tpm2_policynvwritten(1)

okay so this is tpm2_policynvwritten(1) from the General Commands Manual — and it's giving lock down TPM object authorization to the written state of an NV index energy 💅

NAME 💖

tpm2_policynvwritten(1) — restrict TPM object authorization to the written state of an NV index. Periodt.

SYNOPSIS ✨

tpm2_policynvwritten [OPTIONS] [ARGUMENT]

DESCRIPTION 🦋

tpm2_policynvwritten(1) restricts TPM object authorization to the written state of an NV index. It's useful when creating write once NV indexes. 🫶

As an [ARGUMENT] it takes the expected written state of the NV index. It can be specified as s|c|0|1.

OPTIONS 💅

• -S, --session=FILE — A session file from tpm2_startauthsession(1)'s -S option.
• -L, --policy=FILE — File to save the policy digest.
• --cphash=FILE — File path to record the hash of the command parameters. This is commonly termed as cpHash. NOTE: When this option is selected, The tool will not actually execute the command, it simply returns a cpHash.

References 🎀

(The original page has this heading with no extra text under it — keeping it exactly, bestie.)

COMMON OPTIONS 🌸

This collection of options is common to many programs and gives you info many users may expect. 💖

• -h, --help=[man|no-man] — Display the tool's manpage. By default, it attempts to invoke the manpager for the tool, however, on failure will output a short tool summary. This is the same behavior if the man option argument is specified, however if explicit man is requested, the tool will provide errors from man on stderr. If the no-man option is specified, or the manpager fails, the short options will be output to stdout. To successfully use the manpages feature requires the manpages to be installed or on MANPATH, see man(1) for more details.
• -v, --version — Display version information for this tool, supported tctis and exit.
• -V, --verbose — Increase the information that the tool prints to the console during its execution. When using this option the file and line number are printed.
• -Q, --quiet — Silence normal tool output to stdout.
• -Z, --enable-errata — Enable the application of errata fixups. Useful if an errata fixup needs to be applied to commands sent to the TPM. Defining the environment TPM2TOOLS_ENABLE_ERRATA is equivalent.
• -z, --pwd-session — Use password session instead of a HMAC session for authentication. A clear text password is passed to the TPM to authorize the action. This option can be used to avoid problems when unsalted sessions are used in OpenSSL FIPS mode. If auth values are used a salted session should be used for authentication.
• -R, --autoflush — Enable autoflush for transient objects created by the command. If a parent object is loaded from a context file also the transient parent object will be flushed. Autoflush can also be activated if the environment variable TPM2TOOLS_AUTOFLUSH is is set to yes or true. information many users may expect.

TCTI Configuration 🔌

The TCTI or “Transmission Interface” is the communication mechanism with the TPM. TCTIs can be changed for communication with TPMs across different mediums. 🌈

To control the TCTI, the tools respect:

1. The command line option -T or --tcti
2. The environment variable: TPM2TOOLS_TCTI.

Note: The command line option always overrides the environment variable.

The current known TCTIs are:

• tabrmd — The resource manager, called tabrmd. Note that tabrmd and abrmd as a tcti name are synonymous.
• mssim — Typically used for communicating to the TPM software simulator.
• device — Used when talking directly to a TPM device file.
• none — Do not initalize (yes, that spelling in the original) a connection with the TPM. Some tools allow for off-tpm options and thus support not using a TCTI. Tools that do not support it will error when attempted to be used without a TCTI connection. Does not support ANY options and MUST BE presented as the exact text of none.

The arguments to either the command line option or the environment variable are in the form:

<tcti-name>:<tcti-option-config>

Specifying an empty string for either the <tcti-name> or <tcti-option-config> results in the default being used for that portion respectively.

TCTI Defaults 🛟

When a TCTI is not specified, the default TCTI is searched for using dlopen(3) semantics. The tools will search for tabrmd, device and mssim TCTIs IN THAT ORDER and USE THE FIRST ONE FOUND. You can query what TCTI will be chosen as the default by using the -v option to print the version information. The default-tcti key-value pair will indicate which of the aforementioned TCTIs is the default.

Custom TCTIs 🧚

Any TCTI that implements the dynamic TCTI interface can be loaded. The tools internally use dlopen(3), and the raw tcti-name value is used for the lookup. Thus, this could be a path to the shared library, or a library name as understood by dlopen(3) semantics.

TCTI OPTIONS ⚙️

This collection of options are used to configure the various known TCTI modules available:

• device — For the device TCTI, the TPM character device file for use by the device TCTI can be specified. The default is /dev/tpm0.

Example:

-T device:/dev/tpm0

or:

export TPM2TOOLS_TCTI=“device:/dev/tpm0”

• mssim — For the mssim TCTI, the domain name or IP address and port number used by the simulator can be specified. The default are 127.0.0.1 and 2321.

Example:

-T mssim:host=localhost,port=2321

or:

export TPM2TOOLS_TCTI=“mssim:host=localhost,port=2321”

• abrmd — For the abrmd TCTI, the configuration string format is a series of simple key value pairs separated by a , character. Each key and value string are separated by a = character.
  • TCTI abrmd supports two keys:
    1. bus_name : The name of the tabrmd service on the bus (a string).
    2. bus_type : The type of the dbus instance (a string) limited to session and system.

Specify the tabrmd tcti name and a config string of bus_name=com.example.FooBar:

\--tcti=tabrmd:bus_name=com.example.FooBar

Specify the default (abrmd) tcti and a config string of bus_type=session:

\--tcti:bus_type=session

NOTE: abrmd and tabrmd are synonymous. the various known TCTI modules.

EXAMPLES 🍓

Create a write once NV index. To do this the NV index is defined with a write policy that is valid only if the NV index attribute TPMA_NV_WRITTEN was never set.

Define the NV index write policy

tpm2_startauthsession -S session.dat
tpm2_policycommandcode -S session.dat TPM2_CC_NV_Write
tpm2_policynvwritten -S session.dat -L nvwrite.policy c
tpm2_flushcontext session.dat

Define the NV index with the policy

tpm2_nvdefine -s 1 -a "authread|policywrite" -p nvrdpass -L nvwrite.policy

Write the NV index by satisfying the policy

tpm2_startauthsession -S session.dat --policy-session
tpm2_policycommandcode -S session.dat TPM2_CC_NV_Write
tpm2_policynvwritten -S session.dat c
echo 0xAA | xxd -r -p | tpm2_nvwrite 0x01000000 -i- -P session:session.dat
tpm2_flushcontext session.dat

Returns 😌

Tools can return any of the following codes:

• 0 — Success.
• 1 — General non-specific error.
• 2 — Options handling error.
• 3 — Authentication error.
• 4 — TCTI related error.
• 5 — Non supported scheme. Applicable to tpm2_testparams.

Limitations ⚠️

It expects a session to be already established via tpm2_startauthsession(1) and requires one of the following:

• direct device access
• extended session support with tpm2-abrmd.

Without it, most resource managers will not save session state between command invocations.

BUGS 💀

Github Issues

HELP 🫶

See the Mailing List

License 👑

This work is licensed under the Creative Commons Attribution 4.0 International License (CC BY 4.0).

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mwah 💜 — source/version: tpm2-tools; manual page: tpm2_policynvwritten(1). The original footer didn't show a date, so I'm not inventing one, bestie. 💅