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

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Message-ID: <ZfisGSPX9et2hGzx@zx2c4.com>
Date: Mon, 18 Mar 2024 22:03:21 +0100
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: mhklinux@...look.com
Cc: haiyangz@...rosoft.com, wei.liu@...nel.org, decui@...rosoft.com,
	catalin.marinas@....com, will@...nel.org, tglx@...utronix.de,
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
	hpa@...or.com, arnd@...db.de, tytso@....edu, x86@...nel.org,
	linux-kernel@...r.kernel.org, linux-hyperv@...r.kernel.org,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH v3 1/1] x86/hyperv: Use Hyper-V entropy to seed guest
 random number generator

Hi Michael,

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,

 Reviewed-by: Jason A. Donenfeld <Jason@...c4.com>

Thanks for the v3.

Jason