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Subject[RFC][PATCH 0b/3] TXT: Intel(R) Trusted Execution Technology support for Linux - Details
(re-posted with apologies for formatting and email addresses)

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
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 first
(ttyS0/COM1) serial port; this can be disabled by a command line
It can also be compiled to put the information into a hardcoded memory
location in low memory so that systems without a serial port can still
get the log.
o The GETSEC[SENTER] instruction will return control to tboot and it 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
than having to insert special code into the kernel's MP wakeup
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
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 exists,
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 launch,
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 the
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 shortly
(the current suspend code brings the APs out of suspend with an INIT,
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
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 2.6.27-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
Trusted Boot SourceForge project.

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_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 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.

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

Joseph Cihula
Shane Wang
Gang Wei
Intel Corp.

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