From af6b54e2b5baa54c844573b6d49cc91157bcdd7e Mon Sep 17 00:00:00 2001 From: "Jason A. Donenfeld" Date: Wed, 23 Feb 2022 13:46:24 +0100 Subject: virt: vmgenid: notify RNG of VM fork and supply generation ID MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit VM Generation ID is a feature from Microsoft, described at , and supported by Hyper-V and QEMU. Its usage is described in Microsoft's RNG whitepaper, , 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 . And there are hooks for this in libvirt as well, described in . Note, however, that the treatment of this as a UUID is considered to be an accidental QEMU nuance, per , 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 Cc: Adrian Catangiu Cc: Daniel P. Berrangé Cc: Dominik Brodowski Cc: Wei Yongjun Tested-by: Souradeep Chakrabarti # With Hyper-V's virtual hardware Reviewed-by: Ard Biesheuvel Reviewed-by: Greg Kroah-Hartman Reviewed-by: Laszlo Ersek Signed-off-by: Jason A. Donenfeld --- drivers/virt/Kconfig | 11 +++++++++++ 1 file changed, 11 insertions(+) (limited to 'drivers/virt/Kconfig') 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 -- cgit v1.2.3