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Based on kernel version 4.9. Page generated on 2016-12-21 14:34 EST.

1	Intel(R) TXT Overview:
2	=====================
4	Intel's technology for safer computing, Intel(R) Trusted Execution
5	Technology (Intel(R) TXT), defines platform-level enhancements that
6	provide the building blocks for creating trusted platforms.
8	Intel TXT was formerly known by the code name LaGrande Technology (LT).
10	Intel TXT in Brief:
11	o  Provides dynamic root of trust for measurement (DRTM)
12	o  Data protection in case of improper shutdown
13	o  Measurement and verification of launched environment
15	Intel TXT is part of the vPro(TM) brand and is also available some
16	non-vPro systems.  It is currently available on desktop systems
17	based on the Q35, X38, Q45, and Q43 Express chipsets (e.g. Dell
18	Optiplex 755, HP dc7800, etc.) and mobile systems based on the GM45,
19	PM45, and GS45 Express chipsets.
21	For more information, see http://www.intel.com/technology/security/.
22	This site also has a link to the Intel TXT MLE Developers Manual,
23	which has been updated for the new released platforms.
25	Intel TXT has been presented at various events over the past few
26	years, some of which are:
27	      LinuxTAG 2008:
28	          http://www.linuxtag.org/2008/en/conf/events/vp-donnerstag.html
29	      TRUST2008:
30	          http://www.trust-conference.eu/downloads/Keynote-Speakers/
31	          3_David-Grawrock_The-Front-Door-of-Trusted-Computing.pdf
32	      IDF, Shanghai:
33	          http://www.prcidf.com.cn/index_en.html
34	      IDFs 2006, 2007 (I'm not sure if/where they are online)
36	Trusted Boot Project Overview:
37	=============================
39	Trusted Boot (tboot) is an open source, pre-kernel/VMM module that
40	uses Intel TXT to perform a measured and verified launch of an OS
41	kernel/VMM.
43	It is hosted on SourceForge at http://sourceforge.net/projects/tboot.
44	The mercurial source repo is available at http://www.bughost.org/
45	repos.hg/tboot.hg.
47	Tboot currently supports launching Xen (open source VMM/hypervisor
48	w/ TXT support since v3.2), and now Linux kernels.
51	Value Proposition for Linux or "Why should you care?"
52	=====================================================
54	While there are many products and technologies that attempt to
55	measure or protect the integrity of a running kernel, they all
56	assume the kernel is "good" to begin with.  The Integrity
57	Measurement Architecture (IMA) and Linux Integrity Module interface
58	are examples of such solutions.
60	To get trust in the initial kernel without using Intel TXT, a
61	static root of trust must be used.  This bases trust in BIOS
62	starting at system reset and requires measurement of all code
63	executed between system reset through the completion of the kernel
64	boot as well as data objects used by that code.  In the case of a
65	Linux kernel, this means all of BIOS, any option ROMs, the
66	bootloader and the boot config.  In practice, this is a lot of
67	code/data, much of which is subject to change from boot to boot
68	(e.g. changing NICs may change option ROMs).  Without reference
69	hashes, these measurement changes are difficult to assess or
70	confirm as benign.  This process also does not provide DMA
71	protection, memory configuration/alias checks and locks, crash
72	protection, or policy support.
74	By using the hardware-based root of trust that Intel TXT provides,
75	many of these issues can be mitigated.  Specifically: many
76	pre-launch components can be removed from the trust chain, DMA
77	protection is provided to all launched components, a large number
78	of platform configuration checks are performed and values locked,
79	protection is provided for any data in the event of an improper
80	shutdown, and there is support for policy-based execution/verification.
81	This provides a more stable measurement and a higher assurance of
82	system configuration and initial state than would be otherwise
83	possible.  Since the tboot project is open source, source code for
84	almost all parts of the trust chain is available (excepting SMM and
85	Intel-provided firmware).
87	How Does it Work?
88	=================
90	o  Tboot is an executable that is launched by the bootloader as
91	   the "kernel" (the binary the bootloader executes).
92	o  It performs all of the work necessary to determine if the
93	   platform supports Intel TXT and, if so, executes the GETSEC[SENTER]
94	   processor instruction that initiates the dynamic root of trust.
95	   -  If tboot determines that the system does not support Intel TXT
96	      or is not configured correctly (e.g. the SINIT AC Module was
97	      incorrect), it will directly launch the kernel with no changes
98	      to any state.
99	   -  Tboot will output various information about its progress to the
100	      terminal, serial port, and/or an in-memory log; the output
101	      locations can be configured with a command line switch.
102	o  The GETSEC[SENTER] instruction will return control to tboot and
103	   tboot then verifies certain aspects of the environment (e.g. TPM NV
104	   lock, e820 table does not have invalid entries, etc.).
105	o  It will wake the APs from the special sleep state the GETSEC[SENTER]
106	   instruction had put them in and place them into a wait-for-SIPI
107	   state.
