The interest in securing the TPM against interposers, both active and passive has risen to fever pitch with the demonstration of key recovery against windows bitlocker: https://dolosgroup.io/blog/2021/7/9/from-stolen-laptop-to-inside-the-company-network And subsequently the same attack being successful against all the Linux TPM based security solutions: https://www.secura.com/blog/tpm-sniffing-attacks-against-non-bitlocker-targets The attacks fall into two categories: 1. Passive Interposers, which sit on the bus and merely observe 2. Active Interposers, which try to manipulate TPM transactions on the bus using man in the middle and packet stealing to create TPM state the interposer owner desires. Our broadest interposer target is the use of TPM_RS_PW for password authorization which sends the actual password to the TPM without any obfuscation and effectively hands it to any interposer. The way to fix this is to use real sessions for HMAC capabilities to ensure integrity and to use parameter and response encryption to ensure confidentiality of the data flowing over the TPM bus. HMAC sessions by agreeing a challenge with the TPM and then giving a response which is a HMAC of the password and the challenge, so the application proves knowledge of the password to the TPM without ever transmitting the password itself. Using HMAC sessions when sending commands to the TPM also provides some measure of protection against active interposers, since the interposer can't interfere with or delete a HMAC'd command (because they can't manufacture a response with the correct HMAC). To protect TPM transactions where there isn't a shared secret (i.e. the command is something like a PCR extension which doesn't involve a TPM object with a password) we have to do a bit more work to set up sessions with a passed in encrypted secret (called a salt) to act in place of the shared secret in the HMAC. This secret salt is effectively a random number encrypted to a public key of the TPM. The final piece of the puzzle is using parameter input and response return encryption, so any interposer can't see the data passing from the application to the TPM and vice versa. The most insidious interposer attack of all is a reset attack: since the interposer has access to the TPM bus, it can assert the TPM reset line any time it wants. When a TPM resets it mostly comes back in the same state except that all the PCRs are reset to their initial values. Controlling the reset line allows the interposer to change the PCR state after the fact by resetting the TPM and then replaying PCR extends to get the PCRs into a valid state to release secrets, so even if an attack event was recorded, the record is erased. This reset attack violates the fundamental princible of non-repudiability of TPM logs. Defeating the reset attack involves tying all TPM operations within the kernel to a property which will change detectably if the TPM is reset. For that reason, we tie all TPM sessions to the null hierarchy we obtain at start of day and whose seed changes on every reset. If an active interposer asserts a TPM reset, the new null primary won't match the kernel's stored one and all TPM operations will start failing because of HMAC mismatches in the sessions. So if the kernel TPM code keeps operating, it guarantees that a reset hasn't occurred. The final part of the puzzle is that the machine owner must have a fixed idea of the EK of their TPM and should have certified this with the TPM manufacturer. On every boot, the certified EK public key should be used to do a make credential/activate credential attestation key insertion and then the null key certified with the attestation key. We can follow a trust on first use model where an OS installation will extract and verify a public EK and save it to a read only file. This patch series adds a simple API which can ensure the above properties as a layered addition to the existing TPM handling code. This series now includes protections for PCR extend, getting random numbers from the TPM and data sealing and unsealing. It therefore eliminates all uses of TPM2_RS_PW in the kernel and adds encryption protection to sensitive data flowing into and out of the TPM. The first four patches add more sophisticated buffer handling to the TPM which is needed to build the more complex encryption and authentication based commands. Patch 6 adds all the generic cryptography primitives and patches 7-9 use them in critical TPM operations where we want to avoid or detect interposers. Patch 10 exports the name of the null key we used for boot/run time verification and patch 11 documents the security guarantees and expectations. This was originally sent over four years ago, with the last iteration being: https://lore.kernel.org/linux-integrity/1568031515.6613.31.camel@xxxxxxxxxxxxxxxxxxxxx/ I'm dusting it off now because various forces at Microsoft and Google via the Open Compute Platform are making a lot of noise about interposers and we in the linux kernel look critically lacking in that regard, particularly for TPM trusted keys. --- v2 fixes the problems smatch reported and adds more explanation about the code motion in the first few patches James --- James Bottomley (11): tpm: move buffer handling from static inlines to real functions tpm: add buffer handling for TPM2B types tpm: add cursor based buffer functions for response parsing tpm: add buffer function to point to returned parameters tpm: export the context save and load commands tpm: Add full HMAC and encrypt/decrypt session handling code tpm: add hmac checks to tpm2_pcr_extend() tpm: add session encryption protection to tpm2_get_random() KEYS: trusted: Add session encryption protection to the seal/unseal path tpm: add the null key name as a sysfs export Documentation: add tpm-security.rst Documentation/security/tpm/tpm-security.rst | 214 ++++ drivers/char/tpm/Kconfig | 13 + drivers/char/tpm/Makefile | 2 + drivers/char/tpm/tpm-buf.c | 197 +++ drivers/char/tpm/tpm-sysfs.c | 14 + drivers/char/tpm/tpm.h | 14 + drivers/char/tpm/tpm2-cmd.c | 54 +- drivers/char/tpm/tpm2-sessions.c | 1216 +++++++++++++++++++ drivers/char/tpm/tpm2-space.c | 8 +- include/linux/tpm.h | 252 ++-- security/keys/trusted-keys/trusted_tpm2.c | 85 +- 11 files changed, 1944 insertions(+), 125 deletions(-) create mode 100644 Documentation/security/tpm/tpm-security.rst create mode 100644 drivers/char/tpm/tpm-buf.c create mode 100644 drivers/char/tpm/tpm2-sessions.c -- 2.35.3
