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Subject[RFC][PATCH 0b/3] TXT: Intel(R) Trusted Execution Technology support for Linux - Details
How Does it Work?

o Tboot is an executable that is launched by the bootloader as the
(the binary the bootloader executes).
o It performs all of the work necessary to determine if the platform
Intel TXT and, if so, executes the GETSEC[SENTER] processor
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
directly launch the kernel with no changes to any state.
- Tboot will output various information about its progress to the
(ttyS0/COM1) serial port; this can be disabled by a command line
It can also be compiled to put the information into a hardcoded
location in low memory so that systems without a serial port can
get the log.
o The GETSEC[SENTER] instruction will return control to tboot and it
verifies certain aspects of the environment (e.g. TPM NV lock, e820
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
- Because the processors will not respond to an INIT or SIPI when in
TXT environment, it is necessary to create a small VT-x guest for
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
than having to insert special code into the kernel's MP wakeup
o Tboot then applies an (optional) user-defined launch policy to verify
kernel and initrd.
- This policy is rooted in TPM NV and is described in the tboot
The tboot project also contains code for tools to create and
the policy.
- Policies are completely under user control and if not present then
kernel will be launched.
- Policy action is flexible and can include halting on failures or
logging them an 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
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
map it.
o At this point, tboot and TXT are out of the picture until a shutdown
o In order to put a system into any of the sleep states after a TXT
TXT must first be exited. This is to prevent attacks that attempt to
the system to gain control on reboot and steal data left in memory.
- The kernel will perform all of its sleep preparation and populate
shared page with the ACPI data needed to put the platform in the
sleep state.
- Then the kernel jumps into tboot via the vector specified in the
- 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
This is necessary because it must re-establish the measured
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.
- S4 is not supported by this patch, but we expect to fix that
(the current suspend code brings the APs out of suspend with an
which doesn't work under TXT).

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
these to be individually enabled/disabled and the screens in which to
them are BIOS-specific.

grub.conf needs to be modified as follows:
title Linux 2.6.27-rc9 w/ tboot
root (hd0,0)
kernel /tboot.gz
module /vmlinuz-2.6.27-rc9 ro root=LABEL=/ rhgb
console=ttyS0,115200 3
module /initrd-2.6.27-rc9.img
module /Q35_SINIT_16.BIN

As mentioned previously, the tboot source can be downloaded from the
Boot SourceForge project.

The kernel option for enabling Intel TXT support is found under the
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

The Q35_SINIT_16.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
that is used as part of the DRTM process to verify and configure the
It is signed because it operates at a higher privilege level in the
than any other macrocode and its correct operation is critical to the
establishment of the DRTM.

We welcome your comments and suggestions and genuinely value your

Joseph Cihula
Shane Wang
Gang Wei
Intel Corp.

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