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verifiers: Add TPM documentation 

Describe the behaviour of GRUB when the TPM module is in use.

Signed-off-by: Matthew Garrett <mjg59@google.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
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Matthew Garrett 2018-11-29 11:28:10 -08:00 committed by Daniel Kiper
parent d6ca0a90ca
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@ -5545,6 +5545,7 @@ environment variables and commands are listed in the same order.
* Authentication and authorisation:: Users and access control
* Using digital signatures:: Booting digitally signed code
* UEFI secure boot and shim:: Booting digitally signed PE files
* Measured Boot:: Measuring boot components
@end menu
@node Authentication and authorisation
@ -5721,6 +5722,43 @@ mentioned requirements are enforced by the shim_lock module. And itself it
is a persistent module which means that it cannot be unloaded if it was
loaded into the memory.
@node Measured Boot
@section Measuring boot components
If the tpm module is loaded and the platform has a Trusted Platform Module
installed, GRUB will log each command executed and each file loaded into the
TPM event log and extend the PCR values in the TPM correspondingly. All events
will be logged into the PCR described below with a type of EV_IPL and an
event description as described below.
@multitable @columnfractions 0.3 0.1 0.6
@headitem Event type @tab PCR @tab Description
@item Command
@tab 8
@tab All executed commands (including those from configuration files) will be
logged and measured as entered with a prefix of ``grub_cmd: ``
@item Kernel command line
@tab 8
@tab Any command line passed to a kernel will be logged and measured as entered
with a prefix of ``kernel_cmdline: ''
@item Module command line
@tab 8
@tab Any command line passed to a kernel module will be logged and measured as
entered with a prefix of ``module_cmdline: ``
@item Files
@tab 9
@tab Any file read by GRUB will be logged and measured with a descriptive text
corresponding to the filename.
@end multitable
GRUB will not measure its own @file{core.img} - it is expected that firmware
will carry this out. GRUB will also not perform any measurements until the
tpm module is loaded. As such it is recommended that the tpm module be built
into @file{core.img} in order to avoid a potential gap in measurement between
@file{core.img} being loaded and the tpm module being loaded.
Measured boot is currently only supported on EFI platforms.
@node Platform limitations
@chapter Platform limitations