Documentation/intel_txt.txt

   1 =====================
   2 Intel(R) TXT Overview
   3 =====================
   4
   5 Intel's technology for safer computing, Intel(R) Trusted Execution
   6 Technology (Intel(R) TXT), defines platform-level enhancements that
   7 provide the building blocks for creating trusted platforms.
   8
   9 Intel TXT was formerly known by the code name LaGrande Technology (LT).
  10
  11 Intel TXT in Brief:
  12
  13 -  Provides dynamic root of trust for measurement (DRTM)
  14 -  Data protection in case of improper shutdown
  15 -  Measurement and verification of launched environment
  16
  17 Intel TXT is part of the vPro(TM) brand and is also available some
  18 non-vPro systems.  It is currently available on desktop systems
  19 based on the Q35, X38, Q45, and Q43 Express chipsets (e.g. Dell
  20 Optiplex 755, HP dc7800, etc.) and mobile systems based on the GM45,
  21 PM45, and GS45 Express chipsets.
  22
  23 For more information, see http://www.intel.com/technology/security/.
  24 This site also has a link to the Intel TXT MLE Developers Manual,
  25 which has been updated for the new released platforms.
  26
  27 Intel TXT has been presented at various events over the past few
  28 years, some of which are:
  29
  30       - LinuxTAG 2008:
  31           http://www.linuxtag.org/2008/en/conf/events/vp-donnerstag.html
  32
  33       - TRUST2008:
  34           http://www.trust-conference.eu/downloads/Keynote-Speakers/
  35           3_David-Grawrock_The-Front-Door-of-Trusted-Computing.pdf
  36
  37       - IDF, Shanghai:
  38           http://www.prcidf.com.cn/index_en.html
  39
  40       - IDFs 2006, 2007
  41           (I'm not sure if/where they are online)
  42
  43 Trusted Boot Project Overview
  44 =============================
  45
  46 Trusted Boot (tboot) is an open source, pre-kernel/VMM module that
  47 uses Intel TXT to perform a measured and verified launch of an OS
  48 kernel/VMM.
  49
  50 It is hosted on SourceForge at http://sourceforge.net/projects/tboot.
  51 The mercurial source repo is available at http://www.bughost.org/
  52 repos.hg/tboot.hg.
  53
  54 Tboot currently supports launching Xen (open source VMM/hypervisor
  55 w/ TXT support since v3.2), and now Linux kernels.
  56
  57
  58 Value Proposition for Linux or "Why should you care?"
  59 =====================================================
  60
  61 While there are many products and technologies that attempt to
  62 measure or protect the integrity of a running kernel, they all
  63 assume the kernel is "good" to begin with.  The Integrity
  64 Measurement Architecture (IMA) and Linux Integrity Module interface
  65 are examples of such solutions.
  66
  67 To get trust in the initial kernel without using Intel TXT, a
  68 static root of trust must be used.  This bases trust in BIOS
  69 starting at system reset and requires measurement of all code
  70 executed between system reset through the completion of the kernel
  71 boot as well as data objects used by that code.  In the case of a
  72 Linux kernel, this means all of BIOS, any option ROMs, the
  73 bootloader and the boot config.  In practice, this is a lot of
  74 code/data, much of which is subject to change from boot to boot
  75 (e.g. changing NICs may change option ROMs).  Without reference
  76 hashes, these measurement changes are difficult to assess or
  77 confirm as benign.  This process also does not provide DMA
  78 protection, memory configuration/alias checks and locks, crash
  79 protection, or policy support.
  80
  81 By using the hardware-based root of trust that Intel TXT provides,
  82 many of these issues can be mitigated.  Specifically: many
  83 pre-launch components can be removed from the trust chain, DMA
  84 protection is provided to all launched components, a large number
  85 of platform configuration checks are performed and values locked,
  86 protection is provided for any data in the event of an improper
  87 shutdown, and there is support for policy-based execution/verification.
  88 This provides a more stable measurement and a higher assurance of
  89 system configuration and initial state than would be otherwise
  90 possible.  Since the tboot project is open source, source code for
  91 almost all parts of the trust chain is available (excepting SMM and
  92 Intel-provided firmware).
  93
  94 How Does it Work?
  95 =================
  96
  97 -  Tboot is an executable that is launched by the bootloader as
  98    the "kernel" (the binary the bootloader executes).
  99 -  It performs all of the work necessary to determine if the
 100    platform supports Intel TXT and, if so, executes the GETSEC[SENTER]
 101    processor instruction that initiates the dynamic root of trust.
 102
 103    -  If tboot determines that the system does not support Intel TXT
 104       or is not configured correctly (e.g. the SINIT AC Module was
 105       incorrect), it will directly launch the kernel with no changes
 106       to any state.
 107    -  Tboot will output various information about its progress to the
 108       terminal, serial port, and/or an in-memory log; the output
 109       locations can be configured with a command line switch.
 110
 111 -  The GETSEC[SENTER] instruction will return control to tboot and
 112    tboot then verifies certain aspects of the environment (e.g. TPM NV
 113    lock, e820 table does not have invalid entries, etc.).
 114 -  It will wake the APs from the special sleep state the GETSEC[SENTER]
 115    instruction had put them in and place them into a wait-for-SIPI
 116    state.
