linux-kernel - RE: [PATCH v3 1/1] x86/hyperv: Use Hyper-V entropy to seed guest random number generator

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Date: Mon, 18 Mar 2024 22:49:58 +0000
From: Michael Kelley <mhklinux@...look.com>
To: "Jason A. Donenfeld" <Jason@...c4.com>
CC: "haiyangz@...rosoft.com" <haiyangz@...rosoft.com>, "wei.liu@...nel.org"
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Subject: RE: [PATCH v3 1/1] x86/hyperv: Use Hyper-V entropy to seed guest
 random number generator

From: Jason A. Donenfeld <Jason@...c4.com> Sent: Monday, March 18, 2024 2:03 PM
> 
> On Mon, Mar 18, 2024 at 08:54:08AM -0700, mhkelley58@...il.com wrote:
> > From: Michael Kelley <mhklinux@...look.com>
> >
> > A Hyper-V host provides its guest VMs with entropy in a custom ACPI
> > table named "OEM0".  The entropy bits are updated each time Hyper-V
> > boots the VM, and are suitable for seeding the Linux guest random
> > number generator (rng). See a brief description of OEM0 in [1].
> >
> > Generation 2 VMs on Hyper-V use UEFI to boot. Existing EFI code in
> > Linux seeds the rng with entropy bits from the EFI_RNG_PROTOCOL.
> > Via this path, the rng is seeded very early during boot with good
> > entropy. The ACPI OEM0 table provided in such VMs is an additional
> > source of entropy.
> >
> > Generation 1 VMs on Hyper-V boot from BIOS. For these VMs, Linux
> > doesn't currently get any entropy from the Hyper-V host. While this
> > is not fundamentally broken because Linux can generate its own entropy,
> > using the Hyper-V host provided entropy would get the rng off to a
> > better start and would do so earlier in the boot process.
> >
> > Improve the rng seeding for Generation 1 VMs by having Hyper-V specific
> > code in Linux take advantage of the OEM0 table to seed the rng. For
> > Generation 2 VMs, use the OEM0 table to provide additional entropy
> > beyond the EFI_RNG_PROTOCOL. Because the OEM0 table is custom to
> > Hyper-V, parse it directly in the Hyper-V code in the Linux kernel
> > and use add_bootloader_randomness() to add it to the rng. Once the
> > entropy bits are read from OEM0, zero them out in the table so
> > they don't appear in /sys/firmware/acpi/tables/OEM0 in the running
> > VM. The zero'ing is done out of an abundance of caution to avoid
> > potential security risks to the rng. Also set the OEM0 data length
> > to zero so a kexec or other subsequent use of the table won't try
> > to use the zero'ed bits.
> >
> > [1] https://download.microsoft.com/download/1/c/9/1c9813b8-089c-4fef-b2ad-ad80e79403ba/Whitepaper%20-%20The%20Windows%2010%20random%20number%20generation%20infrastructure.pdf 
> 
> Looks good to me. Assuming you've tested this and it works,

Yes, tested on both x86 and arm64.  Thanks.

Michael

> 
>  Reviewed-by: Jason A. Donenfeld <Jason@...c4.com>
> 
> Thanks for the v3.
> 
> Jason