Merge tag 'tpmdd-next-6.10-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/jarkko/linux-tpmdd

Pull TPM updates from Jarkko Sakkinen:
 "These are the changes for the TPM driver with a single major new
  feature: TPM bus encryption and integrity protection. The key pair on
  TPM side is generated from so called null random seed per power on of
  the machine [1]. This supports the TPM encryption of the hard drive by
  adding layer of protection against bus interposer attacks.

  Other than that, a few minor fixes and documentation for tpm_tis to
  clarify basics of TPM localities for future patch review discussions
  (will be extended and refined over times, just a seed)"

Link: https://lore.kernel.org/linux-integrity/20240429202811.13643-1-James.Bottomley@HansenPartnership.com/ [1]

* tag 'tpmdd-next-6.10-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/jarkko/linux-tpmdd: (28 commits)
  Documentation: tpm: Add TPM security docs toctree entry
  tpm: disable the TPM if NULL name changes
  Documentation: add tpm-security.rst
  tpm: add the null key name as a sysfs export
  KEYS: trusted: Add session encryption protection to the seal/unseal path
  tpm: add session encryption protection to tpm2_get_random()
  tpm: add hmac checks to tpm2_pcr_extend()
  tpm: Add the rest of the session HMAC API
  tpm: Add HMAC session name/handle append
  tpm: Add HMAC session start and end functions
  tpm: Add TCG mandated Key Derivation Functions (KDFs)
  tpm: Add NULL primary creation
  tpm: export the context save and load commands
  tpm: add buffer function to point to returned parameters
  crypto: lib - implement library version of AES in CFB mode
  KEYS: trusted: tpm2: Use struct tpm_buf for sized buffers
  tpm: Add tpm_buf_read_{u8,u16,u32}
  tpm: TPM2B formatted buffers
  tpm: Store the length of the tpm_buf data separately.
  tpm: Update struct tpm_buf documentation comments
  ...

Documentation/security/tpm/index.rst

@@ -5,6 +5,8 @@ Trusted Platform Module documentation
 .. toctree::
 
    tpm_event_log
+   tpm-security
+   tpm_tis
    tpm_vtpm_proxy
    xen-tpmfront
    tpm_ftpm_tee

