From: Arnd Bergmann <arnd@arndb.de>
To: "Russell King - ARM Linux" <linux@arm.linux.org.uk>
Subject: Re: [RFC] ARM DMA mapping TODO, v1
Date: Thu, 28 Apr 2011 14:48:17 +0200
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Cc: Catalin Marinas <catalin.marinas@arm.com>, linaro-mm-sig@lists.linaro.org,
        Valdis.Kletnieks@vt.edu, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org
References: <201104212129.17013.arnd@arndb.de> <201104281412.40991.arnd@arndb.de> <20110428123616.GI17290@n2100.arm.linux.org.uk>
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On Thursday 28 April 2011, Russell King - ARM Linux wrote:
> On Thu, Apr 28, 2011 at 02:12:40PM +0200, Arnd Bergmann wrote:
> > On Thursday 28 April 2011, Catalin Marinas wrote:
> > > On Thu, 2011-04-28 at 01:15 +0100, Valdis.Kletnieks@vt.edu wrote:
> > > > On Wed, 27 Apr 2011 12:08:28 BST, Catalin Marinas said:
> > > > 
> > > > > The current version of the ARM ARM says "unpredictable". But this
> > > > > general definition of "unpredictable" does not allow it to deadlock
> > > > > (hardware) or have security implications. It is however allowed to
> > > > > corrupt data.
> > > > 
> > > > Not allowed to have security implications, but is allowed to corrupt data.
> > > 
> > > By security I was referring to TrustZone extensions. IOW, unpredictable
> > > in normal (non-secure) world should not cause data corruption in the
> > > secure world.
> > 
> > That definition is rather useless for operating systems that don't use
> > Trustzone then, right?
> 
> I'm not sure what you're implying.  By running on a device with Trustzone
> extensions, Linux is using them whether it knows it or not.
> 
> Linux on ARMs evaluation boards runs on the secure size of the Trustzone
> dividing line.  Linux on OMAP SoCs runs on the insecure size of that,
> and has to make secure monitor calls to manipulate certain registers
> (eg, to enable workarounds for errata etc).  As SMC calls are highly
> implementation specific, there is and can be no "trustzone" driver.

My point was that when Linux runs in the secure partition (ok, I didn't
know we did that, but still), anything that corrupts Linux data has
security implications. If Linux runs outside of Trustzone, you can
also currupt Linux and the security is completely pointless because
after Linux is gone, you have nothing left that drives your devices
or runs user processes.

The only case where TrustZone would help is when you have an operating
system running in the secure partition as some sort of microkernel
(a.k.a. hypervisor) and have the "unpredictable" behavior isolated
in nonessential parts of the system.

	Arnd
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