[PATCH RFC v1 2/2] drivers/virt: add vmgenid driver for reinitializing RNG

Jason A. Donenfeld
2022-02-23T13:13:10Z

VM Generation ID is a feature from Microsoft, described at
<https://go.microsoft.com/fwlink/?LinkId=260709>, and supported by
Hyper-V and QEMU. Its usage is described in Microsoft's RNG whitepaper,
<https://aka.ms/win10rng>, as:

If the OS is running in a VM, there is a problem that most
hypervisors can snapshot the state of the machine and later rewind
the VM state to the saved state. This results in the machine running
a second time with the exact same RNG state, which leads to serious
security problems. To reduce the window of vulnerability, Windows
10 on a Hyper-V VM will detect when the VM state is reset, retrieve
a unique (not random) value from the hypervisor, and reseed the root
RNG with that unique value. This does not eliminate the
vulnerability, but it greatly reduces the time during which the RNG
system will produce the same outputs as it did during a previous
instantiation of the same VM state.

Linux has the same issue, and given that vmgenid is supported already by
multiple hypervisors, we can implement more or less the same solution.
So this commit wires up the vmgenid ACPI notification to the RNG's newly
added add_vmfork_randomness() function.

This driver builds on prior work from Adrian Catangiu at Amazon, and it
is my hope that that team can resume maintenance of this driver.

Cc: Adrian Catangiu <adrian@parity.io>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
drivers/virt/Kconfig | 8 +++
drivers/virt/Makefile | 1 +
drivers/virt/vmgenid.c | 133 +++++++++++++++++++++++++++++++++++++++++
3 files changed, 142 insertions(+)
create mode 100644 drivers/virt/vmgenid.c

diff --git a/drivers/virt/Kconfig b/drivers/virt/Kconfig
index 8061e8ef449f..e7f9c1bca02b 100644
--- a/drivers/virt/Kconfig
+++ b/drivers/virt/Kconfig
@@ -13,6 +13,14 @@ menuconfig VIRT_DRIVERS

if VIRT_DRIVERS

+config VMGENID
+ bool "Virtual Machine Generation ID driver"
+ default y
+ depends on ACPI
+ help
+ Say Y here to use the hypervisor provided Virtual Machine Generation ID
+ to reseed the RNG when the VM is cloned.
+
config FSL_HV_MANAGER
tristate "Freescale hypervisor management driver"
depends on FSL_SOC
diff --git a/drivers/virt/Makefile b/drivers/virt/Makefile
index 3e272ea60cd9..108d0ffcc9aa 100644
--- a/drivers/virt/Makefile
+++ b/drivers/virt/Makefile
@@ -4,6 +4,7 @@
#

obj-$(CONFIG_FSL_HV_MANAGER) += fsl_hypervisor.o
+obj-$(CONFIG_VMGENID) += vmgenid.o
obj-y += vboxguest/

