On Wed, 11 Jan 2023 at 17:03, Ard Biesheuvel <a...@kernel.org> wrote: > > On Wed, 11 Jan 2023 at 16:23, Laszlo Ersek <ler...@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. -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#98300): https://edk2.groups.io/g/devel/message/98300 Mute This Topic: https://groups.io/mt/94935839/21656 Group Owner: devel+ow...@edk2.groups.io Unsubscribe: https://edk2.groups.io/g/devel/unsub [arch...@mail-archive.com] -=-=-=-=-=-=-=-=-=-=-=-