 117
 118    -  Because the processors will not respond to an INIT or SIPI when
 119       in the TXT environment, it is necessary to create a small VT-x
 120       guest for the APs.  When they run in this guest, they will
 121       simply wait for the INIT-SIPI-SIPI sequence, which will cause
 122       VMEXITs, and then disable VT and jump to the SIPI vector.  This
 123       approach seemed like a better choice than having to insert
 124       special code into the kernel's MP wakeup sequence.
 125
 126 -  Tboot then applies an (optional) user-defined launch policy to
 127    verify the kernel and initrd.
 128
 129    -  This policy is rooted in TPM NV and is described in the tboot
 130       project.  The tboot project also contains code for tools to
 131       create and provision the policy.
 132    -  Policies are completely under user control and if not present
 133       then any kernel will be launched.
 134    -  Policy action is flexible and can include halting on failures
 135       or simply logging them and continuing.
 136
 137 -  Tboot adjusts the e820 table provided by the bootloader to reserve
 138    its own location in memory as well as to reserve certain other
 139    TXT-related regions.
 140 -  As part of its launch, tboot DMA protects all of RAM (using the
 141    VT-d PMRs).  Thus, the kernel must be booted with 'intel_iommu=on'
 142    in order to remove this blanket protection and use VT-d's
 143    page-level protection.
 144 -  Tboot will populate a shared page with some data about itself and
 145    pass this to the Linux kernel as it transfers control.
 146
 147    -  The location of the shared page is passed via the boot_params
 148       struct as a physical address.
 149
 150 -  The kernel will look for the tboot shared page address and, if it
 151    exists, map it.
 152 -  As one of the checks/protections provided by TXT, it makes a copy
 153    of the VT-d DMARs in a DMA-protected region of memory and verifies
 154    them for correctness.  The VT-d code will detect if the kernel was
 155    launched with tboot and use this copy instead of the one in the
 156    ACPI table.
 157 -  At this point, tboot and TXT are out of the picture until a
 158    shutdown (S<n>)
 159 -  In order to put a system into any of the sleep states after a TXT
 160    launch, TXT must first be exited.  This is to prevent attacks that
 161    attempt to crash the system to gain control on reboot and steal
 162    data left in memory.
 163
 164    -  The kernel will perform all of its sleep preparation and
 165       populate the shared page with the ACPI data needed to put the
 166       platform in the desired sleep state.
 167    -  Then the kernel jumps into tboot via the vector specified in the
 168       shared page.
 169    -  Tboot will clean up the environment and disable TXT, then use the
 170       kernel-provided ACPI information to actually place the platform
 171       into the desired sleep state.
 172    -  In the case of S3, tboot will also register itself as the resume
 173       vector.  This is necessary because it must re-establish the
 174       measured environment upon resume.  Once the TXT environment
 175       has been restored, it will restore the TPM PCRs and then
 176       transfer control back to the kernel's S3 resume vector.
 177       In order to preserve system integrity across S3, the kernel
 178       provides tboot with a set of memory ranges (RAM and RESERVED_KERN
 179       in the e820 table, but not any memory that BIOS might alter over
 180       the S3 transition) that tboot will calculate a MAC (message
 181       authentication code) over and then seal with the TPM. On resume
 182       and once the measured environment has been re-established, tboot
 183       will re-calculate the MAC and verify it against the sealed value.
 184       Tboot's policy determines what happens if the verification fails.
 185       Note that the c/s 194 of tboot which has the new MAC code supports
 186       this.
 187
 188 That's pretty much it for TXT support.
 189
 190
 191 Configuring the System
 192 ======================
 193
 194 This code works with 32bit, 32bit PAE, and 64bit (x86_64) kernels.
 195
 196 In BIOS, the user must enable:  TPM, TXT, VT-x, VT-d.  Not all BIOSes
 197 allow these to be individually enabled/disabled and the screens in
 198 which to find them are BIOS-specific.
 199
 200 grub.conf needs to be modified as follows::
 201
 202         title Linux 2.6.29-tip w/ tboot
 203           root (hd0,0)
 204                 kernel /tboot.gz logging=serial,vga,memory
 205                 module /vmlinuz-2.6.29-tip intel_iommu=on ro
 206                        root=LABEL=/ rhgb console=ttyS0,115200 3
 207                 module /initrd-2.6.29-tip.img
 208                 module /Q35_SINIT_17.BIN
 209
 210 The kernel option for enabling Intel TXT support is found under the
 211 Security top-level menu and is called "Enable Intel(R) Trusted
 212 Execution Technology (TXT)".  It is considered EXPERIMENTAL and
 213 depends on the generic x86 support (to allow maximum flexibility in
 214 kernel build options), since the tboot code will detect whether the
 215 platform actually supports Intel TXT and thus whether any of the
 216 kernel code is executed.
 217
 218 The Q35_SINIT_17.BIN file is what Intel TXT refers to as an
 219 Authenticated Code Module.  It is specific to the chipset in the
 220 system and can also be found on the Trusted Boot site.  It is an
 221 (unencrypted) module signed by Intel that is used as part of the
 222 DRTM process to verify and configure the system.  It is signed
 223 because it operates at a higher privilege level in the system than
 224 any other macrocode and its correct operation is critical to the
 225 establishment of the DRTM.  The process for determining the correct
 226 SINIT ACM for a system is documented in the SINIT-guide.txt file
 227 that is on the tboot SourceForge site under the SINIT ACM downloads.