From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-1.mimecast.com (us-smtp-1.mimecast.com [207.211.31.120]) by mx.groups.io with SMTP id smtpd.web12.2218.1573121469366091862 for ; Thu, 07 Nov 2019 02:11:09 -0800 Authentication-Results: mx.groups.io; dkim=pass header.i=@redhat.com header.s=mimecast20190719 header.b=er4dyb2M; spf=pass (domain: redhat.com, ip: 207.211.31.120, mailfrom: lersek@redhat.com) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1573121468; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=yixX++/py6sBiSFVMQI6sWWGNBpLZUyIFn2qkegtNzk=; b=er4dyb2MgWEVXHApUvDuWQxRRFvOq5nGijwISAHXU6kd9V3bWNMtSPt6Vfb0pkSRJRQUqb UYw3ZsmPuyOJCynkcKabYMjy8Q1K9PagwBfk0W+yfGbXD7VsTQ/WKMjRCI/G7VkkHSil9+ xgQ0UtjoNakre8oFFyB0HwzF8teNMRM= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-169-s2mQSKbhPb2xPe-Wi4EvOg-1; Thu, 07 Nov 2019 05:11:05 -0500 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id C2FBB477; Thu, 7 Nov 2019 10:11:03 +0000 (UTC) Received: from lacos-laptop-7.usersys.redhat.com (unknown [10.36.118.71]) by smtp.corp.redhat.com (Postfix) with ESMTP id 647E3608B4; Thu, 7 Nov 2019 10:10:58 +0000 (UTC) To: qemu devel list Cc: Ard Biesheuvel , "Daniel P. Berrange" , Paolo Bonzini , edk2-devel-groups-io , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= , Bret Barkelew , Sean Brogan , Jian J Wang , Erik Bjorge From: "Laszlo Ersek" Subject: privileged entropy sources in QEMU/KVM guests Message-ID: <03e769cf-a5ad-99ce-cd28-690e0a72a310@redhat.com> Date: Thu, 7 Nov 2019 11:10:57 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-MC-Unique: s2mQSKbhPb2xPe-Wi4EvOg-1 X-Mimecast-Spam-Score: 0 Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Hi, related TianoCore BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3D1871 (I'm starting this thread separately because at least some of the topics are specific to QEMU, and I didn't want to litter the BZ with a discussion that may not be interesting to all participants CC'd on the BZ. I am keeping people CC'd on this initial posting; please speak up if you'd like to be dropped from the email thread.) QEMU provides guests with the virtio-rng device, and the OVMF and ArmVirtQemu* edk2 platforms build EFI_RNG_PROTOCOL on top of that device. But, that doesn't seem enough for all edk2 use cases. Also, virtio-rng (hence EFI_RNG_PROTOCOL too) is optional, and its absence may affect some other use cases. (1) For UEFI HTTPS boot, TLS would likely benefit from good quality entropy. If the VM config includes virtio-rng (hence the guest firmware has EFI_RNG_PROTOCOL), then it should be used as a part of HTTPS boot. However, what if virtio-rng (hence EFI_RNG_PROTOCOL) are absent? Should UEFI HTTPS boot be disabled completely (or prevented / rejected somehow), blaming lack of good entropy? Or should TLS silently fall back to "mixing some counters [such as TSC] together and applying a deterministic cryptographic transformation"? IOW, knowing that the TLS setup may not be based on good quality entropy, should we allow related firmware services to "degrade silently" (not functionally, but potentially in security), or should we deny the services altogether? (2) It looks like the SMM driver implementing the privileged part of the UEFI variable runtime service could need access to good quality entropy, while running in SMM; in the future. This looks problematic on QEMU. Entropy is a valuable resource, and whatever resource SMM drivers depend on, should not be possible for e.g. a 3rd party UEFI driver (or even for the runtime OS) to exhaust. Therefore, it's not *only* the case that SMM drivers must not consume EFI_RNG_PROTOCOL (which exists at a less critical privilege level, i.e. outside of SMM/SMRAM), but also that SMM drivers must not depend on the same piece of *hardware* that feeds EFI_RNG_PROTOCOL. Furthermore, assuming we dedicate a hardware entropy device specifically to SMM drivers, such a device cannot be PCI(e). It would have to be a platform device at a fixed location (IO port or MMIO) that is only accessible to such guest code that executes in SMM. IOW, device access would have to be restricted similarly to pflash. (In fact the variable SMM driver will need, AIUI, the entropy for encrypting various variable contents, which are then written into pflash.) Alternatively, CPU instructions could exist that return entropy, and are executable only inside SMM. It seems that e.g. RDRAND can be trapped in guests ("A VMEXIT due to RDRAND will have exit reason 57 (decimal)"). Then KVM / QEMU could provide any particular implementation we wanted -- for example an exception could be injected unless RDRAND had been executed from within SMM. Unfortunately, such an arbitrary restriction (of RDRAND to SMM) would diverge from the Intel SDM, and would likely break other (non-SMM) guest code. Does a platform device that is dynamically detectable and usable in SMM only seem like an acceptable design for QEMU? Thanks, Laszlo