[RFC PATCH v2 02/11] KVM: guest_memfd: Let get_pfn() report read-only pages
Fred Griffoul
griffoul at gmail.com
Wed Oct 14 02:37:24 PDT 2026
From: Fred Griffoul <fgriffo at amazon.co.uk>
A guest_memfd backing cannot map a page read-only for the guest:
get_pfn() returns only a frame, and KVM_MEM_READONLY is refused on
guest_memfd slots. So a hypervisor cannot give a guest a page it may
read but not write.
get_pfn() gets a writable output, which it may clear. The x86 and arm64
fault paths then map the page without write permission, and a guest
write exits with KVM_EXIT_MEMORY_FAULT. A read-only answer applies to
the whole block that max_order allows.
Callers that hand the page to something that writes it must refuse a
read-only page: the arm64 VNCR page and the SEV-SNP VMSA. populate() has
no writable output, so a backing cannot report read-only state through
it. The output does not cover KVM's own writes through the slot's host
address.
The memory provider backing, later in this series, is the first user.
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Fred Griffoul <fgriffo at amazon.co.uk>
---
arch/arm64/kvm/mmu.c | 13 ++++++++-----
arch/arm64/kvm/nested.c | 16 ++++++++++++----
arch/x86/kvm/mmu/mmu.c | 18 ++++++++++++++++--
arch/x86/kvm/svm/sev.c | 13 +++++++++++--
include/linux/kvm_host.h | 16 +++++++++++++---
virt/kvm/guest_memfd.c | 21 ++++++++++++++++-----
6 files changed, 76 insertions(+), 21 deletions(-)
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 6c941aaa10c6..32e591edc69d 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1607,7 +1607,7 @@ struct kvm_s2_fault_desc {
static int gmem_abort(const struct kvm_s2_fault_desc *s2fd)
{
- bool write_fault, exec_fault;
+ bool write_fault, exec_fault, writable;
bool perm_fault = kvm_vcpu_trap_is_permission_fault(s2fd->vcpu);
enum kvm_pgtable_walk_flags flags = KVM_PGTABLE_WALK_SHARED;
enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R;
@@ -1641,14 +1641,17 @@ 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);
- if (ret) {
+ ret = kvm_gmem_get_pfn(kvm, s2fd->memslot, gfn, &pfn, &page, NULL,
+ &writable);
+ if (ret || (write_fault && !writable)) {
kvm_prepare_memory_fault_exit(s2fd->vcpu, s2fd->fault_ipa, PAGE_SIZE,
write_fault, exec_fault, false);
- return ret;
+ if (!ret)
+ kvm_release_faultin_page(kvm, page, true, false);
+ return ret ?: -EFAULT;
}
- if (!(s2fd->memslot->flags & KVM_MEM_READONLY))
+ if (!(s2fd->memslot->flags & KVM_MEM_READONLY) && writable)
prot |= KVM_PGTABLE_PROT_W;
if (s2fd->nested)
diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
index fb54f6dad995..fa86705df949 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -1411,11 +1411,19 @@ 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);
- if (ret) {
+ ret = kvm_gmem_get_pfn(vcpu->kvm, memslot, gfn, &pfn, &page, NULL,
+ &writable);
+ /*
+ * The VNCR page is always written, so a read-only page cannot
+ * back it. Exit on the first access, even a read, rather than
+ * map it read-only and fault on the next write.
+ */
+ if (ret || !writable) {
kvm_prepare_memory_fault_exit(vcpu, vt->wr.pa, PAGE_SIZE,
- write_fault, false, false);
- return ret;
+ true, false, false);
+ if (!ret)
+ kvm_release_faultin_page(vcpu->kvm, page, true, false);
+ return ret ?: -EFAULT;
}
}
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 234d0a95abf5..f689ef5c2b46 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4612,6 +4612,7 @@ static void kvm_mmu_finish_page_fault(struct kvm_vcpu *vcpu,
static int kvm_mmu_faultin_pfn_gmem(struct kvm_vcpu *vcpu,
struct kvm_page_fault *fault)
{
+ bool writable;
int max_order, r;
if (!kvm_slot_has_gmem(fault->slot)) {
@@ -4620,13 +4621,26 @@ 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);
+ &fault->refcounted_page, &max_order, &writable);
if (r) {
kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
return r;
}
- fault->map_writable = !(fault->slot->flags & KVM_MEM_READONLY);
+ /*
+ * get_pfn() may clear writable, on top of the memslot's read-only flag:
+ * a read-only page is mapped read-only, and a guest write to it exits to
+ * userspace rather than being installed.