Documentation/security/tpm/tpm-security.rst

@@ -0,0 +1,216 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+
+TPM Security
+============
+
+The object of this document is to describe how we make the kernel's
+use of the TPM reasonably robust in the face of external snooping and
+packet alteration attacks (called passive and active interposer attack
+in the literature).  The current security document is for TPM 2.0.
+
+Introduction
+------------
+
+The TPM is usually a discrete chip attached to a PC via some type of
+low bandwidth bus.  There are exceptions to this such as the Intel
+PTT, which is a software TPM running inside a software environment
+close to the CPU, which are subject to different attacks, but right at
+the moment, most hardened security environments require a discrete
+hardware TPM, which is the use case discussed here.
+
+Snooping and Alteration Attacks against the bus
+-----------------------------------------------
+
+The current state of the art for snooping the `TPM Genie`_ hardware
+interposer which is a simple external device that can be installed in
+a couple of seconds on any system or laptop.  Recently attacks were
+successfully demonstrated against the `Windows Bitlocker TPM`_ system.
+Most recently the same `attack against TPM based Linux disk
+encryption`_ schemes.  The next phase of research seems to be hacking
+existing devices on the bus to act as interposers, so the fact that
+the attacker requires physical access for a few seconds might
+evaporate.  However, the goal of this document is to protect TPM
+secrets and integrity as far as we are able in this environment and to
+try to insure that if we can't prevent the attack then at least we can
+detect it.
+
+Unfortunately, most of the TPM functionality, including the hardware
+reset capability can be controlled by an attacker who has access to
+the bus, so we'll discuss some of the disruption possibilities below.
+
+Measurement (PCR) Integrity
+---------------------------
+
+Since the attacker can send their own commands to the TPM, they can
+send arbitrary PCR extends and thus disrupt the measurement system,
+which would be an annoying denial of service attack.  However, there
+are two, more serious, classes of attack aimed at entities sealed to
+trust measurements.
+
+1. The attacker could intercept all PCR extends coming from the system
+   and completely substitute their own values, producing a replay of
+   an untampered state that would cause PCR measurements to attest to
+   a trusted state and release secrets
+
+2. At some point in time the attacker could reset the TPM, clearing
+   the PCRs and then send down their own measurements which would
+   effectively overwrite the boot time measurements the TPM has
+   already done.
+
+The first can be thwarted by always doing HMAC protection of the PCR
+extend and read command meaning measurement values cannot be
+substituted without producing a detectable HMAC failure in the
+response.  However, the second can only really be detected by relying
+on some sort of mechanism for protection which would change over TPM
+reset.
+
+Secrets Guarding
+----------------
+
+Certain information passing in and out of the TPM, such as key sealing
+and private key import and random number generation, is vulnerable to
+interception which HMAC protection alone cannot protect against, so
+for these types of command we must also employ request and response
+encryption to prevent the loss of secret information.
+
+Establishing Initial Trust with the TPM
+---------------------------------------
+
+In order to provide security from the beginning, an initial shared or
+asymmetric secret must be established which must also be unknown to
+the attacker.  The most obvious avenues for this are the endorsement
+and storage seeds, which can be used to derive asymmetric keys.
+However, using these keys is difficult because the only way to pass
+them into the kernel would be on the command line, which requires
+extensive support in the boot system, and there's no guarantee that
+either hierarchy would not have some type of authorization.
+
+The mechanism chosen for the Linux Kernel is to derive the primary
+elliptic curve key from the null seed using the standard storage seed
+parameters.  The null seed has two advantages: firstly the hierarchy
+physically cannot have an authorization, so we are always able to use
+it and secondly, the null seed changes across TPM resets, meaning if
+we establish trust on the null seed at start of day, all sessions
+salted with the derived key will fail if the TPM is reset and the seed
+changes.
+
+Obviously using the null seed without any other prior shared secrets,
+we have to create and read the initial public key which could, of
+course, be intercepted and substituted by the bus interposer.
+However, the TPM has a key certification mechanism (using the EK
+endorsement certificate, creating an attestation identity key and
+certifying the null seed primary with that key) which is too complex
+to run within the kernel, so we keep a copy of the null primary key
+name, which is what is exported via sysfs so user-space can run the
+full certification when it boots.  The definitive guarantee here is
+that if the null primary key certifies correctly, you know all your
+TPM transactions since start of day were secure and if it doesn't, you
+know there's an interposer on your system (and that any secret used
+during boot may have been leaked).
+
+Stacking Trust
+--------------
+
+In the current null primary scenario, the TPM must be completely
+cleared before handing it on to the next consumer.  However the kernel
+hands to user-space the name of the derived null seed key which can
+then be verified by certification in user-space.  Therefore, this chain
+of name handoff can be used between the various boot components as
+well (via an unspecified mechanism).  For instance, grub could use the
+null seed scheme for security and hand the name off to the kernel in
+the boot area.  The kernel could make its own derivation of the key
+and the name and know definitively that if they differ from the handed
+off version that tampering has occurred.  Thus it becomes possible to
+chain arbitrary boot components together (UEFI to grub to kernel) via
+the name handoff provided each successive component knows how to
+collect the name and verifies it against its derived key.
+
+Session Properties
+------------------
+
+All TPM commands the kernel uses allow sessions.  HMAC sessions may be
+used to check the integrity of requests and responses and decrypt and
+encrypt flags may be used to shield parameters and responses.  The
+HMAC and encryption keys are usually derived from the shared
+authorization secret, but for a lot of kernel operations that is well
+known (and usually empty).  Thus, every HMAC session used by the
+kernel must be created using the null primary key as the salt key
+which thus provides a cryptographic input into the session key
+derivation.  Thus, the kernel creates the null primary key once (as a
+volatile TPM handle) and keeps it around in a saved context stored in
+tpm_chip for every in-kernel use of the TPM.  Currently, because of a
+lack of de-gapping in the in-kernel resource manager, the session must
+be created and destroyed for each operation, but, in future, a single
+session may also be reused for the in-kernel HMAC, encryption and
+decryption sessions.
+
+Protection Types
+----------------
+
+For every in-kernel operation we use null primary salted HMAC to
+protect the integrity.  Additionally, we use parameter encryption to
+protect key sealing and parameter decryption to protect key unsealing
+and random number generation.
+
+Null Primary Key Certification in Userspace
+===========================================
+
+Every TPM comes shipped with a couple of X.509 certificates for the
+primary endorsement key.  This document assumes that the Elliptic
+Curve version of the certificate exists at 01C00002, but will work
+equally well with the RSA certificate (at 01C00001).
+
+The first step in the certification is primary creation using the
+template from the `TCG EK Credential Profile`_ which allows comparison
+of the generated primary key against the one in the certificate (the
+public key must match).  Note that generation of the EK primary
+requires the EK hierarchy password, but a pre-generated version of the
+EC primary should exist at 81010002 and a TPM2_ReadPublic() may be
+performed on this without needing the key authority.  Next, the
+certificate itself must be verified to chain back to the manufacturer
+root (which should be published on the manufacturer website).  Once
+this is done, an attestation key (AK) is generated within the TPM and
+it's name and the EK public key can be used to encrypt a secret using
+TPM2_MakeCredential.  The TPM then runs TPM2_ActivateCredential which
+will only recover the secret if the binding between the TPM, the EK
+and the AK is true. the generated AK may now be used to run a
+certification of the null primary key whose name the kernel has
+exported.  Since TPM2_MakeCredential/ActivateCredential are somewhat
+complicated, a more simplified process involving an externally
+generated private key is described below.
+
+This process is a simplified abbreviation of the usual privacy CA
+based attestation process.  The assumption here is that the
+attestation is done by the TPM owner who thus has access to only the
+owner hierarchy.  The owner creates an external public/private key
+pair (assume elliptic curve in this case) and wraps the private key
+for import using an inner wrapping process and parented to the EC
+derived storage primary.  The TPM2_Import() is done using a parameter
+decryption HMAC session salted to the EK primary (which also does not
+require the EK key authority) meaning that the inner wrapping key is
+the encrypted parameter and thus the TPM will not be able to perform
+the import unless is possesses the certified EK so if the command
+succeeds and the HMAC verifies on return we know we have a loadable
+copy of the private key only for the certified TPM.  This key is now
+loaded into the TPM and the Storage primary flushed (to free up space
+for the null key generation).
+
+The null EC primary is now generated using the Storage profile
+outlined in the `TCG TPM v2.0 Provisioning Guidance`_; the name of
+this key (the hash of the public area) is computed and compared to the
+null seed name presented by the kernel in
+/sys/class/tpm/tpm0/null_name.  If the names do not match, the TPM is
+compromised.  If the names match, the user performs a TPM2_Certify()
+using the null primary as the object handle and the loaded private key
+as the sign handle and providing randomized qualifying data.  The
+signature of the returned certifyInfo is verified against the public
+part of the loaded private key and the qualifying data checked to
+prevent replay.  If all of these tests pass, the user is now assured
+that TPM integrity and privacy was preserved across the entire boot
+sequence of this kernel.
+
+.. _TPM Genie: https://www.nccgroup.trust/globalassets/about-us/us/documents/tpm-genie.pdf
+.. _Windows Bitlocker TPM: https://dolosgroup.io/blog/2021/7/9/from-stolen-laptop-to-inside-the-company-network
+.. _attack against TPM based Linux disk encryption: https://www.secura.com/blog/tpm-sniffing-attacks-against-non-bitlocker-targets
+.. _TCG EK Credential Profile: https://trustedcomputinggroup.org/resource/tcg-ek-credential-profile-for-tpm-family-2-0/
+.. _TCG TPM v2.0 Provisioning Guidance: https://trustedcomputinggroup.org/resource/tcg-tpm-v2-0-provisioning-guidance/

