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.
- 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
- 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.