+ */
+ fault->map_writable = !(fault->slot->flags & KVM_MEM_READONLY) &&
+ writable;
+ if (fault->write && !fault->map_writable) {
+ kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
+ kvm_release_faultin_page(vcpu->kvm, fault->refcounted_page,
+ true, false);
+ fault->refcounted_page = NULL;
+ return -EFAULT;
+ }
fault->max_level = kvm_max_level_for_order(max_order);
return RET_PF_CONTINUE;
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 427229347876..e121441bc89c 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3988,6 +3988,7 @@ static void sev_snp_init_protected_guest_state(struct kvm_vcpu *vcpu)
struct vcpu_svm *svm = to_svm(vcpu);
struct kvm_memory_slot *slot;
struct page *page;
+ bool writable;
kvm_pfn_t pfn;
gfn_t gfn;
@@ -4025,9 +4026,17 @@ static void sev_snp_init_protected_guest_state(struct kvm_vcpu *vcpu)
* 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, &page, NULL,
+ &writable))
return;
+ /* The CPU writes the VMSA on every VMRUN: a read-only page cannot be one. */
+ if (!writable) {
+ if (page)
+ kvm_release_page_clean(page);
+ return;
+ }
+
/*
* From this point forward, the VMSA will always be a guest-mapped page
* rather than the initial one allocated by KVM in svm->sev_es.vmsa. In
@@ -4958,7 +4967,7 @@ 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, &page, &order, NULL);
if (ret) {
pr_warn_ratelimited("SEV: Unexpected RMP fault, no backing page for private GPA 0x%llx\n",
gpa);
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index b33d2f475042..a15abd0f7fb7 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -649,6 +649,15 @@ static inline bool kvm_slot_has_gmem(const struct kvm_memory_slot *slot)
* is responsible for put_page() after use. If *page is left NULL the PFN
* is treated as non-refcounted, and its lifetime is owned by the
* implementation across bind()/unbind().
+ * - *writable, when the caller passes it, is true on entry. Clear it to
+ * have KVM map the page read-only; a guest write then exits as a memory
+ * fault. It covers the whole block that *max_order allows, so that block
+ * must be all writable or all read-only. It applies to stage-2 mappings
+ * only, not to KVM's own writes through the memslot's host address. A
+ * caller that hands the page to hardware that writes it passes @writable
+ * and refuses a read-only page. Only a caller that never writes the page
+ * itself passes NULL. populate() has no writable result, so KVM cannot
+ * learn read-only state through it; it is used only to fill a page.
*
* Memory intended to back guest RAM MUST be reported as E820_TYPE_RAM by the
* host so KVM maps it write-back (and applies the memory-encryption bit on
@@ -662,7 +671,8 @@ struct kvm_gmem_ops {
struct kvm_memory_slot *slot);
int (*get_pfn)(struct file *file, 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, struct page **page, int *max_order,
+ bool *writable);
int (*populate)(struct file *file, struct kvm *kvm,
struct kvm_memory_slot *slot, gfn_t gfn,
kvm_pfn_t *pfn, struct page *src_page, int order);
@@ -2642,12 +2652,12 @@ 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);
+ int *max_order, bool *writable);
#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)
+ int *max_order, bool *writable)
{
KVM_BUG_ON(1, kvm);
return -EIO;
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 057f90eb835a..c9595cc9d56e 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -624,7 +624,7 @@ static void kvm_gmem_native_unbind(struct file *slot_file, struct kvm *kvm,
static int kvm_gmem_native_get_pfn(struct file *file, struct kvm *kvm,
struct kvm_memory_slot *slot, gfn_t gfn,
kvm_pfn_t *pfn, struct page **page,
- int *max_order);
+ int *max_order, bool *writable);
static void kvm_gmem_native_release(struct file *file);
static int kvm_gmem_native_mmap(struct file *file,
struct vm_area_struct *vma);
@@ -936,7 +936,7 @@ static struct folio *__kvm_gmem_get_pfn(struct file *file,
static int kvm_gmem_native_get_pfn(struct file *file, struct kvm *kvm,
struct kvm_memory_slot *slot, gfn_t gfn,
kvm_pfn_t *pfn, struct page **page,
- int *max_order)
+ int *max_order, bool *writable)
{
pgoff_t index = kvm_gmem_get_index(slot, gfn);
struct folio *folio;
@@ -964,7 +964,7 @@ static int kvm_gmem_native_get_pfn(struct file *file, struct kvm *kvm,
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)
+ int *max_order, bool *writable)
{
const struct kvm_gmem_ops *ops;
@@ -977,7 +977,10 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
return -EFAULT;
*page = NULL;
- return ops->get_pfn(file, kvm, slot, gfn, pfn, page, max_order);
+ if (writable)
+ *writable = true;
+ return ops->get_pfn(file, kvm, slot, gfn, pfn, page, max_order,
+ writable);
}
EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_gmem_get_pfn);
@@ -1028,6 +1031,7 @@ static int kvm_gmem_populate_one(const struct kvm_gmem_ops *ops,
void *opaque)
{
struct page *ignored_page = NULL;
+ bool writable = true;
kvm_pfn_t pfn;
int ret;
@@ -1041,10 +1045,17 @@ static int kvm_gmem_populate_one(const struct kvm_gmem_ops *ops,
src_page, 0);
else
ret = ops->get_pfn(file, kvm, slot, gfn, &pfn,
- &ignored_page, NULL);
+ &ignored_page, NULL, &writable);
if (ret)
return ret;
+ /* post_populate() writes the page, so it cannot be read-only. */
+ if (!writable) {
+ if (ignored_page)
+ put_page(ignored_page);
+ return -EPERM;
+ }
+
ret = post_populate(kvm, gfn, pfn, src_page, opaque);
/*
--
2.47.3
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