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[RFC v2][PATCH 0b/1] intel_txt: Intel(R) Trusted Execution Technology support for Linux - Details

From:  Joseph Cihula <>
Subject:  [RFC v2][PATCH 0b/1] intel_txt: Intel(R) Trusted Execution Technology support for Linux - Details
Date:  Mon, 30 Mar 2009 22:14:13 -0700
Message-ID:  <>
Archive-link:  Article

How Does it Work?

o  Tboot is an executable that is launched by the bootloader as the "kernel"
   (the binary the bootloader executes).
o  It performs all of the work necessary to determine if the platform supports
   Intel TXT and, if so, executes the GETSEC[SENTER] processor instruction
   that initiates the dynamic root of trust.
   -  If tboot determines that the system does not support Intel TXT or is not
      configured correctly (e.g. the SINIT AC Module was incorrect), it will
      directly launch the kernel with no changes to any state.
   -  Tboot will output various information about its progress to the terminal,
      serial port, and/or an in-memory log; the output locations can be configured
      with a command line switch.
o  The GETSEC[SENTER] instruction will return control to tboot and tboot then
   verifies certain aspects of the environment (e.g. TPM NV lock, e820 table
   does not have invalid entries, etc.).
o  It will wake the APs from the special sleep state the GETSEC[SENTER]
   instruction had put them in and place them into a wait-for-SIPI state.
   -  Because the processors will not respond to an INIT or SIPI when in the
      TXT environment, it is necessary to create a small VT-x guest for the
      APs.  When they run in this guest, they will simply wait for the
      INIT-SIPI-SIPI sequence, which will cause VMEXITs, and then disable VT
      and jump to the SIPI vector.  This approach seemed like a better choice
      than having to insert special code into the kernel's MP wakeup sequence.
o  Tboot then applies an (optional) user-defined launch policy to verify the
   kernel and initrd.
   -  This policy is rooted in TPM NV and is described in the tboot project.
      The tboot project also contains code for tools to create and provision
      the policy.
   -  Policies are completely under user control and if not present then any
      kernel will be launched.
   -  Policy action is flexible and can include halting on failures or simply
      logging them and continuing.
o  Tboot adjusts the e820 table provided by the bootloader to reserve its own
   location in memory as well as to reserve certain other TXT-related regions.
o  As part of it's launch, tboot DMA protects all of RAM (using the VT-d PMRs).
   Thus, the kernel must be booted with 'intel_iommu=on' in order to remove this
   blanket protection and use VT-d's page-level protection.
o  Tboot will populate a shared page with some data about itself and pass this
   to the Linux kernel as it transfers control.
   -  The location of the shared page is passed via the boot_params struct as
      a physical address.
o  The kernel will look for the tboot shared page address and, if it exists,
   map it.
o  As one of the checks/protections provided by TXT, it makes a copy of the VT-d
   DMARs in a DMA-protected region of memory and verifies them for correctness.
   The VT-d code will detect if the kernel was launched with tboot and use this
   copy instead of the one in the ACPI table.
o  At this point, tboot and TXT are out of the picture until a shutdown (S<n>)
o  In order to put a system into any of the sleep states after a TXT launch,
   TXT must first be exited.  This is to prevent attacks that attempt to crash
   the system to gain control on reboot and steal data left in memory.
   -  The kernel will perform all of its sleep preparation and populate the
      shared page with the ACPI data needed to put the platform in the desired
      sleep state.
   -  Then the kernel jumps into tboot via the vector specified in the shared
   -  Tboot will clean up the environment and disable TXT, then use the
      kernel-provided ACPI information to actually place the platform into the
      desired sleep state.
   -  In the case of S3, tboot will also register itself as the resume vector.
      This is necessary because it must re-establish the measured environment
      upon resume.  Once the TXT environment has been restored, it will
      restore the TPM PCRs and then transfer control back to the kernel's S3
      resume vector.
      In order to preserve system integrity across S3, the kernel provides tboot
      with a set of memory ranges (kernel code/data/bss, S3 resume code, and AP
      trampoline) that tboot will calculate a MAC (message authentication code)
      over and then seal with the TPM.  On resume and once the measured environment
      has been re-established, tboot will re-calculate the MAC and verify it
      against the sealed value.  Tboot's policy determines what happens if the
      verification fails.

That's pretty much it for TXT support.

Configuring the System:

This code works with 32bit, 32bit PAE, and 64bit (x86_64) kernels.

In BIOS, the user must enable:  TPM, TXT, VT-x, VT-d.  Not all BIOSes allow these to be
individually enabled/disabled and the screens in which to find them are BIOS-specific.

grub.conf needs to be modified as follows:
        title Linux 2.6.29-tip w/ tboot
          root (hd0,0)
                kernel /tboot.gz logging=serial,vga,memory
                module /vmlinuz-2.6.29-tip intel_iommu=on ro root=LABEL=/ rhgb console=ttyS0,115200
                module /initrd-2.6.29-tip.img
                module /Q35_SINIT_17.BIN

The kernel option for enabling Intel TXT support is found under the Security top-level menu and is
called "Enable Intel(R) Trusted Execution Technology (TXT)".  It is marked as EXPERIMENTAL and
depends only on the generic x86 support (to allow maximum flexibility in kernel build options),
since the tboot code will detect whether the platform actually supports Intel TXT and thus whether
any of the kernel code is executed.

The Q35_SINIT_17.BIN file is what Intel TXT refers to as an Authenticated Code Module.  It is
specific to the chipset in the system and can also be found on the Trusted Boot site.  It is an
(unencrypted) binary module signed by Intel that is used as part of the DRTM process to verify and
configure the system.  It is signed because it operates at a higher privilege level in the system
than any other macrocode and its correct operation is critical to the establishment of the DRTM.
The process for determining the correct SINIT ACM for a system is documented in the SINIT-guide.txt
file that is on the tboot SourceForge site under the SINIT ACM downloads.


This patch implements the basic TXT support as described above.  There will be additional patches
sent to add functionality and improve the security of the launched kernel.  Some of those tasks
o  userspace S3 integrity protection
   The current patch only provides memory integrity protection of the kernel.
   Complete S3 protection requires providing integrity over userspace as well.
   We have this implemented for 64b builds and are working out some issues on 32b.
o  automatic forcing of intel_iommu
   Since enabling intel_iommu is required both for security and correct operation,
   it should be forced on if the kernel is launched with TXT and any failures during
   enablement should cause a boot failure rather than booting insecurely.
o  BIOS call scrubbing
   One of the goals of TXT is to reduce the TCB by eliminating pre-kernel code from
   the trust boundary.  This includes BIOS (with the exception of SMM).  We will see
   how many of the BIOS calls can be eliminated on a TXT boot and whether the remainder
   of BIOS-provided code and data can be measured or "protected" (e.g. validate ACPI
   pointers before using).

We welcome your comments and suggestions and genuinely value your feedback.

Joseph Cihula
Shane Wang
Intel Corp.

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