tpm2: fix dangling loaded session after tpm2_createpolicy

[oldium]

Clevis cleans the context regurarly from transient objects by invoking tpm2_flushcontext -t, but there is one case which Clevis omits. The tpm2_createpolicy command loads new session and this needs to be cleaned by a call to tpm2_flushcontext -l, see tpm2-software/tpm2-tools#3427.

This is usually not an issue when resource manager is used (i.e. with /dev/tpmrm0 device), but when raw TPM device /dev/tpm0 or software TPM (swtpm) without resource manager is used, the usage of clevis (multiple calls are required) ends with the following error:

WARNING:esys:src/tss2-esys/api/Esys_StartAuthSession.c:390:Esys_StartAuthSession_Finish() Received TPM Error
ERROR:esys:src/tss2-esys/api/Esys_StartAuthSession.c:136:Esys_StartAuthSession() Esys Finish ErrorCode (0x00000903)
ERROR: Esys_StartAuthSession(0x903) - tpm:warn(2.0): out of memory for session contexts
ERROR: Unable to run tpm2_create

[oldium]

It seems they will be fixing the behaviour of tpm2-createpolicy (see tpm2-software/tpm2-tools#3427 (comment)), but in order to have this working with older versions, I would keep the workaround with tpm2_flushcontext -l for now.

[oldium]

Reproduction with clevis is actually simple - either use swtpm (I discovered this with swtpm originally) or raw TPM device (/dev/tpm0) and run tpm2 pin tests. Or the following:

# export TPM2TOOLS_TCTI=device:/dev/tpm0
# tpm2_getcap pcrs
selected-pcrs:
  - sha1: [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 ]
  - sha256: [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 ]
# echo "test" | clevis encrypt tpm2 '{"pcr_ids":"23"}' | clevis decrypt
test
# echo "test" | clevis encrypt tpm2 '{"pcr_ids":"23"}' | clevis decrypt
test
# echo "test" | clevis encrypt tpm2 '{"pcr_ids":"23"}' | clevis decrypt
WARNING:esys:src/tss2-esys/api/Esys_StartAuthSession.c:390:Esys_StartAuthSession_Finish() Received TPM Error
ERROR:esys:src/tss2-esys/api/Esys_StartAuthSession.c:136:Esys_StartAuthSession() Esys Finish ErrorCode (0x00000903)
ERROR: Esys_StartAuthSession(0x903) - tpm:warn(2.0): out of memory for session contexts
ERROR: Unable to run tpm2_create
Creating TPM2 object for jwk failed!

[sarroutbi]

Thanks for your PR, @oldium . Sorry for my ignorance but ... is it necessary to call it twice? Is not a call to this command valid?:

tpm2_flushcontext -t -l

[oldium]

Thanks for your PR, @oldium . Sorry for my ignorance but ... is it necessary to call it twice? Is not a call to this command valid?:

tpm2_flushcontext -t -l

Your question is right on target, I have checked exactly that. 😁 This is supported since tpm2-tools 5.3, but unfortunately Debian 11 ships tpm2-tools 5.0...

[oldium]

@sarroutbi But meson could check for that, safe command to execute is:

TPM2TOOLS_TCTI=none tpm2_flushcontext -t -l

When it reports ERROR: Options -t, -l and -s are mutually exclusive, the flushing combo is not supported. And it is forwards compatible, so any update to tpm2-tools in future cannot break the current clevis release.

The correct command/sequence can be generated during build (with configure_file). What do you think?

[sarroutbi]

@sarroutbi But meson could check for that, safe command to execute is:

TPM2TOOLS_TCTI=none tpm2_flushcontext -t -l

When it reports ERROR: Options -t, -l and -s are mutually exclusive, the flushing combo is not supported. And it is forwards compatible, so any update to tpm2-tools in future cannot break the current clevis release.

The correct command/sequence can be generated during build (with configure_file). What do you think?

Thanks for clarifying. In that case, I would keep it as it is now.

[sarroutbi]

LGTM