Re: [PATCH v5 06/11] security: keys: trusted: Verify creation data

[Eric Biggers]

On Fri, Nov 11, 2022 at 03:16:31PM -0800, Evan Green wrote:
> security: keys: trusted: Verify creation data
>
> If a loaded key contains creation data, ask the TPM to verify that
> creation data. This allows users like encrypted hibernate to know that
> the loaded and parsed creation data has not been tampered with.

I don't understand what the purpose of this is.

I thought that the way to "seal" a key to a TPM PCR is to include the PCR in the
"policy".

Are you doing that too?  What is the purpose of using the "creation data"?

> +	/* Auth */
> +	tpm_buf_append_u32(&buf, 9);
> +	tpm_buf_append_u32(&buf, TPM2_RS_PW);
> +	tpm_buf_append_u16(&buf, 0);
> +	tpm_buf_append_u8(&buf, 0);
> +	tpm_buf_append_u16(&buf, 0);

This is struct tpm2_null_auth_area, so this is another place that could take
advantage of a new helper function to append it.

> +	/* Creation data hash */
> +	if (payload->creation_hash_len < 2) {
> +		rc = -EINVAL;
> +		goto out;
> +	}
> +
> +	tpm_buf_append_u16(&buf, payload->creation_hash_len - 2);
> +	tpm_buf_append(&buf, payload->creation_hash + 2,
> +		       payload->creation_hash_len - 2);

So the first two bytes of creation_hash are a redundant length field that needs
to be ignored here?  Perhaps tpm2_key_encode() shouldn't include that redundant
length field?

> +
> +	/* signature scheme */
> +	tpm_buf_append_u16(&buf, TPM_ALG_NULL);
> +
> +	/* creation ticket */
> +	tpm_buf_append(&buf, payload->tk, payload->tk_len);
> +
> +	rc = tpm_transmit_cmd(chip, &buf, 6, "certifying creation data");
> +	if (rc)
> +		goto out;

This is another instance of the bug where a positive TPM2_RC_* code is being
returned from a function that is supposed to return a negative errno value.

- Eric