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[54.240.197.234]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48de3f7f8b8sm9365477f8f.35.2026.10.14.02.37.36 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Wed, 14 Oct 2026 02:37:37 -0700 (PDT) From: Fred Griffoul To: Paolo Bonzini , Sean Christopherson , Marc Zyngier , Oliver Upton , Catalin Marinas , Andrew Morton , David Hildenbrand , Alexander Viro , Christian Brauner , Jan Kara , Jason Gunthorpe , Kevin Tian , Joerg Roedel , Will Deacon , Robin Murphy , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H . Peter Anvin" , Jonathan Corbet , Shuah Khan , Shuah Khan Cc: David Woodhouse , Ackerley Tng , "Aneesh Kumar K . V" , Lorenzo Stoakes , "Liam R . Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Joey Gouly , Suzuki K Poulose , Zenghui Yu , Steffen Eiden , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, iommu@lists.linux.dev, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, linux-doc@vger.kernel.org Subject: [RFC PATCH v2 02/11] KVM: guest_memfd: Let get_pfn() report read-only pages Date: Wed, 14 Oct 2026 09:37:24 +0000 Message-ID: <20261014093733.90312-3-griffoul@gmail.com> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20261014093733.90312-1-griffoul@gmail.com> References: <20261014093733.90312-1-griffoul@gmail.com> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Fred Griffoul 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 --- 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