diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 7ba92f2ced38..754b662a453c 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6438,6 +6438,11 @@ specified via KVM_CREATE_GUEST_MEMFD.  Currently defined flags:
   ============================ ================================================
   GUEST_MEMFD_FLAG_MMAP        Enable using mmap() on the guest_memfd file
                                descriptor.
+  GUEST_MEMFD_FLAG_INIT_SHARED Make all memory in the file shared during
+                               KVM_CREATE_GUEST_MEMFD (memory files created
+                               without INIT_SHARED will be marked private).
+                               Shared memory can be faulted into host userspace
+                               page tables. Private memory cannot.
   ============================ ================================================
 
 When the KVM MMU performs a PFN lookup to service a guest fault and the backing
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index b1d52d0c56ec..52f6000ab020 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1599,7 +1599,8 @@ struct kvm_memory_attributes {
 #define KVM_MEMORY_ATTRIBUTE_PRIVATE           (1ULL << 3)
 
 #define KVM_CREATE_GUEST_MEMFD	_IOWR(KVMIO,  0xd4, struct kvm_create_guest_memfd)
-#define GUEST_MEMFD_FLAG_MMAP	(1ULL << 0)
+#define GUEST_MEMFD_FLAG_MMAP		(1ULL << 0)
+#define GUEST_MEMFD_FLAG_INIT_SHARED	(1ULL << 1)
 
 struct kvm_create_guest_memfd {
 	__u64 size;
diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
index 3e58bd496104..0de56ce3c4e2 100644
--- a/tools/testing/selftests/kvm/guest_memfd_test.c
+++ b/tools/testing/selftests/kvm/guest_memfd_test.c
@@ -239,8 +239,9 @@ static void test_create_guest_memfd_multiple(struct kvm_vm *vm)
 	close(fd1);
 }
 
-static void test_guest_memfd_flags(struct kvm_vm *vm, uint64_t valid_flags)
+static void test_guest_memfd_flags(struct kvm_vm *vm)
 {
+	uint64_t valid_flags = vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_FLAGS);
 	size_t page_size = getpagesize();
 	uint64_t flag;
 	int fd;
@@ -274,6 +275,10 @@ static void test_guest_memfd(unsigned long vm_type)
 	vm = vm_create_barebones_type(vm_type);
 	flags = vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_FLAGS);
 
+	/* This test doesn't yet support testing mmap() on private memory. */
+	if (!(flags & GUEST_MEMFD_FLAG_INIT_SHARED))
+		flags &= ~GUEST_MEMFD_FLAG_MMAP;
+
 	test_create_guest_memfd_multiple(vm);
 	test_create_guest_memfd_invalid_sizes(vm, flags, page_size);
 
@@ -292,7 +297,7 @@ static void test_guest_memfd(unsigned long vm_type)
 	test_fallocate(fd, page_size, total_size);
 	test_invalid_punch_hole(fd, page_size, total_size);
 
-	test_guest_memfd_flags(vm, flags);
+	test_guest_memfd_flags(vm);
 
 	close(fd);
 	kvm_vm_free(vm);
@@ -334,9 +339,13 @@ static void test_guest_memfd_guest(void)
 	TEST_ASSERT(vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_FLAGS) & GUEST_MEMFD_FLAG_MMAP,
 		    "Default VM type should support MMAP, supported flags = 0x%x",
 		    vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_FLAGS));
+	TEST_ASSERT(vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_FLAGS) & GUEST_MEMFD_FLAG_INIT_SHARED,
+		    "Default VM type should support INIT_SHARED, supported flags = 0x%x",
+		    vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_FLAGS));
 
 	size = vm->page_size;
-	fd = vm_create_guest_memfd(vm, size, GUEST_MEMFD_FLAG_MMAP);
+	fd = vm_create_guest_memfd(vm, size, GUEST_MEMFD_FLAG_MMAP |
+					     GUEST_MEMFD_FLAG_INIT_SHARED);
 	vm_set_user_memory_region2(vm, slot, KVM_MEM_GUEST_MEMFD, gpa, size, NULL, fd, 0);
 
 	mem = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 94bafd6c558c..cf3afba23a6b 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -328,6 +328,9 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
 	if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
 		return VM_FAULT_SIGBUS;
 
+	if (!((u64)inode->i_private & GUEST_MEMFD_FLAG_INIT_SHARED))
+		return VM_FAULT_SIGBUS;
+
 	folio = kvm_gmem_get_folio(inode, vmf->pgoff);
 	if (IS_ERR(folio)) {
 		int err = PTR_ERR(folio);
@@ -525,7 +528,8 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
 	u64 valid_flags = 0;
 
 	if (kvm_arch_supports_gmem_mmap(kvm))
-		valid_flags |= GUEST_MEMFD_FLAG_MMAP;
+		valid_flags |= GUEST_MEMFD_FLAG_MMAP |
+			       GUEST_MEMFD_FLAG_INIT_SHARED;
 
 	if (flags & ~valid_flags)
 		return -EINVAL;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index e3a268757621..5f644ca54af3 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4930,7 +4930,8 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
 		return 1;
 	case KVM_CAP_GUEST_MEMFD_FLAGS:
 		if (!kvm || kvm_arch_supports_gmem_mmap(kvm))
-			return GUEST_MEMFD_FLAG_MMAP;
+			return GUEST_MEMFD_FLAG_MMAP |
+			       GUEST_MEMFD_FLAG_INIT_SHARED;
 
 		return 0;
 #endif