Re: [Linaro-acpi] [PATCH v5 18/18] Documentation: ACPI for ARM64

[Arnd Bergmann <arnd@arndb.de>]

On Wednesday 07 January 2015 17:59:04 Jason Cooper wrote:
> On Wed, Jan 07, 2015 at 03:05:14PM -0500, Jon Masters wrote:
> > On 01/07/2015 02:58 PM, Jon Masters wrote:
> > > On 01/07/2015 01:41 PM, Jason Cooper wrote:
> > 
> > >> One of the reasons I've really enjoyed working with ARM platforms and DT
> > >> is the absence of this type of 'feature'.  I honestly don't care whether
> > >> the kernel gets the board configuration info from DT or ACPI or FOO, as
> > >> long as we can avoid the security mistakes of the past:
> > >>
> > >>   http://www.spiegel.de/international/world/catalog-reveals-nsa-has-back-doors-for-numerous-devices-a-940994.html
> > > 
> > > ACPI is not the great satan. I'm aware certain others in the community
> > > have written missinformed blog posts and G+ rants equating ACPI with SMI
> > > and even with various other system firmware. I can't force someone to
> > > become informed on a topic, especially if it's politically useful to
> > > them to hate on ACPI and use the security paranoia handwavy argument.
> > 
> > To clarify, and this is not directed at you Jason, it is politically
> > useful to some who have written rants those business models are built
> > upon being paid to enable platforms. For those folks, standardized
> > platforms which allow a common OS approach are seen as threatening.
> 
> Ahh, thanks for clarifying.
> 
> > In the previous rants (which were really instigated as a result of the
> > above) ACPI was equated with SMM (System Management Mode), which is a
> > bit like the Secure/Trusted world on AArch64 in which you might run
> > another "Trusted" OS. These are the places where you want to watch out
> > to malware of the kind cited in your link, not in ACPI tables.
> 
> fwiw, I *am* concerned about those spaces.  It seems we agree on their
> vulnerability to attack (plus being meaty targets).
> 
> To more concisely state my other reply to you, wrt to AML, I'm primarily
> concerned about a malicious update to the ACPI tables.  The ACPI tables
> in the update would be otherwise normal, except for the AML blob that
> contains some extra code.  The malicious payload.  Then, a routine call
> into an AML (for pinctrl, say) executes the malicious code.
> 
> The plausibility and preventability of that style of attack is what I'm
> hoping to nail down with this discussion.

If you want to run hidden code through the firmware, then doing an attack
on Intel SMM or ARM TrustZone would be much harder to detect and as easy
to get in, as long as you have the ability to flash arbitrary firmware.
I think this has been shown to happen in the wild. That code could
go and manipulate the running kernel image to do something else.

Running code inside of the AML interpreter is fairly limited for the
purpose of an attack [*], but there might be bugs in the interpreter that
allow arbitrary code execution from malicious firmware. I think this
case would be similar to constructing a malicious DT blob that exploits
a bug in the DT parser for arbitrary code execution. The AML interpreter
is a relatively large chunk of code, but it's self-contained. In
comparison, the DT parser is much smaller, but has the additional
(theoretical) problem of potential buffer overflows in any drivers that
use it incorrectly (e.g. format string attacks on string properties).
Another difference is that the AML code is intended to not be
user-upgradable without a full firmware upgrade, while a DT blob is
meant to be easily replaced if necessary without flashing the firmware,
using the same permissions you need for updating the OS.

I'm deliberately not trying to draw conclusions regarding whether AML
is more or less secure than DT, but the above is my understanding of
the fundamental differences.

	Arnd

[*] I would assume you can get AML code to write to arbitrary physical
memory locations without much effort, but it has rather limited arithmetical
capabilities which makes it hard to know where to write to.