimited("SEV: Unable to split RMP entry for GPA 0x%llx PFN 0x%llx ret %d\n",
-				    gpa, pfn, ret);
+		ret = snp_rmptable_psmash(pfn);
+		if (ret) {
+			/*
+			 * Look it up again. If it's 4K now then the PSMASH may
+			 * have raced with another process and the issue has
+			 * already resolved itself. If it's not assigned, then
+			 * this must have raced with another process that made
+			 * this page shared.
+			 */
+			if (!snp_lookup_rmpentry(pfn, &assigned, &rmp_level) &&
+			    ((assigned && rmp_level == PG_LEVEL_4K) || !assigned))
+				goto out;
+
+			pr_warn_ratelimited("SEV: Unable to split RMP entry for GPA 0x%llx PFN 0x%llx ret %d\n",
+					    gpa, pfn, ret);
+		}
 	}
 
 	kvm_zap_gfn_range(kvm, gfn, gfn + PTRS_PER_PMD);
 out:
 	trace_kvm_rmp_fault(vcpu, gpa, pfn, error_code, rmp_level, ret);
-out_no_trace:
-	kvm_release_page_unused(page);
 }
 
 static bool is_pfn_range_shared(kvm_pfn_t start, kvm_pfn_t end)

-- 
2.55.0.699.gb54405d56f-goog

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

[Ackerley Tng]

When reloading the guest VMSA for an SEV-SNP vCPU, KVM retrieves the PFN
from guest_memfd.

Drop the page reference immediately after retrieving the PFN instead of
holding it across MMU lock acquisition in preparation for a follow-up patch
to stop returning page pointers from guest_memfd PFN lookups.

This is safe because the page's validity and presence are governed by KVM's
MMU invalidation protocol rather than the page reference.

No functional change intended.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 arch/x86/kvm/svm/sev.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index b34b11d7f8fad..d3d620bc04dce 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -4062,6 +4062,7 @@ static void __sev_snp_reload_vmsa(struct kvm_vcpu *vcpu, gpa_t gpa)
 	 */
 	if (kvm_gmem_get_pfn(vcpu->kvm, slot, gfn, &pfn, &page, NULL))
 		return;
+	kvm_release_page_clean(page);
 
 	read_lock(&kvm->mmu_lock);
 	/*
@@ -4076,8 +4077,6 @@ static void __sev_snp_reload_vmsa(struct kvm_vcpu *vcpu, gpa_t gpa)
 	else
 		svm->vmcb->control.vmsa_pa = pfn_to_hpa(pfn);
 	read_unlock(&kvm->mmu_lock);
-
-	kvm_release_page_clean(page);
 }
 
 /*

-- 
2.55.0.699.gb54405d56f-goog

[PATCH v2 4/4] KVM: guest_memfd: Stop returning struct page from PFN lookup

[Ackerley Tng]

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 d3d620bc04dce..32db979daaaf0 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);
 	/*
@@ -5003,7 +5001,6 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
 	struct kvm *kvm = vcpu->kvm;
 	int order, rmp_level, ret;
 	unsigned long mmu_seq;
-	struct page *page;
 	bool assigned;
 	kvm_pfn_t pfn;
 	gfn_t gfn;
@@ -5033,13 +5030,12 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
 	mmu_seq = kvm->mmu_invalidate_seq;
 	smp_rmb();
 
-	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

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

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [Critical] A preexisting vulnerability in `sev_gmem_make_shared` allows a malicious SEV-SNP guest to cause a host kernel panic by transitioning a `guest_memfd` page to VMSA state. When the page is subsequently freed, KVM fails to reclaim it and returns the still-firmware-owned page to the host page allocator.
--

--- Patch [3]: [PATCH v2 3/4] KVM: SEV: Drop page refcount early in VMSA reload ---
commit 597ff1c52e82911967f2122ff1d77aa79a4a2d9b
Author: Ackerley Tng <ackerleytng@google.com>

KVM: SEV: Drop page refcount early in VMSA reload

This commit modifies the guest VMSA reload path for SEV-SNP vCPUs to drop
the page reference immediately after retrieving the PFN from guest_memfd.
This avoids holding the reference across MMU lock acquisition and prepares
for future changes. The page's presence remains governed by KVM's MMU
invalidation protocol.

> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index b34b11d7f8fad..d3d620bc04dce 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -4062,6 +4062,7 @@ static void __sev_snp_reload_vmsa(struct kvm_vcpu *vcpu, gpa_t gpa)
>  	 */
>  	if (kvm_gmem_get_pfn(vcpu->kvm, slot, gfn, &pfn, &page, NULL))
>  		return;
> +	kvm_release_page_clean(page);
>  
>  	read_lock(&kvm->mmu_lock);
>  	/*
> @@ -4076,8 +4077,6 @@ static void __sev_snp_reload_vmsa(struct kvm_vcpu *vcpu, gpa_t gpa)
>  	else
>  		svm->vmcb->control.vmsa_pa = pfn_to_hpa(pfn);
>  	read_unlock(&kvm->mmu_lock);
> -
> -	kvm_release_page_clean(page);
>  }

[Severity: Critical]
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.

