summaryrefslogtreecommitdiffstats
path: root/drivers
diff options
context:
space:
mode:
authorJason A. Donenfeld <Jason@zx2c4.com>2022-02-23 13:46:24 +0100
committerJason A. Donenfeld <Jason@zx2c4.com>2022-03-13 02:00:56 +0100
commitaf6b54e2b5baa54c844573b6d49cc91157bcdd7e (patch)
tree66f6361aa18a0c1fe6f5416c55b97f2917a9b6fe /drivers
parentACPI: allow longer device IDs (diff)
downloadlinux-af6b54e2b5baa54c844573b6d49cc91157bcdd7e.tar.xz
linux-af6b54e2b5baa54c844573b6d49cc91157bcdd7e.zip
virt: vmgenid: notify RNG of VM fork and supply generation ID
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> Tested-by: Souradeep Chakrabarti <souradch.linux@gmail.com> # With Hyper-V's virtual hardware 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>
Diffstat (limited to 'drivers')
-rw-r--r--drivers/virt/Kconfig11
-rw-r--r--drivers/virt/Makefile1
-rw-r--r--drivers/virt/vmgenid.c100
3 files changed, 112 insertions, 0 deletions
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>");