drivers/virt/vmgenid.c

// 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[] = {
	{ "VMGENCTR", 0 },
	{ "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>");