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doc: trusted-encrypted: updates with TEE as a new trust source

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Update documentation for Trusted and Encrypted Keys with TEE as a new
trust source. Following is brief description of updates:

- Add a section to demonstrate a list of supported devices along with
  their security properties/guarantees.
- Add a key generation section.
- Updates for usage section including differences specific to a trust
  source.

Co-developed-by: Elaine Palmer <erpalmer@us.ibm.com>
Signed-off-by: Elaine Palmer <erpalmer@us.ibm.com>
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
This commit is contained in:
Sumit Garg 2021-03-01 18:41:26 +05:30 committed by Jarkko Sakkinen
parent 0a95ebc913
commit c429805fdf
1 changed files with 138 additions and 33 deletions
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171
Documentation/security/keys/trusted-encrypted.rst
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@ -6,30 +6,127 @@ Trusted and Encrypted Keys are two new key types added to the existing kernel
key ring service. Both of these new types are variable length symmetric keys,
and in both cases all keys are created in the kernel, and user space sees,
stores, and loads only encrypted blobs. Trusted Keys require the availability
of a Trusted Platform Module (TPM) chip for greater security, while Encrypted
Keys can be used on any system. All user level blobs, are displayed and loaded
in hex ascii for convenience, and are integrity verified.
of a Trust Source for greater security, while Encrypted Keys can be used on any
system. All user level blobs, are displayed and loaded in hex ASCII for
convenience, and are integrity verified.
Trusted Keys use a TPM both to generate and to seal the keys. Keys are sealed
under a 2048 bit RSA key in the TPM, and optionally sealed to specified PCR
(integrity measurement) values, and only unsealed by the TPM, if PCRs and blob
integrity verifications match. A loaded Trusted Key can be updated with new
(future) PCR values, so keys are easily migrated to new pcr values, such as
when the kernel and initramfs are updated. The same key can have many saved
blobs under different PCR values, so multiple boots are easily supported.
TPM 1.2
-------
Trust Source
============
By default, trusted keys are sealed under the SRK, which has the default
authorization value (20 zeros). This can be set at takeownership time with the
trouser's utility: "tpm_takeownership -u -z".
A trust source provides the source of security for Trusted Keys. This
section lists currently supported trust sources, along with their security
considerations. Whether or not a trust source is sufficiently safe depends
on the strength and correctness of its implementation, as well as the threat
environment for a specific use case. Since the kernel doesn't know what the
environment is, and there is no metric of trust, it is dependent on the
consumer of the Trusted Keys to determine if the trust source is sufficiently
safe.
TPM 2.0
-------
* Root of trust for storage
The user must first create a storage key and make it persistent, so the key is
available after reboot. This can be done using the following commands.
(1) TPM (Trusted Platform Module: hardware device)
Rooted to Storage Root Key (SRK) which never leaves the TPM that
provides crypto operation to establish root of trust for storage.
(2) TEE (Trusted Execution Environment: OP-TEE based on Arm TrustZone)
Rooted to Hardware Unique Key (HUK) which is generally burnt in on-chip
fuses and is accessible to TEE only.
* Execution isolation
(1) TPM
Fixed set of operations running in isolated execution environment.
(2) TEE
Customizable set of operations running in isolated execution
environment verified via Secure/Trusted boot process.
* Optional binding to platform integrity state
(1) TPM
Keys can be optionally sealed to specified PCR (integrity measurement)
values, and only unsealed by the TPM, if PCRs and blob integrity
verifications match. A loaded Trusted Key can be updated with new
(future) PCR values, so keys are easily migrated to new PCR values,
such as when the kernel and initramfs are updated. The same key can
have many saved blobs under different PCR values, so multiple boots are
easily supported.
(2) TEE
Relies on Secure/Trusted boot process for platform integrity. It can
be extended with TEE based measured boot process.
* Interfaces and APIs
(1) TPM
TPMs have well-documented, standardized interfaces and APIs.
(2) TEE
TEEs have well-documented, standardized client interface and APIs. For
more details refer to ``Documentation/staging/tee.rst``.
* Threat model