Documentation/security/tpm/tpm_tis.rst

@@ -0,0 +1,46 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=========================
+TPM FIFO interface driver
+=========================
+
+TCG PTP Specification defines two interface types: FIFO and CRB. The former is
+based on sequenced read and write operations,  and the latter is based on a
+buffer containing the full command or response.
+
+FIFO (First-In-First-Out) interface is used by the tpm_tis_core dependent
+drivers. Originally Linux had only a driver called tpm_tis, which covered
+memory mapped (aka MMIO) interface but it was later on extended to cover other
+physical interfaces supported by the TCG standard.
+
+For historical reasons above the original MMIO driver is called tpm_tis and the
+framework for FIFO drivers is named as tpm_tis_core. The postfix "tis" in
+tpm_tis comes from the TPM Interface Specification, which is the hardware
+interface specification for TPM 1.x chips.
+
+Communication is based on a 20 KiB buffer shared by the TPM chip through a
+hardware bus or memory map, depending on the physical wiring. The buffer is
+further split into five equal-size 4 KiB buffers, which provide equivalent
+sets of registers for communication between the CPU and TPM. These
+communication endpoints are called localities in the TCG terminology.
+
+When the kernel wants to send commands to the TPM chip, it first reserves
+locality 0 by setting the requestUse bit in the TPM_ACCESS register. The bit is
+cleared by the chip when the access is granted. Once it completes its
+communication, the kernel writes the TPM_ACCESS.activeLocality bit. This
+informs the chip that the locality has been relinquished.
+
+Pending localities are served in order by the chip in descending order, one at
+a time:
+
+- Locality 0 has the lowest priority.
+- Locality 5 has the highest priority.
+
+Further information on the purpose and meaning of the localities can be found
+in section 3.2 of the TCG PC Client Platform TPM Profile Specification.
+
+References
+==========
+
+TCG PC Client Platform TPM Profile (PTP) Specification
+https://trustedcomputinggroup.org/resource/pc-client-platform-tpm-profile-ptp-specification/