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tpm2-software / tpm2-tss

OSS implementation of the TCG TPM2 Software Stack (TSS2)
https://tpm2-software.github.io
BSD 2-Clause "Simplified" License
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ESYS: StartAuthSession bind auth trailing zeroes #2836

Closed throwException closed 1 month ago

throwException commented 1 month ago

When StartAuthSession is called with a bind entity with a auth value containing trailing zeroes, the HMAC or policy session computation of ESYS does not match the computation on the TPM2.

The fix is to remove trailing zeroes from the auth value according to the specification (TPM2 Architecture, 19.6.5, Note 2) before computation of the session key.

The fixed bug is especially tricky as a randomly generated auth value of the bind object can cause HMAC or policy session to fail occassionally.

tomoveu commented 1 month ago

good find @throwException

AndreasFuchsTPM commented 1 month ago

I am wondering, why https://github.com/tpm2-software/tpm2-tss/blob/80f87339fb73fd50b581b5927711e45e64279348/src/tss2-esys/esys_tr.c#L510 and https://github.com/tpm2-software/tpm2-tss/blob/a19ac4c61c3f3bbdfc61cf01040a30729d6ba441/src/tss2-esys/esys_iutil.c#L1734 are not effective in this case of bind ?

The idea was and is that the auth values carried in the metadata are always free of trailing zeros to begin with. Thus I don't like the fix at this place but rather want to find out the call path that circumvented the previous cases.

throwException commented 1 month ago

@AndreasFuchsTPM I'm sorry, this bug was already fixed in 4.0.2 by commit d3bcce8c7ed13d7eef6a0fbcb536fcf217804289.

AndreasFuchsTPM commented 1 month ago

No worries, PRs are always welcome !