Re: [PATCH v3 2/2] virt: vmgenid: introduce driver for reinitializing RNG on VM fork

[Laszlo Ersek]

On 02/24/22 14:39, Jason A. Donenfeld wrote:
> 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.
> 
> 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>
> Cc: Laszlo Ersek <lersek@redhat.com>
> Cc: Daniel P. Berrangé <berrange@redhat.com>
> Cc: Dominik Brodowski <linux@dominikbrodowski.net>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>  drivers/virt/Kconfig   |   9 +++
>  drivers/virt/Makefile  |   1 +
>  drivers/virt/vmgenid.c | 121 +++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 131 insertions(+)
>  create mode 100644 drivers/virt/vmgenid.c
> 
> diff --git a/drivers/virt/Kconfig b/drivers/virt/Kconfig
> index 8061e8ef449f..d3276dc2095c 100644
> --- a/drivers/virt/Kconfig
> +++ b/drivers/virt/Kconfig
> @@ -13,6 +13,15 @@ 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.
> +
>  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..5da4dc8f25e3
> --- /dev/null
> +++ b/drivers/virt/vmgenid.c
> @@ -0,0 +1,121 @@
> +// 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.
> + */
> +
> +#include <linux/kernel.h>
> +#include <linux/module.h>
> +#include <linux/acpi.h>
> +#include <linux/random.h>
> +
> +ACPI_MODULE_NAME("vmgenid");
> +
> +enum { VMGENID_SIZE = 16 };
> +
> +static struct {
> +     u8 this_id[VMGENID_SIZE];
> +     u8 *next_id;
> +} state;
> +
> +static int vmgenid_acpi_add(struct acpi_device *device)
> +{
> +     struct acpi_buffer buffer = { ACPI_ALLOCATE_BUFFER };
> +     union acpi_object *pss;
> +     phys_addr_t phys_addr;
> +     acpi_status status;
> +     int ret = 0;
> +
> +     if (!device)
> +             return -EINVAL;
> +
> +     status = acpi_evaluate_object(device->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 ||
> +         pss->package.elements[0].type != ACPI_TYPE_INTEGER ||
> +         pss->package.elements[1].type != ACPI_TYPE_INTEGER) {
> +             ret = -EINVAL;
> +             goto out;
> +     }
> +
> +     phys_addr = (pss->package.elements[0].integer.value << 0) |
> +                 (pss->package.elements[1].integer.value << 32);
> +     state.next_id = acpi_os_map_memory(phys_addr, VMGENID_SIZE);
> +     if (!state.next_id) {
> +             ret = -ENOMEM;
> +             goto out;
> +     }
> +     device->driver_data = &state;
> +
> +     memcpy(state.this_id, state.next_id, sizeof(state.this_id));
> +     add_device_randomness(state.this_id, sizeof(state.this_id));
> +
> +out:
> +     ACPI_FREE(buffer.pointer);
> +     return ret;
> +}
> +
> +static int vmgenid_acpi_remove(struct acpi_device *device)
> +{
> +     if (!device || acpi_driver_data(device) != &state)
> +             return -EINVAL;
> +     device->driver_data = NULL;
> +     if (state.next_id)
> +             acpi_os_unmap_memory(state.next_id, VMGENID_SIZE);
> +     state.next_id = NULL;
> +     return 0;
> +}
> +
> +static void vmgenid_acpi_notify(struct acpi_device *device, u32 event)
> +{
> +     u8 old_id[VMGENID_SIZE];
> +
> +     if (!device || acpi_driver_data(device) != &state)
> +             return;
> +     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[] = {
> +     {"VMGENID", 0},
> +     {"QEMUVGID", 0},
> +     { },
> +};
> +
> +static struct acpi_driver acpi_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_driver);
> +}
> +
> +static void __exit vmgenid_exit(void)
> +{
> +     acpi_bus_unregister_driver(&acpi_driver);
> +}
> +
> +module_init(vmgenid_init);
> +module_exit(vmgenid_exit);
> +
> +MODULE_DEVICE_TABLE(acpi, vmgenid_ids);
> +MODULE_DESCRIPTION("Virtual Machine Generation ID");
> +MODULE_LICENSE("GPL v2");
> 

I'm not an experienced reviewer for the kernel.

I've made an effort to check several -- although not all -- aspects of
this patch, and it looks OK to me.

Reviewed-by: Laszlo Ersek <lersek@redhat.com>

Thanks
Laszlo