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

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Message-ID: <CAHmME9oWUJr3fYKFOy=aRdd0uE8H8ke9o+KXde0zAnAwnWUPMA@mail.gmail.com>
Date:   Wed, 23 Feb 2022 17:36:25 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     "K . Y . Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        linux-hyperv@...r.kernel.org
Cc:     "Woodhouse, David" <dwmw@...zon.co.uk>,
        "Catangiu, Adrian Costin" <acatan@...zon.com>, graf@...zon.com,
        Colm MacCarthaigh <colmmacc@...zon.com>,
        "Singh, Balbir" <sblbir@...zon.com>,
        "Weiss, Radu" <raduweis@...zon.com>, Jann Horn <jannh@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Theodore Ts'o" <tytso@....edu>,
        Igor Mammedov <imammedo@...hat.com>, ehabkost@...hat.com,
        KVM list <kvm@...r.kernel.org>, adrian@...ity.io,
        ben@...portsystems.com, "Michael S. Tsirkin" <mst@...hat.com>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        lersek@...hat.com, linux-s390@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        QEMU Developers <qemu-devel@...gnu.org>
Subject: Re: [PATCH RFC v1 2/2] drivers/virt: add vmgenid driver for
 reinitializing RNG

Adding the Hyper-V people to this:

On Wed, Feb 23, 2022 at 2:13 PM Jason A. Donenfeld <Jason@...c4.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