TPM2 Support in bootloader / kernel in order to retrieve GELI passphrase
Date: Sat, 27 Nov 2021 14:34:54 UTC
Dear All, Could you please guide me so that we can together integrate the following piece of work into the FreeBSD base system? Thank you for your time and consideration. I have created the following bundle of work [1]. The referenced patch implements on top of releng/13.0, the support for TPM2 in the EFI bootloader and in the kernel in order to allow for storage and retrievel of a GELI passphrase in a TPM2 module, secured with a PCR policy. The way the bootloader behavior is modified is the following: 1) before calling efipart_inithandles() an attempt to retrieve the passphrase from a TPM2 module might be performed - how this is achieved is described later on. 2) if a passphrase is indeed retrieved, then after determining currdev, the currdev is checked for the presence of a /.passphrase_marker file which must contain the same passphrase as retrieved from the TPM. This is supposed to ensure that we do not end up booting an environment not on the device we just unlocked with the passphrase. 3a) If all is go, the autoboot_delay is set to -1 in order to prevent any interaction and continue the boot process of the "safe" environment, a 'kern.geom.eli.passphrase.from_tpm2.passphrase' variable is set to the passphrase from TPM in order for kernel use later, as well as a kern.geom.eli.passphrase.from_tpm2.was_retrieved'='1' variable. 3b) If the passphrase marker does not match, the bootloader cleans up GELI keys, the TPM passphrase and kern.geom.eli.passphrase and exits. The way the kernel behavior is modified is the following: 1) In init_main.c, after vfs_mountroot() a check is added 2a) If kern.geom.eli.passphrase.from_tpm2.was_retrieved is not set to 1, then we do nothing and continue the boot process 2b) If the was_retrieved variable is set to '1' then we check for the same passphrase marker as the bootloader, its content compared against the 'kern.geom.eli.passphrase.from_tpm2.passphrase' variable. 3a) If all is go, the passphrase variable is unset and the boot process continues, 3c) If the passphrase marker does not match, we panic. The configuration of the bootloader for this procedure looks the following: 1) We set an efivar KernGeomEliPassphraseFromTpm2NvIndex to contain the TPM2 NV Index we store our passphrase in, e.g. 0x1000001 2) We set an efivar KernGeomEfiPassphraseFromTpm2PolicyPcr to contain the PCR policy used to secure the passphrase, e.g. sha256:0,2,4,7 3a) If both are set, the bootloader will attempt to retrieve the passphrase and behave in the modified way described above 3b) Otherwise, it behaves as the vanilla version and will ask for GELI passphrases if necessary The configuration of the TPM and the passphrase marker looks the following: 1) echo -n "passphrase" >/.passphrase_marker 2) chmod 600 /.passphrase_marker 3) tpm2_createpolicy -L policy.digest --policy-pcr -l sha256:0,2,4,7 4) tpm2_nvdefine -Q 0x1000001 -s `wc -c /.passphrase_marker` -L policy.digest -A "policyread|policywrite" 5) tpm2_nvwrite -Q 0x1000001 -i /.passphrase_marker -P pcr:sha256:0,2,4,7 [1] https://github.com/sadaszewski/freebsd-src/compare/releng/13.0...tpm-support-in-stand Kind regards, -- Stanislaw