Re: [PATCH RFC v1 2/2] drivers/virt: add vmgenid driver for reinitializing RNG

Jason A. Donenfeld
2022-02-23T16:36:50Z

Adding the Hyper-V people to this:

On Wed, Feb 23, 2022 at 2:13 PM Jason A. Donenfeld <Jason@zx2c4.com> 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.
>
> This driver builds on prior work from Adrian Catangiu at Amazon, and it
> is my hope that that team can resume maintenance of this driver.

If any of you have some experience with the Hyper-V side of this
protocol, could you take a look at this and see if it matches the way
this is supposed to work? It appears to work fine with QEMU's
behavior, at least, but I know Hyper-V has a lot of additional
complexities.

Thanks,
Jason