On Thu, Feb 16, 2023 at 03:13:58PM -0500, James Bottomley wrote: > 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 > v3 rebases the encryption to be against Ard's new library function, the > aescfb addition of which appears as patch 1. > > James > > --- > > Ard Biesheuvel (1): > crypto: lib - implement library version of AES in CFB mode > > 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 | 216 ++++ > drivers/char/tpm/Kconfig | 13 + > drivers/char/tpm/Makefile | 2 + > drivers/char/tpm/tpm-buf.c | 196 ++++ > drivers/char/tpm/tpm-chip.c | 3 + > drivers/char/tpm/tpm-sysfs.c | 18 + > drivers/char/tpm/tpm.h | 14 + > drivers/char/tpm/tpm2-cmd.c | 52 +- > drivers/char/tpm/tpm2-sessions.c | 1160 +++++++++++++++++++ > drivers/char/tpm/tpm2-space.c | 8 +- > include/crypto/aes.h | 5 + > include/linux/tpm.h | 257 ++-- > lib/crypto/Kconfig | 5 + > lib/crypto/Makefile | 3 + > lib/crypto/aescfb.c | 75 ++ > security/keys/trusted-keys/trusted_tpm2.c | 82 +- > 16 files changed, 1984 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 > create mode 100644 lib/crypto/aescfb.c > > -- > 2.35.3 > Overally looks much better than earlier version! I'll aim to test this next week [*]. [*] The startup I was in went out of business, meaning that I'm ATM independent contributor doing this on my free time until I find a new job. That said, I spend 4 hours a day with kernel every working day so should not affect much as far reviewing and testing goes. BR, Jarkko