obj-$(CONFIG_NITRO_ENCLAVES) += nitro_enclaves/
diff --git a/drivers/virt/vmgenid.c b/drivers/virt/vmgenid.c
new file mode 100644
index 000000000000..11ed7f668bb1
--- /dev/null
+++ b/drivers/virt/vmgenid.c
@@ -0,0 +1,133 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Virtual Machine Generation ID driver
+ *
+ * Copyright (C) 2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ * Copyright (C) 2020 Amazon. All rights reserved.
+ * Copyright (C) 2018 Red Hat Inc. All rights reserved.
+ */
+
+#define pr_fmt(fmt) "vmgenid: " fmt
+
+#include <linux/acpi.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/random.h>
+#include <linux/uuid.h>
+
+#define DEV_NAME "vmgenid"
+ACPI_MODULE_NAME(DEV_NAME);
+
+static struct {
+ uuid_t uuid;
+ void *uuid_iomap;
+} vmgenid_data;
+
+static int vmgenid_acpi_map(acpi_handle handle)
+{
+ phys_addr_t phys_addr = 0;
+ struct acpi_buffer buffer = { ACPI_ALLOCATE_BUFFER, NULL };
+ acpi_status status;
+ union acpi_object *pss;
+ union acpi_object *element;
+ int i;
+
+ status = acpi_evaluate_object(handle, "ADDR", NULL, &buffer);
+ if (ACPI_FAILURE(status)) {
+ ACPI_EXCEPTION((AE_INFO, status, "Evaluating ADDR"));
+ return -ENODEV;
+ }
+ pss = buffer.pointer;
+ if (!pss || pss->type != ACPI_TYPE_PACKAGE || pss->package.count != 2)
+ return -EINVAL;
+
+ for (i = 0; i < pss->package.count; ++i) {
+ element = &pss->package.elements[i];
+ if (element->type != ACPI_TYPE_INTEGER)
+ return -EINVAL;
+ phys_addr |= element->integer.value << i * 32;
+ }
+
+ vmgenid_data.uuid_iomap = acpi_os_map_memory(phys_addr, sizeof(vmgenid_data.uuid));
+ if (!vmgenid_data.uuid_iomap) {
+ pr_err("failed to map memory at %pa, size %zu\n",
+ &phys_addr, sizeof(vmgenid_data.uuid));
+ return -ENOMEM;
+ }
+
+ memcpy_fromio(&vmgenid_data.uuid, vmgenid_data.uuid_iomap, sizeof(vmgenid_data.uuid));
+
+ return 0;
+}
+
+static int vmgenid_acpi_add(struct acpi_device *device)
+{
+ int ret;
+
+ if (!device)
+ return -EINVAL;
+ ret = vmgenid_acpi_map(device->handle);
+ if (ret < 0) {
+ pr_err("failed to map acpi device: %d\n", ret);
+ return ret;
+ }
+ device->driver_data = &vmgenid_data;
+ add_device_randomness(&vmgenid_data.uuid, sizeof(vmgenid_data.uuid));
+ return 0;
+}
+
+static int vmgenid_acpi_remove(struct acpi_device *device)
+{
+ if (!device || acpi_driver_data(device) != &vmgenid_data)
+ return -EINVAL;
+ device->driver_data = NULL;
+ if (vmgenid_data.uuid_iomap)
+ acpi_os_unmap_memory(vmgenid_data.uuid_iomap, sizeof(vmgenid_data.uuid));
+ vmgenid_data.uuid_iomap = NULL;
+ return 0;
+}
+
+static void vmgenid_acpi_notify(struct acpi_device *device, u32 event)
+{
+ uuid_t old_uuid = vmgenid_data.uuid;
+
+ if (!device || acpi_driver_data(device) != &vmgenid_data)
+ return;
+ memcpy_fromio(&vmgenid_data.uuid, vmgenid_data.uuid_iomap, sizeof(vmgenid_data.uuid));
+ if (!memcmp(&old_uuid, &vmgenid_data.uuid, sizeof(vmgenid_data.uuid)))
+ return;
+ add_vmfork_randomness(&vmgenid_data.uuid, sizeof(vmgenid_data.uuid));
+}
+
+static const struct acpi_device_id vmgenid_ids[] = {
+ {"VMGENID", 0},
+ {"QEMUVGID", 0},
+ {"", 0},
+};
+
+static struct acpi_driver acpi_vmgenid_driver = {
+ .name = "vm_generation_id",
+ .ids = vmgenid_ids,
+ .owner = THIS_MODULE,
+ .ops = {
+ .add = vmgenid_acpi_add,
+ .remove = vmgenid_acpi_remove,
+ .notify = vmgenid_acpi_notify,
+ }
+};
+
+static int __init vmgenid_init(void)
+{
+ return acpi_bus_register_driver(&acpi_vmgenid_driver);
+}
+
+static void __exit vmgenid_exit(void)
+{
+ acpi_bus_unregister_driver(&acpi_vmgenid_driver);
+}
+
+module_init(vmgenid_init);
+module_exit(vmgenid_exit);
+
+MODULE_DESCRIPTION("Virtual Machine Generation ID");
+MODULE_LICENSE("GPL");
--
2.35.1