Re: [PATCH v2 1/1] x86/hyperv: Use Hyper-V entropy to seed guest random number generator

[Jason A. Donenfeld]

Hi Michael,

On Thu, Mar 07, 2024 at 10:48:20AM -0800, mhkelley58@xxxxxxxxx wrote:
> +	/*
> +	 * Seed the Linux random number generator with entropy provided by
> +	 * the Hyper-V host in ACPI table OEM0.  It would be nice to do this
> +	 * even earlier in ms_hyperv_init_platform(), but the ACPI subsystem
> +	 * isn't set up at that point. Skip if booted via EFI as generic EFI
> +	 * code has already done some seeding using the EFI RNG protocol.
> +	 */
> +	if (!IS_ENABLED(CONFIG_ACPI) || efi_enabled(EFI_BOOT))
> +		return;

Even if EFI seeds the kernel using its own code, if this is available,
it should be used too. So I think you should remove the `|| efi_enabled(EFI_BOOT)`
part and let the add_bootloader_randomness() do what it wants with the
entropy.

> +
> +	status = acpi_get_table("OEM0", 0, &header);
> +	if (ACPI_FAILURE(status) || !header)
> +		return;
> +
> +	/*
> +	 * Since the "OEM0" table name is for OEM specific usage, verify
> +	 * that what we're seeing purports to be from Microsoft.
> +	 */
> +	if (strncmp(header->oem_table_id, "MICROSFT", 8))
> +		goto error;
> +
> +	/*
> +	 * Ensure the length is reasonable.  Requiring at least 32 bytes and
> +	 * no more than 256 bytes is somewhat arbitrary.  Hyper-V currently
> +	 * provides 64 bytes, but allow for a change in a later version.
> +	 */
> +	if (header->length < sizeof(*header) + 32 ||
> +	    header->length > sizeof(*header) + 256)

What's the point of the lower bound? Obviously skip for 0, but if
there's only 16 bytes, cool, 16 bytes is good and can't hurt.

For the upper bound, I understand you need some sanity check. Why not
put it a bit higher, though, at SZ_4K or something? Can't hurt.

> +		goto error;
> +
> +	length = header->length - sizeof(*header);
> +	randomdata = (u8 *)(header + 1);
> +
> +	pr_debug("Hyper-V: Seeding rng with %d random bytes from ACPI table OEM0\n",
> +			length);
> +
> +	add_bootloader_randomness(randomdata, length);
> +
> +	/*
> +	 * To prevent the seed data from being visible in /sys/firmware/acpi,
> +	 * zero out the random data in the ACPI table and fixup the checksum.
> +	 */
> +	for (i = 0; i < length; i++) {
> +		header->checksum += randomdata[i];
> +		randomdata[i] = 0;
> +	}

Seems dangerous for kexec and such. What if, in addition to zeroing out
the actual data, you also set header->length to 0, so that it doesn't
get used again as 32 bytes of known zeros?

Thanks,
Jason