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

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Message-ID: <Y9KG6g0CHlnKwuW+@work-vm>
Date:   Thu, 26 Jan 2023 13:58:02 +0000
From:   "Dr. David Alan Gilbert" <dgilbert@...hat.com>
To:     "Reshetova, Elena" <elena.reshetova@...el.com>
Cc:     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

* Reshetova, Elena (elena.reshetova@...el.com) wrote:
> > On Wed, Jan 25, 2023 at 03:29:07PM +0000, Reshetova, Elena wrote:
> > > Replying only to the not-so-far addressed points.
> > >
> > > > On Wed, Jan 25, 2023 at 12:28:13PM +0000, Reshetova, Elena wrote:
> > > > > Hi Greg,
> > 
> > <...>
> > 
> > > > > 3) All the tools are open-source and everyone can start using them right
> > away
> > > > even
> > > > > without any special HW (readme has description of what is needed).
> > > > > Tools and documentation is here:
> > > > > https://github.com/intel/ccc-linux-guest-hardening
> > > >
> > > > Again, as our documentation states, when you submit patches based on
> > > > these tools, you HAVE TO document that.  Otherwise we think you all are
> > > > crazy and will get your patches rejected.  You all know this, why ignore
> > > > it?
> > >
> > > Sorry, I didn’t know that for every bug that is found in linux kernel when
> > > we are submitting a fix that we have to list the way how it has been found.
> > > We will fix this in the future submissions, but some bugs we have are found by
> > > plain code audit, so 'human' is the tool.
> > 
> > My problem with that statement is that by applying different threat
> > model you "invent" bugs which didn't exist in a first place.
> > 
> > For example, in this [1] latest submission, authors labeled correct
> > behaviour as "bug".
> > 
> > [1] https://lore.kernel.org/all/20230119170633.40944-1-
> > alexander.shishkin@...ux.intel.com/
> 
> Hm.. Does everyone think that when kernel dies with unhandled page fault 
> (such as in that case) or detection of a KASAN out of bounds violation (as it is in some
> other cases we already have fixes or investigating) it represents a correct behavior even if
> you expect that all your pci HW devices are trusted? What about an error in two 
> consequent pci reads? What about just some failure that results in erroneous input? 

I'm not sure you'll get general agreement on those answers for all
devices and situations; I think for most devices for non-CoCo
situations, then people are generally OK with a misbehaving PCI device
causing a kernel crash, since most people are running without IOMMU
anyway, a misbehaving device can cause otherwise undetectable chaos.

I'd say:
  a) For CoCo, a guest (guaranteed) crash isn't a problem - CoCo doesn't
  guarantee forward progress or stop the hypervisor doing something
  truly stupid.

  b) For CoCo, information disclosure, or corruption IS a problem

  c) For non-CoCo some people might care about robustness of the kernel
  against a failing PCI device, but generally I think they worry about
  a fairly clean failure, even in the unexpected-hot unplug case.

  d) It's not clear to me what 'trust' means in terms of CoCo for a PCIe
  device; if it's a device that attests OK and we trust it is the device
  it says it is, do we give it freedom or are we still wary?

Dave


> Best Regards,
> Elena.
> 
-- 
Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK