Re: [PATCH v4 11/15] pci: Add pci_iomap_shared{,_range}

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Andi Kleen <ak@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>,
	"Kuppuswamy,
	Sathyanarayanan"  <sathyanarayanan.kuppuswamy@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Andy Lutomirski <luto@kernel.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Richard Henderson <rth@twiddle.net>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
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	Helge Deller <deller@gmx.de>,
	"David S . Miller" <davem@davemloft.net>,
	Arnd Bergmann <arnd@arndb.de>, Jonathan Corbet <corbet@lwn.net>,
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Subject: Re: [PATCH v4 11/15] pci: Add pci_iomap_shared{,_range}
Date: Sun, 29 Aug 2021 11:27:42 -0400	[thread overview]
Message-ID: <20210829112105-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <d21a2a2d-4670-ba85-ce9a-fc8ea80ef1be@linux.intel.com>

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.

Right. So given that, why do we need to tweak a random API like the map?
If you just make all maps be shared then the user is in control.
Seems sensible to me.

> 
> > 
> > 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

Let's be frank, even without encryption disabling most drivers -
especially weird ones that poke at hardware before probe -
is far safer than keeping them, but one loses a bunch of features.
IOW all this hardening is nice but which security/feature tradeoff
to take it a policy decision, not something kernel should do
imho.

-- 
MST