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Documentation: add tpm-security.rst
Document how the new encrypted secure interface for TPM2 works and how security can be assured after boot by certifying the NULL seed. Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com> Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org> Tested-by: Jarkko Sakkinen <jarkko@kernel.org> Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
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Jarkko Sakkinen
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Documentation/security/tpm/tpm-security.rst
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.. SPDX-License-Identifier: GPL-2.0-only
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TPM Security
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============
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The object of this document is to describe how we make the kernel's
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use of the TPM reasonably robust in the face of external snooping and
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packet alteration attacks (called passive and active interposer attack
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in the literature). The current security document is for TPM 2.0.
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Introduction
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------------
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The TPM is usually a discrete chip attached to a PC via some type of
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low bandwidth bus. There are exceptions to this such as the Intel
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PTT, which is a software TPM running inside a software environment
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close to the CPU, which are subject to different attacks, but right at
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the moment, most hardened security environments require a discrete
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hardware TPM, which is the use case discussed here.
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Snooping and Alteration Attacks against the bus
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-----------------------------------------------
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The current state of the art for snooping the `TPM Genie`_ hardware
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interposer which is a simple external device that can be installed in
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a couple of seconds on any system or laptop. Recently attacks were
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successfully demonstrated against the `Windows Bitlocker TPM`_ system.
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Most recently the same `attack against TPM based Linux disk
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encryption`_ schemes. The next phase of research seems to be hacking
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existing devices on the bus to act as interposers, so the fact that
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the attacker requires physical access for a few seconds might
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evaporate. However, the goal of this document is to protect TPM
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secrets and integrity as far as we are able in this environment and to
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try to insure that if we can't prevent the attack then at least we can
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detect it.
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Unfortunately, most of the TPM functionality, including the hardware
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reset capability can be controlled by an attacker who has access to
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the bus, so we'll discuss some of the disruption possibilities below.
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Measurement (PCR) Integrity
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---------------------------
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Since the attacker can send their own commands to the TPM, they can
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send arbitrary PCR extends and thus disrupt the measurement system,
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which would be an annoying denial of service attack. However, there
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are two, more serious, classes of attack aimed at entities sealed to
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trust measurements.
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1. The attacker could intercept all PCR extends coming from the system
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and completely substitute their own values, producing a replay of
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an untampered state that would cause PCR measurements to attest to
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a trusted state and release secrets
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2. At some point in time the attacker could reset the TPM, clearing
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the PCRs and then send down their own measurements which would
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effectively overwrite the boot time measurements the TPM has
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already done.
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The first can be thwarted by always doing HMAC protection of the PCR
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extend and read command meaning measurement values cannot be
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substituted without producing a detectable HMAC failure in the
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response. However, the second can only really be detected by relying
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on some sort of mechanism for protection which would change over TPM
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reset.
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Secrets Guarding
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----------------
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Certain information passing in and out of the TPM, such as key sealing
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and private key import and random number generation, is vulnerable to
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interception which HMAC protection alone cannot protect against, so
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for these types of command we must also employ request and response
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encryption to prevent the loss of secret information.
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Establishing Initial Trust with the TPM
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---------------------------------------
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In order to provide security from the beginning, an initial shared or
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asymmetric secret must be established which must also be unknown to
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the attacker. The most obvious avenues for this are the endorsement
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and storage seeds, which can be used to derive asymmetric keys.
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However, using these keys is difficult because the only way to pass
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them into the kernel would be on the command line, which requires
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extensive support in the boot system, and there's no guarantee that
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either hierarchy would not have some type of authorization.
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The mechanism chosen for the Linux Kernel is to derive the primary
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elliptic curve key from the null seed using the standard storage seed
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parameters. The null seed has two advantages: firstly the hierarchy
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physically cannot have an authorization, so we are always able to use
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it and secondly, the null seed changes across TPM resets, meaning if
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we establish trust on the null seed at start of day, all sessions
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salted with the derived key will fail if the TPM is reset and the seed
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changes.
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Obviously using the null seed without any other prior shared secrets,
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we have to create and read the initial public key which could, of
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course, be intercepted and substituted by the bus interposer.
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However, the TPM has a key certification mechanism (using the EK
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endorsement certificate, creating an attestation identity key and
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certifying the null seed primary with that key) which is too complex
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to run within the kernel, so we keep a copy of the null primary key
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name, which is what is exported via sysfs so user-space can run the
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full certification when it boots. The definitive guarantee here is
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that if the null primary key certifies correctly, you know all your
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TPM transactions since start of day were secure and if it doesn't, you
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know there's an interposer on your system (and that any secret used
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during boot may have been leaked).
