Re: [PATCH 0/3] OVMF: support EFI_RNG_PROTOCOL without virtio-rng

["Ard Biesheuvel" <ardb@kernel.org>]

On Fri, 11 Nov 2022 at 03:41, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> Hi Ard,
>
> On Thu, Nov 10, 2022 at 2:48 PM Ard Biesheuvel <ardb@kernel.org> wrote:
> >
> > Currently, we only expose EFI_RNG_PROTOCOL when running under QEMU if it
> > exposes a virtio-rng device. This means that generic EFI apps or
> > loaders have no access to an entropy source if this device is
> > unavailable, unless they implement their own arch-specific handling to
> > figure out whether any CPU instructions or monitor calls can be used
> > instead.
> >
> > So let's wire those up as EFI_RNG_PROTOCOL implementations as well,
> > using the existing drivers and libraries.
>
> I tested this series on x86 and it appears to work as expected. Thanks
> for putting this together. So,
>
>     Tested-by: Jason A. Donenfeld <Jason@zx2c4.com>
>
> On very brief inspection, this also looks good, though I'm not really
> an EDK2 expert and my review isn't very thorough. But in case it
> helps, which you can take or leave,
>
>     Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
>

Thanks.

> My only question is how it chooses which RNG source to use in the
> event that multiple are available. I would think preferring virtio-rng
> if available is the right thing there. If it's based on the order of
> the items in the .dsc file, then it looks like this series is doing
> the right thing.
>

No, it is essentially arbitrary (but not random :-))

We already have special handling for the virtio RNG device in the BDS
code, because normally, EFI only dispatches drivers for devices that
it needs to boot (i..e, it walks the device path of the boot entry and
only connects a device to its driver at each stage if it needs to do
so to get to the next one)

So connecting the virtio-rng device to its driver needs to be done
explicitly, or it may not be connected at all. We handle this in
ConnectVirtioPciRng() for x86 and some similar code exists in
ArmVirtPkg/Library/PlatformBootManagerLib/PlatformBm.c

On ARM, the RngDxe wired up by this patch is backed by a hypervisor or
secure world firmware service, rather than by the VMM, so in the ARM
case, I think this one is the preferred one given that the VMM is
generally less trusted (although that distinction really only matters
for confidential compute).

On x86, we use the RdRand instruction, which is also independent from
the VMM, so I'd assume this is the preferred choice, no? Or do you
have concerns about broken implementations?

Another distinction is that the ARM version only implements
EFI_RNG_ALGORITHM_RAW, whereas x86 also implements
EFI_RNG_ALGORITHM_SP800_90_CTR_256_GUID. (virtio-rng also only
implements EFI_RNG_ALGORITHM_RAW). This likely does not matter at all,
but it is nevertheless good to call out while we decide which driver
to give precedence.

Another thing to note is that we generally try very hard to do as
little as possible at boot time (although you might get a different
impression when looking at the code :-)). So simply skipping the
virtio-rng driver dispatch if some implementation of EFI_RNG_PROTOCOL
is already available seems appropriate from that angle as well.