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@xxxxxxxxxxxxxxxxxxxxx> --- v6: replace kernel space with null seed context save --- Documentation/security/tpm/tpm-security.rst | 204 ++++++++++++++++++++++++++++ 1 file changed, 204 insertions(+) create mode 100644 Documentation/security/tpm/tpm-security.rst diff --git a/Documentation/security/tpm/tpm-security.rst b/Documentation/security/tpm/tpm-security.rst new file mode 100644 index 000000000000..3411030505a1 --- /dev/null +++ b/Documentation/security/tpm/tpm-security.rst @@ -0,0 +1,204 @@ +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. 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 https://www.nccgroup.trust/us/our-research/tpm-genie/ which +is a simple external device that can be installed in a couple of +seconds on any system or laptop. However, 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, 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 certified 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, the generated EK primary key may now be used to run an +attestation on the null seed. The specific problem here is that the +EK primary is not a signing key so cannot on its own be used to sign +the key certification, hence the complex process below. + +Note: 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. + +.. _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/ -- 2.16.4
