[PATCH v3 5/5] tpm: ensure that output of PCR read contains the correct digest size

[Roberto Sassu]

On 10/30/2018 8:52 PM, Jarkko Sakkinen wrote:
> On Tue, 30 Oct 2018, Roberto Sassu wrote:
>> This patch ensures that the digest size returned by the TPM during a PCR
>> read matches the size of the algorithm passed as argument to
>> tpm2_pcr_read(). The check is performed after information about the PCR
>> banks has been retrieved.
>>
>> Signed-off-by: Roberto Sassu <roberto.sassu at huawei.com>
> 
> What is the scenarion when this can happen (should be explained in
> the commit message)?

Without an HMAC session, the request/response payload can be modified.
This patch ensures that the digest size in the payload is equal to the
size of the algorithm specified by the caller.

Patch 3/5 only ensures that there is no buffer overflow when data is
copied to the tpm_digest structure passed by the caller.

Patch 5/5 uses the PCR bank information introduced in patch 4/5 to
ensure that the exact amount of data is copied from the response
payload. However, the patch may not help because an attacker can modify
the algorithm in the request payload so that the TPM returns a shorter
digest.

For me it is ok to remove this patch from the set. It was requested by
Mimi.

Roberto

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