by Daniel P. Smith

[permalink] [raw]

Subject: Re: [PATCH v5 00/12] x86: Trenchboot secure dynamic launch Linux kernel support

Hi Dave!

Please find a response that will hopefully address the questions raised.
The answers were meant to be thorough but succinct, though if there is
any areas that are not clear for anyone, please feel free to
ask. This response and any further questions for clarity will be
incorporated into the documentation patch and the cover letter for the
next version of the series.

On 2/23/22 12:45, Dave Hansen wrote:
> On 2/16/22 19:54, Ross Philipson wrote:
>> The larger focus of the TrenchBoot project (https://github.com/TrenchBoot) is to
>> enhance the boot security and integrity in a unified manner. The first area of
>> focus has been on the Trusted Computing Group's Dynamic Launch for establishing
>> a hardware Root of Trust for Measurement, also know as DRTM (Dynamic Root of
>> Trust for Measurement). The project has been and continues to work on providing
>> a unified means to Dynamic Launch that is a cross-platform (Intel and AMD) and
>> cross-architecture (x86 and Arm), with our recent involvment in the upcoming
>> Arm DRTM specification. The order of introducing DRTM to the Linux kernel
>> follows the maturity of DRTM in the architectures. Intel's Trusted eXecution
>> Technology (TXT) is present today and only requires a preamble loader, e.g. a
>> boot loader, and an OS kernel that is TXT-aware. AMD DRTM implementation has
>> been present since the introduction of AMD-V but requires an additional
>> component that is AMD specific and referred to in the specification as the
>> Secure Loader, which the TrenchBoot project has an active prototype in
>> development. Finally Arm's implementation is in specification development stage
>> and the project is looking to support it when it becomes available.
>
> What problem is this patch series solving? Is the same problem solved
> in a different way in the kernel today? What is wrong with that
> solution? What effects will end users see if they apply this series?

What problem is the Secure Launch patch series solving?
-------------------------------------------------------

* This patch series begins solving the problem of maintaining a robust
multi-architecture path of entry from DRTM into the Linux kernel.
* DRTM (Dynamic Root of Trust for Measurement) is a strong security
capability that has been used in niche OS environments, including
OpenXT and Qubes. For more than a decade, some have successfully
deployed Linux with DRTM, but popular Linux distros have not yet used
DRTM.
* The TrenchBoot project was started to improve the usability of DRTM
with Open-Source systems software (e.g. Linux, Xen) on hardware
architectures that provide a DRTM capability, e.g Intel x86, AMD x86,
Arm, and OpenPOWER.
* Microsoft Secured Core enterprise PCs use DRTM as a cornerstone of
establishing device integrity, optionally validated by Azure
Attestation. Devices with DRTM and Linux Secure Launch will have
necessary building blocks for attestation to local and remote
services, including Azure.
* TrenchBoot contributors have collaborated with Arm on the development
of their recently released DRTM specification [1], which can enable
Windows VBS (virtualization-based security) and Linux Secure Launch
capabilities, on DRTM-capable Arm devices such as Microsoft Secured
Core PCs.
* From 2018-2020, possibly motivated by DRTM requirements in MS Secured
Core, Intel Hardware Shield[2] introduced vPro hardware and firmware
features for SMM (System Management Mode) trustworthiness via
attestable isolation between operating systems and SMM. DRTM prevents
any DMA interference during the Intel Hardware Shield PPAM integrity
report exchange with Linux.

[1] https://developer.arm.com/documentation/den0113/latest
[2]
https://www.intel.com/content/dam/www/central-libraries/us/en/documents/drtm-
based-computing-whitepaper.pdf

Is the same problem solved in a different way in the kernel today?
------------------------------------------------------------------

* Today the only way to start Linux via DRTM is with Intel's tboot
exokernel.
* The Secure Launch patch series was designed to co-exist with the
existing tboot extensions in the Linux kernel as to not to disrupt
existing users of tboot.
* The first beta release of the Arm DRTM specification was just made
public on February 17th, 2022. Obviously there are currently no
implementations available yet.

What is wrong with that solution?
---------------------------------

* A short discussion over tboot can be found in the Overview section of
secure_launch_overview.rst in the documentation patch, which is v5
patch 02/12.
* Functionally tboot's primary deficiency is that it is an Intel-only
solution.
* There is no support for the AMD/Hygon, whose SKINIT capability has
been around nearly as long as Intel TXT.
* The security merits of tboot's approach could be debated endlessly by
security researchers depending on their view of trust and security.

What effects will end users see if they apply this series?
----------------------------------------------------------

* To provide a full answer, the capability can be completely disabled
via the Kconfig system resulting in no new code paths.
* The other case is when a kernel is built with Secure Launch enabled
and in this case there are two relevant aspects, impacts to user
experience and the benefits the user will gain.
* As to the impacts to user experience, the end users should see no
effects in the launch of the kernel from this series.
* One of the primary goals for this series was to minimize change to the
kernel boot flow and to ensure the capability was benign if compiled
in but not enabled/used.
* When the bootloader is configured to launch the kernel via DRTM, again
there will be little to no effect on the user experience. There are a
few CPU behavior differences that result from doing a DRTM but their
effect is only seen by Linux internals, for which this series makes
the kernel aware.
* The benefit is that it removes having to trust all the second and
third party code in the UEFI boot chain. For instance during the
Boothole vulnerability situation, Boothole had a near zero if not a
zero impact for DRTM systems, i.e. it could not be used to compromise
nor load a bad kernel.
* Removing the need to trust every driver, service, and setup code in
firmware is not the only benefit as DRTM provides several use cases
that are not otherwise possible. Please see my response to Paul Moore
or visit trenchboot.org/events to see the numerous talks on usages and
capabilities that are possible because of DRTM.

V/r,
Daniel P. Smith