Re: [edk2-devel] [PATCH 0/3] OVMF: support EFI_RNG_PROTOCOL without virtio-rng

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

On Wed, 11 Jan 2023 at 17:03, Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Wed, 11 Jan 2023 at 16:23, Laszlo Ersek <lersek@redhat.com> wrote:
> >
> > On 1/10/23 19:19, Jason A. Donenfeld via groups.io wrote:
> > > Could we get this merged?
> >
> > Sorry to barge in -- I have *zero* complaints regarding this particular
> > series, so whatever I'm about to say regards *further* BDS
> > customizations. Please feel free to go ahead with merging this one, as
> > far as I'm concerned.
> >
>
> Thanks.
>
> > So, picking up the thread at
> > <https://listman.redhat.com/archives/edk2-devel-archive/2022-November/055607.html>.
> > The argument in that thread was made that "RDRAND-based protocol is
> > better than nothing". However, the most recent idea, favoring the
> > RDRAND-based protocol implementation over the virtio-rng-based one,
> > seems to enable a degradation too, of EFI-time randomness.
> >
> > Most commonly, virtio-rng is fed on the host side from /dev/urandom,
> > which *I think* means that the EFI_RNG_PROTOCOL from VirtioRngDxe will
> > expose all the "good quality entropy", pre-boot, that the host-side
> > Linux kernel collects from *multiple* sources. If the consumer of
> > EFI_RNG_PROTOCOL in the guest doesn't do its own mixing, it sill gets
> > the good stuff. That could potentially be degraded by relying on RDRAND
> > only, in the guest.
> >
>
> Indeed.
>
> > I can't propose any particular priority ordering mechanism for the
> > platform firmware to produce exactly one EFI_RNG_PROTOCOL.
> >
> > Normally I'd suggest any viable mechanism for the platform to block or
> > to delay "SecurityPkg/RandomNumberGenerator/RngDxe/RngDxe.inf" --
> > introducing a new dynamic PCD for early exit, adding a new protocol
> > dependency to its DEPEX, postponing its protocol installation to an
> > event group notification function or a protocol installation
> > notification. Note that RngDxe.inf is a DXE_DRIVER, so it produces its
> > protocol in its entry point function, so for blocking it or
> > short-circuiting it, one of these measures would be needed. It could
> > even be turned into a UEFI_DRIVER, one that would bind a synthetic VenHw
> > device path.
> >
> > But, I'm not proposing any of those right now, because I imagine there
> > are advantages to having EFI_RNG_PROTOCOL in the DXE phase, that is,
> > *before* the BDS phase.
> >
> > VirtioRngDxe is a UEFI_DRIVER module that follows the UEFI driver model;
> > in other words, it won't do anything beyond exposing the
> > EFI_DRIVER_BINDING_PROTOCOL until BDS connects it. I think that should
> > be sufficient for most cases, even (for example) possibly providing
> > randomness for TLS in UEFI HTTPS Boot. But I vaguely remember we had
> > wished for randomness being available earlier than BDS.
> > "SecurityPkg/RandomNumberGenerator/RngDxe/RngDxe.inf" can fill that
> > role, VirtioRngDxe can't.
> >
> > So best would be if both could coexist, and VirtioRngDxe took effect
> > *whenever* it were available. Of course the UEFI spec allows for a
> > client to collect all instances of EFI_RNG_PROTOCOL, and then to call
> > GetInfo() on each, but that's hardly enough for a client to pick the one
> > it thinks is "more secure". So one way or another we might want to
> > control this still at the platform level, where we can form ideas about
> > both protocol providers, *and* perhaps even determine if we *actually
> > need* pre-BDS randomness.
> >
> > BDS could try connecting the virtio-rng device. If that failed, it could
> > try "unblocking" RngDxe. If RngDxe were a UEFI driver following the UEFI
> > driver model (see the VenHw option above), this would not be hard to do,
> > with a "fallback" gBS->ConnectController() call.
> >
> > (Regarding VenHw vs. VenMedia vs. VenMsg -- RngDxe uses an RNG that's
> > built into the processor, wich is arguably "inside the resource domain"
> > of the system. So VenHw seems the right choice.)
> >
> > RngDxe could perhaps be restructured for the addition of a new entry
> > point (new INF file and new entry point C file), so that it remain
> > compatible with existent platforms that already consume it (and want it
> > to remain a DXE_DRIVER).
> >
> > BDS could also signal an event group or install a synthetic protocol, so
> > that the notification function in RngDxe expose EFI_RNG_PROTOCOL in
> > response.
> >
> > Unblocking a DXE_DRIVER's DEPEX from BDS seems more cumbersome, by
> > installing a dependend-upon synthetic protocol; I believe we might have
> > to call gDS->Dispatch() manually then.
> >
> > And if a dynamic PCD caused RngDxe to exit early, we couldn't undo that
> > from BDS at all.
> >
>
> One option that might be feasible would be to modify VIrtioRngDxe so it:
> - installs a RNG protocol implementation solely based on [Base]RngLib
> when it is dispatched
> - uninstalls it again when it binds to the first virtio-rng device
> - reinstalls it when it unbinds from the last virtio-rng device it was bound to
> - installs the virtio-rng backed flavor of the RNG protocol when
> binding to a device

(Un)installing the protocol is a bit problematic, as a caller may hold
a reference. Probably better to expose a single implementation from
VirtioRngDxe, and back it with whatever is available at the time of
the call.

> (- mixes the output of the latter with the RngLIb based implementation)
>
> I think this would address all of these concerns, assuming that the
> mixing is done correctly.
>
> *However*, I am not convinced that any of this is worth the hassle,
> tbh. If you don't trust your CPU, all bets are off anyway - the only
> thing we'd need to cater for is an explicit opt-out for known broken
> implementations of RdRand.