[PATCH v5 3/3] virt: vmgenid: introduce driver for reinitializing RNG on VM fork

Jason A. Donenfeld
2022-02-26T22:07:13Z

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.

It can be used from qemu via the `-device vmgenid,guid=auto` parameter.
After setting that, use `savevm` in the monitor to save the VM state,
then quit QEMU, start it again, and use `loadvm`. That will trigger this
driver's notify function, which hands the new UUID to the RNG. This is
described in <https://git.qemu.org/?p=qemu.git;a=blob;f=docs/specs/vmgenid.txt>.
And there are hooks for this in libvirt as well, described in
<https://libvirt.org/formatdomain.html#general-metadata>.

Note, however, that the treatment of this as a UUID is considered to be
an accidental QEMU nuance, per
<https://github.com/libguestfs/virt-v2v/blob/master/docs/vm-generation-id-across-hypervisors.txt>,
so this driver simply treats these bytes as an opaque 128-bit binary
blob, as per the spec. This doesn't really make a difference anyway,
considering that's how it ends up when handed to the RNG in the end.

Cc: Alexander Graf <graf@amazon.com>
Cc: Adrian Catangiu <adrian@parity.io>
Cc: Daniel P. Berrangé <berrange@redhat.com>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
Changes v4->v5:
- [Greg] Use module_acpi_driver instead of writing my own code.
- [Alex] Match on _CID instead of _HID.
- Prefer Y over M but still allow M, to handle initramfs reseeds.
- [Wei] Use IS_ERR instead of NULL check with devm_memremap.

MAINTAINERS | 1 +
drivers/virt/Kconfig | 11 +++++
drivers/virt/Makefile | 1 +
drivers/virt/vmgenid.c | 100 +++++++++++++++++++++++++++++++++++++++++
4 files changed, 113 insertions(+)
create mode 100644 drivers/virt/vmgenid.c

diff --git a/MAINTAINERS b/MAINTAINERS
index 777cd6fa2b3d..a10997e15146 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -16211,6 +16211,7 @@ M: Jason A. Donenfeld <Jason@zx2c4.com>
T: git https://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git
S: Maintained
F: drivers/char/random.c
+F: drivers/virt/vmgenid.c

RAPIDIO SUBSYSTEM
M: Matt Porter <mporter@kernel.crashing.org>
diff --git a/drivers/virt/Kconfig b/drivers/virt/Kconfig
index 8061e8ef449f..121b9293c737 100644
--- a/drivers/virt/Kconfig
+++ b/drivers/virt/Kconfig
@@ -13,6 +13,17 @@ menuconfig VIRT_DRIVERS

if VIRT_DRIVERS

+config VMGENID
+ tristate "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. This is highly recommended if
+ you intend to do any rollback / cloning / snapshotting of VMs.
+
+ Prefer Y to M so that this protection is activated very early.
+
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..0ae1a39f2e28
--- /dev/null
+++ b/drivers/virt/vmgenid.c
@@ -0,0 +1,100 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ *
+ * The "Virtual Machine Generation ID" is exposed via ACPI and changes when a
+ * virtual machine forks or is cloned. This driver exists for shepherding that
+ * information to random.c.
+ */
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/acpi.h>
+#include <linux/random.h>
+
+ACPI_MODULE_NAME("vmgenid");
+
+enum { VMGENID_SIZE = 16 };
+
+struct vmgenid_state {
+ u8 *next_id;
+ u8 this_id[VMGENID_SIZE];
+};
+
+static int vmgenid_add(struct acpi_device *device)
+{
+ struct acpi_buffer parsed = { ACPI_ALLOCATE_BUFFER };
+ struct vmgenid_state *state;
+ union acpi_object *obj;
+ phys_addr_t phys_addr;
+ acpi_status status;
+ int ret = 0;
+
+ state = devm_kmalloc(&device->dev, sizeof(*state), GFP_KERNEL);
+ if (!state)
+ return -ENOMEM;
+
+ status = acpi_evaluate_object(device->handle, "ADDR", NULL, &parsed);
+ if (ACPI_FAILURE(status)) {
+ ACPI_EXCEPTION((AE_INFO, status, "Evaluating ADDR"));
+ return -ENODEV;
+ }
+ obj = parsed.pointer;
+ if (!obj || obj->type != ACPI_TYPE_PACKAGE || obj->package.count != 2 ||
+ obj->package.elements[0].type != ACPI_TYPE_INTEGER ||
+ obj->package.elements[1].type != ACPI_TYPE_INTEGER) {
+ ret = -EINVAL;
+ goto out;
+ }
+
+ phys_addr = (obj->package.elements[0].integer.value << 0) |
+ (obj->package.elements[1].integer.value << 32);
+ state->next_id = devm_memremap(&device->dev, phys_addr, VMGENID_SIZE, MEMREMAP_WB);
+ if (IS_ERR(state->next_id)) {
+ ret = PTR_ERR(state->next_id);
+ goto out;
+ }
+
+ memcpy(state->this_id, state->next_id, sizeof(state->this_id));
+ add_device_randomness(state->this_id, sizeof(state->this_id));
+
+ device->driver_data = state;
+
+out:
+ ACPI_FREE(parsed.pointer);
+ return ret;
+}
+
+static void vmgenid_notify(struct acpi_device *device, u32 event)
+{
+ struct vmgenid_state *state = acpi_driver_data(device);
+ u8 old_id[VMGENID_SIZE];
+
+ memcpy(old_id, state->this_id, sizeof(old_id));
+ memcpy(state->this_id, state->next_id, sizeof(state->this_id));
+ if (!memcmp(old_id, state->this_id, sizeof(old_id)))
+ return;
+ add_vmfork_randomness(state->this_id, sizeof(state->this_id));
+}
+
+static const struct acpi_device_id vmgenid_ids[] = {
+ { "VM_GEN_COUNTER", 0 },
+ { }
+};
+
+static struct acpi_driver vmgenid_driver = {
+ .name = "vmgenid",
+ .ids = vmgenid_ids,
+ .owner = THIS_MODULE,
+ .ops = {
+ .add = vmgenid_add,
+ .notify = vmgenid_notify
+ }
+};
+
+module_acpi_driver(vmgenid_driver);
+
+MODULE_DEVICE_TABLE(acpi, vmgenid_ids);
+MODULE_DESCRIPTION("Virtual Machine Generation ID");
+MODULE_LICENSE("GPL v2");
+MODULE_AUTHOR("Jason A. Donenfeld <Jason@zx2c4.com>");
--
2.35.1