[RFC PATCH 1/6] KVM: guest_memfd: Add a writable result to get_pfn()
Fred Griffoul
griffoul at gmail.com
Mon Oct 5 02:55:47 PDT 2026
From: Fred Griffoul <fgriffo at amazon.co.uk>
A guest_memfd backing has no way to tell KVM that a page must stay
mapped but must not be written by the guest. get_pfn() returns only a
frame, and KVM takes write access from the memslot flags.
KVM_MEM_READONLY is not an option: KVM refuses it on guest_memfd slots,
because it emulates a write to a read-only slot as MMIO, which private
memory cannot support.
Add a writable output to get_pfn(). KVM sets it to true before the
call, and the backing may clear it. When it is false, the x86 and arm64
stage-2 fault paths map the page without write permission. A guest
write to that page then exits to userspace as a memory fault; KVM first
releases any page that the backing returned. Callers that do not map
the page pass NULL.
The result applies to stage-2 mappings only. When KVM writes guest
memory through the memslot's host address, the permissions of the host
mapping apply.
Signed-off-by: Fred Griffoul <fgriffo at amazon.co.uk>
---
arch/arm64/kvm/mmu.c | 13 ++++++++-----
arch/arm64/kvm/nested.c | 10 +++++++---
arch/x86/kvm/mmu/mmu.c | 18 ++++++++++++++++--
arch/x86/kvm/svm/sev.c | 4 ++--
include/linux/kvm_host.h | 11 ++++++++---
samples/kvm/gmem_provider.c | 3 ++-
virt/kvm/guest_memfd.c | 13 ++++++++-----
7 files changed, 51 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..9ad1fa029835 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -1411,12 +1411,16 @@ 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);
+ if (ret || (write_fault && !writable)) {
kvm_prepare_memory_fault_exit(vcpu, vt->wr.pa, PAGE_SIZE,
write_fault, false, false);
- return ret;
+ if (!ret)
+ kvm_release_faultin_page(vcpu->kvm, page, true, false);
+ return ret ?: -EFAULT;
}
+ vt->wr.pw &= writable;
}
scoped_guard(write_lock, &vcpu->kvm->mmu_lock) {
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 234d0a95abf5..17ac2bfdc164 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);
+ /*
+ * The memory's owner has the final say on writability, on top of the
+ * memslot flag: a page it reports read-only 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 125779c82bc4..983e19f7dbd2 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -4063,7 +4063,7 @@ 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, NULL))
return;
/*
@@ -4996,7 +4996,7 @@ 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 04fa0cb126f6..f84f3ab44acf 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -660,9 +660,14 @@ struct kvm_gmem_ops {
struct kvm_memory_slot *slot, loff_t offset);
void (*unbind)(struct file *file, struct kvm *kvm,
struct kvm_memory_slot *slot);
+ /*
+ * @writable: [out] clear to have KVM map the page read-only; a guest
+ * write then exits as a memory fault. NULL if the caller does not care.
+ */
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);
@@ -2651,12 +2656,12 @@ 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/samples/kvm/gmem_provider.c b/samples/kvm/gmem_provider.c
index 9728f5a8029b..75197c088762 100644
--- a/samples/kvm/gmem_provider.c
+++ b/samples/kvm/gmem_provider.c
@@ -157,7 +157,8 @@ static int gmem_max_order(struct gmem_info *info, gfn_t gfn, unsigned long index
static int gmem_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)
{
struct gmem_info *info = to_gmem_info(file);
pgoff_t index = gfn - slot->base_gfn + slot->gmem.pgoff;
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index d284bb70fe05..0f4bf2cc5e8e 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);
@@ -988,7 +988,7 @@ 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;
@@ -1016,7 +1016,7 @@ 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;
@@ -1029,7 +1029,10 @@ 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);
@@ -1093,7 +1096,7 @@ 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, NULL);
if (ret)
return ret;
--
2.47.3
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