The strength and appropriateness of a particular TPM or TEE for a given
purpose must be assessed when using them to protect security-relevant data.
Key Generation
==============
Trusted Keys
------------
New keys are created from random numbers generated in the trust source. They
are encrypted/decrypted using a child key in the storage key hierarchy.
Encryption and decryption of the child key must be protected by a strong
access control policy within the trust source.
* TPM (hardware device) based RNG
Strength of random numbers may vary from one device manufacturer to
another.
* TEE (OP-TEE based on Arm TrustZone) based RNG
RNG is customizable as per platform needs. It can either be direct output
from platform specific hardware RNG or a software based Fortuna CSPRNG
which can be seeded via multiple entropy sources.
Encrypted Keys
--------------
Encrypted keys do not depend on a trust source, and are faster, as they use AES
for encryption/decryption. New keys are created from kernel-generated random
numbers, and are encrypted/decrypted using a specified ‘master’ key. The
‘master’ key can either be a trusted-key or user-key type. The main disadvantage
of encrypted keys is that if they are not rooted in a trusted key, they are only
as secure as the user key encrypting them. The master user key should therefore
be loaded in as secure a way as possible, preferably early in boot.
Usage
=====
Trusted Keys usage: TPM
-----------------------
TPM 1.2: By default, trusted keys are sealed under the SRK, which has the
default authorization value (20 bytes of 0s). This can be set at takeownership
time with the TrouSerS utility: "tpm_takeownership -u -z".
TPM 2.0: The user must first create a storage key and make it persistent, so the
key is available after reboot. This can be done using the following commands.
With the IBM TSS 2 stack::
@ -78,14 +175,21 @@ TPM_STORED_DATA format. The key length for new keys are always in bytes.
Trusted Keys can be 32 - 128 bytes (256 - 1024 bits), the upper limit is to fit
within the 2048 bit SRK (RSA) keylength, with all necessary structure/padding.
Encrypted keys do not depend on a TPM, and are faster, as they use AES for
encryption/decryption. New keys are created from kernel generated random
numbers, and are encrypted/decrypted using a specified 'master' key. The
'master' key can either be a trusted-key or user-key type. The main
disadvantage of encrypted keys is that if they are not rooted in a trusted key,
they are only as secure as the user key encrypting them. The master user key
should therefore be loaded in as secure a way as possible, preferably early in
boot.
Trusted Keys usage: TEE
-----------------------
Usage::
keyctl add trusted name "new keylen" ring
keyctl add trusted name "load hex_blob" ring
keyctl print keyid
"keyctl print" returns an ASCII hex copy of the sealed key, which is in format
specific to TEE device implementation. The key length for new keys is always
in bytes. Trusted Keys can be 32 - 128 bytes (256 - 1024 bits).
Encrypted Keys usage
--------------------
The decrypted portion of encrypted keys can contain either a simple symmetric
key or a more complex structure. The format of the more complex structure is
@ -103,8 +207,8 @@ Where::
format:= 'default | ecryptfs | enc32'
key-type:= 'trusted' | 'user'
Examples of trusted and encrypted key usage:
Examples of trusted and encrypted key usage
-------------------------------------------
Create and save a trusted key named "kmk" of length 32 bytes.
@ -150,7 +254,7 @@ Load a trusted key from the saved blob::
f1f8fff03ad0acb083725535636addb08d73dedb9832da198081e5deae84bfaf0409c22b
e4a8aea2b607ec96931e6f4d4fe563ba
Reseal a trusted key under new pcr values::
Reseal (TPM specific) a trusted key under new PCR values::
$ keyctl update 268728824 "update pcrinfo=`cat pcr.blob`"
$ keyctl print 268728824
@ -164,11 +268,12 @@ Reseal a trusted key under new pcr values::
7ef6a24defe4846104209bf0c3eced7fa1a672ed5b125fc9d8cd88b476a658a4434644ef
df8ae9a178e9f83ba9f08d10fa47e4226b98b0702f06b3b8
The initial consumer of trusted keys is EVM, which at boot time needs a high
quality symmetric key for HMAC protection of file metadata. The use of a
quality symmetric key for HMAC protection of file metadata. The use of a
trusted key provides strong guarantees that the EVM key has not been
compromised by a user level problem, and when sealed to specific boot PCR
values, protects against boot and offline attacks. Create and save an
compromised by a user level problem, and when sealed to a platform integrity
state, protects against boot and offline attacks. Create and save an
encrypted key "evm" using the above trusted key "kmk":
option 1: omitting 'format'::