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From:   Rajat Jain <rajatja@google.com>
Date:   Tue, 24 Aug 2021 14:55:24 -0700
Message-ID: <CACK8Z6GaKb58OWu-hxabweFKX8MBPngmWQa6vhDgFJy9pWp_vQ@mail.gmail.com>
Subject: Re: [PATCH v4 11/15] pci: Add pci_iomap_shared{,_range}
To:     Bjorn Helgaas <helgaas@kernel.org>
Cc:     Andi Kleen <ak@linux.intel.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Dan Williams <dan.j.williams@intel.com>,
        "Kuppuswamy, Sathyanarayanan" 
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        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
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Thanks a lot Bjorn for adding me!


On Tue, Aug 24, 2021 at 11:55 AM Bjorn Helgaas <helgaas@kernel.org> wrote:
>
> [+cc Rajat; I still don't know what "shared memory with a hypervisor
> in a confidential guest" means, but now we're talking about hardened
> drivers and allow lists, which Rajat is interested in]
>
> On Tue, Aug 24, 2021 at 10:20:44AM -0700, Andi Kleen wrote:
> >
> > > I see. Hmm. It's a bit of a random thing to do it at the map time
> > > though. E.g. DMA is all handled transparently behind the DMA API.
> > > Hardening is much more than just replacing map with map_shared
> > > and I suspect what you will end up with is basically
> > > vendors replacing map with map shared to make things work
> > > for their users and washing their hands.
> >
> > That concept exists too. There is a separate allow list for the drivers. So
> > just adding shared to a driver is not enough, until it's also added to the
> > allowlist
> >
> > Users can of course chose to disable the allowlist, but they need to
> > understand the security implications.

This is great. I'd be interested in looking at this allowlist
mechanism. Is this something in-kernel or in userspace? Is this
available upstream or are you maintaining this allowlist elsewhere?
(Background: https://lore.kernel.org/linux-pci/CACK8Z6E8pjVeC934oFgr=VB3pULx_GyT2NkzAogdRQJ9TKSX9A@mail.gmail.com/)

Short Summary: we also have our security team that audits drivers, and
we'd like to enable only audited drivers for the untrusted devices.
Currently, we're carrying this allowlist mechanism on our own since
the idea was Nack'ed by upstream. So if there is something available,
we'd like to use it too.

Thanks,

Rajat


> >
> > > I would say an explicit flag in the driver that says "hardened"
> > > and refusing to init a non hardened one would be better.
> >
> > We have that too (that's the device filtering)
> >
> > But the problem is that device filtering just stops the probe functions, not
> > the initcalls, and lot of legacy drivers do MMIO interactions before going
> > into probe. In some cases it's unavoidable because of the device doesn't
> > have a separate enumeration mechanism it needs some kind of probing to even
> > check for its existence And since we don't want to change all of them it's
> > far safer to make the ioremap opt-in.
> >
> >
> > -Andi
> >