Because the error is bypassed without calling snp_leak_pages(), the
firmware-owned page is returned to the host page allocator.

Could this lead to an RMP violation and a host kernel panic when the host
later attempts to access the freed page?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260818-gmem-no-return-page-v2-0-5298f42d49bb@google.com?part=3

Re: [PATCH v2 2/4] KVM: SEV: Drop page refcount early during RMP fault handling

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Dropping the page refcount early introduces a race condition where a concurrent `guest_memfd` invalidation can trigger a false-positive 'Unexpected RMP fault' kernel warning.
--

--- Patch [2]: [PATCH v2 2/4] KVM: SEV: Drop page refcount early during RMP fault handling ---
commit 7d0f356d8592ce1804052b0fdb61fccf3a11ecaa
Author: Sean Christopherson <seanjc@google.com>

KVM: SEV: Drop page refcount early during RMP fault handling

When handling an RMP fault, KVM retrieves the PFN for a private GPA from
guest_memfd. This patch drops the page reference immediately after retrieving
the PFN instead of holding it across the entire handler, and adopts the MMU
invalidation protocol to guard committing anything based on the PFN.

> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index b2738362a928b..b34b11d7f8fad 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c

[ ... ]

> @@ -5030,18 +5031,22 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
>  		return;
>  	}
>  
> +	mmu_seq = kvm->mmu_invalidate_seq;
> +	smp_rmb();
> +
>  	ret = kvm_gmem_get_pfn(kvm, slot, gfn, &pfn, &page, &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) {
>  		pr_warn_ratelimited("SEV: Unexpected RMP fault, no assigned RMP entry found for GPA 0x%llx PFN 0x%llx error %d\n",
>  				    gpa, pfn, ret);
> -		goto out_no_trace;
> +		return;
>  	}

[Severity: Medium]
Since the page reference is dropped before this lookup in
sev_handle_rmp_fault(), could a concurrent guest_memfd hole-punch drop
the file's reference to the page and free it?

If the page is freed, the RMP entry would transition to shared/unassigned.
Could this then cause snp_lookup_rmpentry() to see the page as unassigned
and incorrectly emit this warning before the mmu_invalidate_retry_gfn()
check happens?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260818-gmem-no-return-page-v2-0-5298f42d49bb@google.com?part=2

Re: [PATCH v2 4/4] KVM: guest_memfd: Stop returning struct page from PFN lookup

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [Critical] Concurrent execution of KVM_SEV_SNP_LAUNCH_UPDATE and a vCPU page fault triggers a race condition leading to a host kernel panic.
--

--- Patch [4]: [PATCH v2 4/4] KVM: guest_memfd: Stop returning struct page from PFN lookup ---
commit 766989bc1910148a83bffa481ae22b1d1eb71f01
Author: Sean Christopherson <seanjc@google.com>

KVM: guest_memfd: Stop returning struct page from PFN lookup

This commit stops returning a refcounted struct page from guest_memfd PFN
lookups, dropping the folio reference immediately instead. This prevents
concurrent guest page faults from holding elevated refcounts that would
interfere with in-place memory conversions between shared and private states.

Link: https://lore.kernel.org/all/anZ4W9o5pTWIEgMY@yzhao56-desk.sh.intel.com/

> 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

[ ... ]

> @@ -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;
>  }

[Severity: Critical]
This is a pre-existing issue, but is it possible for a concurrent page
fault to trigger a host kernel panic via a hardware RMP fault here?

Looking at kvm_gmem_get_pfn() in virt/kvm/guest_memfd.c, if the folio is
not up to date, it zeroes the page and marks it up to date:

    if (!folio_test_uptodate(folio)) {
        clear_highpage(folio_page(folio, 0));
        folio_mark_uptodate(folio);
    }

However, in the KVM_SEV_SNP_LAUNCH_UPDATE path, __kvm_gmem_populate()
fetches a folio, unlocks it, and calls post_populate() (which maps to
sev_gmem_post_populate() and calls rmp_make_private()) before finally
marking the folio up to date.

If a malicious userspace process races a vCPU page fault against
KVM_SEV_SNP_LAUNCH_UPDATE, could the vCPU thread running kvm_gmem_get_pfn()
acquire the lock on the not-yet-uptodate folio and call clear_highpage()
after rmp_make_private() has already executed?

If clear_highpage() writes to the page after it has been made RMP-private,
this would trigger a fatal hardware RMP fault on the host CPU. If it writes
just before, it appears it could silently corrupt the encrypted firmware
payload.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260818-gmem-no-return-page-v2-0-5298f42d49bb@google.com?part=4