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Stacking Trust
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--------------
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In the current null primary scenario, the TPM must be completely
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cleared before handing it on to the next consumer. However the kernel
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hands to user-space the name of the derived null seed key which can
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then be verified by certification in user-space. Therefore, this chain
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of name handoff can be used between the various boot components as
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well (via an unspecified mechanism). For instance, grub could use the
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null seed scheme for security and hand the name off to the kernel in
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the boot area. The kernel could make its own derivation of the key
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and the name and know definitively that if they differ from the handed
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off version that tampering has occurred. Thus it becomes possible to
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chain arbitrary boot components together (UEFI to grub to kernel) via
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the name handoff provided each successive component knows how to
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collect the name and verifies it against its derived key.
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Session Properties
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------------------
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All TPM commands the kernel uses allow sessions. HMAC sessions may be
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used to check the integrity of requests and responses and decrypt and
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encrypt flags may be used to shield parameters and responses. The
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HMAC and encryption keys are usually derived from the shared
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authorization secret, but for a lot of kernel operations that is well
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known (and usually empty). Thus, every HMAC session used by the
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kernel must be created using the null primary key as the salt key
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which thus provides a cryptographic input into the session key
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derivation. Thus, the kernel creates the null primary key once (as a
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volatile TPM handle) and keeps it around in a saved context stored in
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tpm_chip for every in-kernel use of the TPM. Currently, because of a
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lack of de-gapping in the in-kernel resource manager, the session must
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be created and destroyed for each operation, but, in future, a single
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session may also be reused for the in-kernel HMAC, encryption and
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decryption sessions.
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Protection Types
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----------------
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For every in-kernel operation we use null primary salted HMAC to
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protect the integrity. Additionally, we use parameter encryption to
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protect key sealing and parameter decryption to protect key unsealing
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and random number generation.
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Null Primary Key Certification in Userspace
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===========================================
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Every TPM comes shipped with a couple of X.509 certificates for the
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primary endorsement key. This document assumes that the Elliptic
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Curve version of the certificate exists at 01C00002, but will work
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equally well with the RSA certificate (at 01C00001).
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The first step in the certification is primary creation using the
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template from the `TCG EK Credential Profile`_ which allows comparison
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of the generated primary key against the one in the certificate (the
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public key must match). Note that generation of the EK primary
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requires the EK hierarchy password, but a pre-generated version of the
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EC primary should exist at 81010002 and a TPM2_ReadPublic() may be
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performed on this without needing the key authority. Next, the
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certificate itself must be verified to chain back to the manufacturer
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root (which should be published on the manufacturer website). Once
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this is done, an attestation key (AK) is generated within the TPM and
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it's name and the EK public key can be used to encrypt a secret using
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TPM2_MakeCredential. The TPM then runs TPM2_ActivateCredential which
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will only recover the secret if the binding between the TPM, the EK
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and the AK is true. the generated AK may now be used to run a
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certification of the null primary key whose name the kernel has
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exported. Since TPM2_MakeCredential/ActivateCredential are somewhat
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complicated, a more simplified process involving an externally
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generated private key is described below.
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This process is a simplified abbreviation of the usual privacy CA
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based attestation process. The assumption here is that the
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attestation is done by the TPM owner who thus has access to only the
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owner hierarchy. The owner creates an external public/private key
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pair (assume elliptic curve in this case) and wraps the private key
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for import using an inner wrapping process and parented to the EC
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derived storage primary. The TPM2_Import() is done using a parameter
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decryption HMAC session salted to the EK primary (which also does not
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require the EK key authority) meaning that the inner wrapping key is
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the encrypted parameter and thus the TPM will not be able to perform
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the import unless is possesses the certified EK so if the command
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succeeds and the HMAC verifies on return we know we have a loadable
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copy of the private key only for the certified TPM. This key is now
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loaded into the TPM and the Storage primary flushed (to free up space
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for the null key generation).
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The null EC primary is now generated using the Storage profile
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outlined in the `TCG TPM v2.0 Provisioning Guidance`_; the name of
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this key (the hash of the public area) is computed and compared to the
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null seed name presented by the kernel in
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/sys/class/tpm/tpm0/null_name. If the names do not match, the TPM is
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compromised. If the names match, the user performs a TPM2_Certify()
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using the null primary as the object handle and the loaded private key
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as the sign handle and providing randomized qualifying data. The
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signature of the returned certifyInfo is verified against the public
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part of the loaded private key and the qualifying data checked to
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prevent replay. If all of these tests pass, the user is now assured
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that TPM integrity and privacy was preserved across the entire boot
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sequence of this kernel.
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.. _TPM Genie: https://www.nccgroup.trust/globalassets/about-us/us/documents/tpm-genie.pdf
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.. _Windows Bitlocker TPM: https://dolosgroup.io/blog/2021/7/9/from-stolen-laptop-to-inside-the-company-network
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.. _attack against TPM based Linux disk encryption: https://www.secura.com/blog/tpm-sniffing-attacks-against-non-bitlocker-targets
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.. _TCG EK Credential Profile: https://trustedcomputinggroup.org/resource/tcg-ek-credential-profile-for-tpm-family-2-0/
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.. _TCG TPM v2.0 Provisioning Guidance: https://trustedcomputinggroup.org/resource/tcg-tpm-v2-0-provisioning-guidance/
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