Re: [PATCH v4] virt: vmgenid: introduce driver for reinitializing RNG on VM fork

[Greg KH <gregkh@linuxfoundation.org>]

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@parity.io>
> Cc: Daniel P. Berrangé <berrange@redhat.com>
> Cc: Dominik Brodowski <linux@dominikbrodowski.net>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Reviewed-by: Laszlo Ersek <lersek@redhat.com>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>


Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>