linux-kernel - Re: [PATCH 6/7] x86/boot/tboot: Move tboot_force

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Date:   Tue, 17 May 2022 08:13:50 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        David Airlie <airlied@...ux.ie>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Liu Yi L <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Ning Sun <ning.sun@...el.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@....de>,
        Steve Wahl <steve.wahl@....com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/7] x86/boot/tboot: Move tboot_force_iommu() to Intel
 IOMMU

On Tue, May 17, 2022 at 10:05:43AM +0800, Baolu Lu wrote:
> Hi Jason,
> 
> On 2022/5/17 02:06, Jason Gunthorpe wrote:
> > > +static __init int tboot_force_iommu(void)
> > > +{
> > > +	if (!tboot_enabled())
> > > +		return 0;
> > > +
> > > +	if (no_iommu || dmar_disabled)
> > > +		pr_warn("Forcing Intel-IOMMU to enabled\n");
> > Unrelated, but when we are in the special secure IOMMU modes, do we
> > force ATS off? Specifically does the IOMMU reject TLPs that are marked
> > as translated?
> 
> Good question. From IOMMU point of view, I don't see a point to force
> ATS off, but trust boot involves lots of other things that I am not
> familiar with. Anybody else could help to answer?

ATS is inherently not secure, if a rouge device can issue a TLP with
the translated bit set then it has unlimited access to host memory.

Many of these trusted iommu scenarios rely on the idea that a rouge
device cannot DMA to arbitary system memory.

Jason