On Fri, Feb 25, 2022 at 01:48:48PM +0100, 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. > > Cc: Adrian Catangiu <adrian@xxxxxxxxx> > Cc: Daniel P. Berrangé <berrange@xxxxxxxxxx> > Cc: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx> > Cc: Ard Biesheuvel <ardb@xxxxxxxxxx> > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> > Reviewed-by: Laszlo Ersek <lersek@xxxxxxxxxx> > Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx> Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
