linux-kernel - Re: Linux guest kernel threat model for Confidential Computing

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Message-ID: <20230131123033-mutt-send-email-mst@kernel.org>
Date:   Tue, 31 Jan 2023 12:39:48 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Christophe de Dinechin <dinechin@...hat.com>
Cc:     jejb@...ux.ibm.com, "Reshetova, Elena" <elena.reshetova@...el.com>,
        Leon Romanovsky <leon@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Shishkin, Alexander" <alexander.shishkin@...el.com>,
        "Shutemov, Kirill" <kirill.shutemov@...el.com>,
        "Kuppuswamy, Sathyanarayanan" <sathyanarayanan.kuppuswamy@...el.com>,
        "Kleen, Andi" <andi.kleen@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Wunner, Lukas" <lukas.wunner@...el.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "Poimboe, Josh" <jpoimboe@...hat.com>,
        "aarcange@...hat.com" <aarcange@...hat.com>,
        Cfir Cohen <cfir@...gle.com>, Marc Orr <marcorr@...gle.com>,
        "jbachmann@...gle.com" <jbachmann@...gle.com>,
        "pgonda@...gle.com" <pgonda@...gle.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        James Morris <jmorris@...ei.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        "Lange, Jon" <jlange@...rosoft.com>,
        "linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: Linux guest kernel threat model for Confidential Computing

On Tue, Jan 31, 2023 at 04:14:29PM +0100, Christophe de Dinechin wrote:
> Finally, security considerations that apply irrespective of whether the
> platform is confidential or not are also outside of the scope of this
> document. This includes topics ranging from timing attacks to social
> engineering.

Why are timing attacks by hypervisor on the guest out of scope?

> </doc>
> 
> Feel free to comment and reword at will ;-)
> 
> 
> 3/ PCI-as-a-threat: where does that come from
> 
> Isn't there a fundamental difference, from a threat model perspective,
> between a bad actor, say a rogue sysadmin dumping the guest memory (which CC
> should defeat) and compromised software feeding us bad data? I think there
> is: at leats inside the TCB, we can detect bad software using measurements,
> and prevent it from running using attestation.  In other words, we first
> check what we will run, then we run it. The security there is that we know
> what we are running. The trust we have in the software is from testing,
> reviewing or using it.
> 
> This relies on a key aspect provided by TDX and SEV, which is that the
> software being measured is largely tamper-resistant thanks to memory
> encryption. In other words, after you have measured your guest software
> stack, the host or hypervisor cannot willy-nilly change it.
> 
> So this brings me to the next question: is there any way we could offer the
> same kind of service for KVM and qemu? The measurement part seems relatively
> easy. Thetamper-resistant part, on the other hand, seems quite difficult to
> me. But maybe someone else will have a brilliant idea?
> 
> So I'm asking the question, because if you could somehow prove to the guest
> not only that it's running the right guest stack (as we can do today) but
> also a known host/KVM/hypervisor stack, we would also switch the potential
> issues with PCI, MSRs and the like from "malicious" to merely "bogus", and
> this is something which is evidently easier to deal with.

Agree absolutely that's much easier.

> I briefly discussed this with James, and he pointed out two interesting
> aspects of that question:
> 
> 1/ In the CC world, we don't really care about *virtual* PCI devices. We
>    care about either virtio devices, or physical ones being passed through
>    to the guest. Let's assume physical ones can be trusted, see above.
>    That leaves virtio devices. How much damage can a malicious virtio device
>    do to the guest kernel, and can this lead to secrets being leaked?
> 
> 2/ He was not as negative as I anticipated on the possibility of somehow
>    being able to prevent tampering of the guest. One example he mentioned is
>    a research paper [1] about running the hypervisor itself inside an
>    "outer" TCB, using VMPLs on AMD. Maybe something similar can be achieved
>    with TDX using secure enclaves or some other mechanism?

Or even just secureboot based root of trust?

-- 
MST