108	   -  Because the processors will not respond to an INIT or SIPI when
109	      in the TXT environment, it is necessary to create a small VT-x
110	      guest for the APs.  When they run in this guest, they will
111	      simply wait for the INIT-SIPI-SIPI sequence, which will cause
112	      VMEXITs, and then disable VT and jump to the SIPI vector.  This
113	      approach seemed like a better choice than having to insert
114	      special code into the kernel's MP wakeup sequence.
115	o  Tboot then applies an (optional) user-defined launch policy to
116	   verify the kernel and initrd.
117	   -  This policy is rooted in TPM NV and is described in the tboot
118	      project.  The tboot project also contains code for tools to
119	      create and provision the policy.
120	   -  Policies are completely under user control and if not present
121	      then any kernel will be launched.
122	   -  Policy action is flexible and can include halting on failures
123	      or simply logging them and continuing.
124	o  Tboot adjusts the e820 table provided by the bootloader to reserve
125	   its own location in memory as well as to reserve certain other
126	   TXT-related regions.
127	o  As part of its launch, tboot DMA protects all of RAM (using the
128	   VT-d PMRs).  Thus, the kernel must be booted with 'intel_iommu=on'
129	   in order to remove this blanket protection and use VT-d's
130	   page-level protection.
131	o  Tboot will populate a shared page with some data about itself and
132	   pass this to the Linux kernel as it transfers control.
133	   -  The location of the shared page is passed via the boot_params
134	      struct as a physical address.
135	o  The kernel will look for the tboot shared page address and, if it
136	   exists, map it.
137	o  As one of the checks/protections provided by TXT, it makes a copy
138	   of the VT-d DMARs in a DMA-protected region of memory and verifies
139	   them for correctness.  The VT-d code will detect if the kernel was
140	   launched with tboot and use this copy instead of the one in the
141	   ACPI table.
142	o  At this point, tboot and TXT are out of the picture until a
143	   shutdown (S<n>)
144	o  In order to put a system into any of the sleep states after a TXT
145	   launch, TXT must first be exited.  This is to prevent attacks that
146	   attempt to crash the system to gain control on reboot and steal
147	   data left in memory.
148	   -  The kernel will perform all of its sleep preparation and
149	      populate the shared page with the ACPI data needed to put the
150	      platform in the desired sleep state.
151	   -  Then the kernel jumps into tboot via the vector specified in the
152	      shared page.
153	   -  Tboot will clean up the environment and disable TXT, then use the
154	      kernel-provided ACPI information to actually place the platform
155	      into the desired sleep state.
156	   -  In the case of S3, tboot will also register itself as the resume
157	      vector.  This is necessary because it must re-establish the
158	      measured environment upon resume.  Once the TXT environment
159	      has been restored, it will restore the TPM PCRs and then
160	      transfer control back to the kernel's S3 resume vector.
161	      In order to preserve system integrity across S3, the kernel
162	      provides tboot with a set of memory ranges (RAM and RESERVED_KERN
163	      in the e820 table, but not any memory that BIOS might alter over
164	      the S3 transition) that tboot will calculate a MAC (message
165	      authentication code) over and then seal with the TPM. On resume
166	      and once the measured environment has been re-established, tboot
167	      will re-calculate the MAC and verify it against the sealed value.
168	      Tboot's policy determines what happens if the verification fails.
169	      Note that the c/s 194 of tboot which has the new MAC code supports
170	      this.
172	That's pretty much it for TXT support.
175	Configuring the System:
176	======================
178	This code works with 32bit, 32bit PAE, and 64bit (x86_64) kernels.
180	In BIOS, the user must enable:  TPM, TXT, VT-x, VT-d.  Not all BIOSes
181	allow these to be individually enabled/disabled and the screens in
182	which to find them are BIOS-specific.
184	grub.conf needs to be modified as follows:
185	        title Linux 2.6.29-tip w/ tboot
186	          root (hd0,0)
187	                kernel /tboot.gz logging=serial,vga,memory
188	                module /vmlinuz-2.6.29-tip intel_iommu=on ro
189	                       root=LABEL=/ rhgb console=ttyS0,115200 3
190	                module /initrd-2.6.29-tip.img
191	                module /Q35_SINIT_17.BIN
193	The kernel option for enabling Intel TXT support is found under the
194	Security top-level menu and is called "Enable Intel(R) Trusted
195	Execution Technology (TXT)".  It is considered EXPERIMENTAL and
196	depends on the generic x86 support (to allow maximum flexibility in
197	kernel build options), since the tboot code will detect whether the
198	platform actually supports Intel TXT and thus whether any of the
199	kernel code is executed.
201	The Q35_SINIT_17.BIN file is what Intel TXT refers to as an
202	Authenticated Code Module.  It is specific to the chipset in the
203	system and can also be found on the Trusted Boot site.  It is an
204	(unencrypted) module signed by Intel that is used as part of the
205	DRTM process to verify and configure the system.  It is signed
206	because it operates at a higher privilege level in the system than
207	any other macrocode and its correct operation is critical to the
208	establishment of the DRTM.  The process for determining the correct
209	SINIT ACM for a system is documented in the SINIT-guide.txt file
210	that is on the tboot SourceForge site under the SINIT ACM downloads.
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