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).
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:
28 http://www.linuxtag.org/2008/en/conf/events/vp-donnerstag.html
30 http://www.trust-conference.eu/downloads/Keynote-Speakers/
31 3_David-Grawrock_The-Front-Door-of-Trusted-Computing.pdf
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
43 It is hosted on SourceForge at http://sourceforge.net/projects/tboot.
44 The mercurial source repo is available at http://www.bughost.org/
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).
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
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
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
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
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
142 o At this point, tboot and TXT are out of the picture until a
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
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
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
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
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.