ESYS: StartAuthSession bind auth trailing zeroes #2836
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good find @throwException |
| * | ||
| * Remove tailing zeroes from the auth value | ||
| */ | ||
| while ((bindNode->auth.size > 0) && |
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I think the auth.size > 0 check needs to be part of the original if statement and then use a working variable for the removal of trailing zeroes. It is more steps, but safer in my opinion.
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@tomoveu You are right, the bindNode->auth shouldn't be changed. But since this is passed to iesys_compute_bound_entity later, the TPM2B_DIGEST structure needs to be copied into a working variable I name bindNodeAuth.
The computation of secret_size was off in the original merge request, which I corrected by moving the removal of trailing zeros.
Finally, some lines below can be simplied since bindNodeAuth.size is always valid and 0 if bindNode == NULL.
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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. Signed-off-by: Stefan Thöni <stefan.thoeni@gapfruit.com>
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I am wondering, why tpm2-tss/src/tss2-esys/esys_tr.c Line 510 in 80f8733 and tpm2-tss/src/tss2-esys/esys_iutil.c Line 1734 in a19ac4c 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. |
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@AndreasFuchsTPM I'm sorry, this bug was already fixed in 4.0.2 by commit d3bcce8. |
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No worries, PRs are always welcome ! |
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.