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  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
 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:
 29       TRUST2008:
 31           3_David-Grawrock_The-Front-Door-of-Trusted-Computing.pdf
 32       IDF, Shanghai:
 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
 44 The mercurial source repo is available at
 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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