Hello Richard,
On 14.07.21 12:44, Richard Weinberger wrote:
> Ahmad,
>
> ----- Ursprüngliche Mail -----
>> Von: "Ahmad Fatoum" <a.fatoum@xxxxxxxxxxxxxx>
>
> [...]
>
> Sure, why not? It shows that you will also in future take care of it.
Good point. I did that for v3.
>
> [...]
>
>>> +} __packed;
>>> +
>>> +static bool use_otp_key;
>>> +module_param_named(dcp_use_otp_key, use_otp_key, bool, 0);
>>> +MODULE_PARM_DESC(dcp_use_otp_key, "Use OTP instead of UNIQUE key for sealing");
>>
>> Shouldn't these be documented in admin-guide/kernel-parameters.txt as well?
>
> Yes. Will do.
>
>>> +static bool skip_zk_test;
>>> +module_param_named(dcp_skip_zk_test, skip_zk_test, bool, 0);
>>> +MODULE_PARM_DESC(dcp_skip_zk_test, "Don't test whether device keys are
>>> zero'ed");
>>
>> Does this need to be configurible? I'd assume this can only happen when using an
>> unfused OTP. In such a case, it's ok to always warn, so you don't need to make
>> this configurible.
>
> We found such a setting super useful while working with targets where the keys are
> zero'ed for various reasons.
> There are cases where you want to use/test trusted keys even when the master key
> is void. Our detection logic does not only print a warning, it refuses to load
> blobs. So IMHO the config knob makes sense.
Ah, I missed that it refuses to continue in that case.
>
>>> +
>>> +static unsigned int calc_blob_len(unsigned int payload_len)
>>> +{
>>> + return sizeof(struct dcp_blob_fmt) + payload_len + DCP_BLOB_AUTHLEN;
>>> +}
>>> +
>>> +static int do_dcp_crypto(u8 *in, u8 *out, bool is_encrypt)
>>
>> I assume in can't be const because the use with sg APIs?
>
> I'm pretty sure this was the main reason, but I can check again.
>
>>> +{
>>> + int res = 0;
>>> + struct skcipher_request *req = NULL;
>>> + DECLARE_CRYPTO_WAIT(wait);
>>> + struct scatterlist src_sg, dst_sg;
>>> + struct crypto_skcipher *tfm;
>>> + u8 paes_key[DCP_PAES_KEYSIZE];
>>> +
>>> + if (!use_otp_key)
>>
>> I'd invert this. Makes code easier to read.
>
> Ok. :-)
>
>>> + paes_key[0] = DCP_PAES_KEY_UNIQUE;
>>> + else
>>> + paes_key[0] = DCP_PAES_KEY_OTP;
>>> +
>>> + tfm = crypto_alloc_skcipher("ecb-paes-dcp", CRYPTO_ALG_INTERNAL,
>>> + CRYPTO_ALG_INTERNAL);
>>> + if (IS_ERR(tfm)) {
>>> + res = PTR_ERR(tfm);
>>> + pr_err("Unable to request DCP pAES-ECB cipher: %i\n", res);
>>
>> Can you define pr_fmt above? There's also %pe now that can directly print out an
>> error pointer.
>
> pr_fmt is not defined on purpose. include/keys/trusted-type.h defines already one
> and I assumed "trusted_key:" is the desired prefix for all kinds of trusted keys.
Ah, all good then. I didn't define it for CAAM either, but forgot why I didn't
along the way. May've been the same reason.
> [...]
>
>> - payload_len is at offset 33, but MIN_KEY_SIZE == 32 and there are no minimum
>> size checks. Couldn't you read beyond the buffer this way?
>
> The key has a minimum size of MIN_KEY_SIZE, but p->blob (being struct trusted_key_payload->blob[MAX_BLOB_SIZE])
> is much larger.
> So the assumption is that a DCP blob will always be smaller than MAX_BLOB_SIZE.
>
>> - offset 33 is unaligned for payload_len. Please use get_unaligned_le32 here.
>
> Oh yes. Makes sense!
>
> [...]
>
>>
>> jfyi, in the prelude of my CAAM series, I made this the default
>> when .get_random == NULL.
>
> Right. :-)
>
> [...]
>
>>> + ret = do_dcp_crypto(buf, buf, true);
>>> + if (ret)
>>> + goto out;
>>> +
>>> + if (memcmp(buf, bad, AES_BLOCK_SIZE) == 0) {
>>> + pr_err("Device neither in secure nor trusted mode!\n");
>>
>> What's the difference between secure and trusted? Can't this test be skipped
>> if use_otp_key == false?
>
> DCP has many modes of operation. Secure is one level above trusted.
> For the gory details see "Security Reference Manual for the i.MX 6ULL Applications Processor".
> I'm not sure whether all information my manual describes is publicly available so I
> don't dare to copy&paste from it.
>
> As David and I understood the logic, both OTP and UNIQUE keys can be zero'ed.
> It is also possible that DCP has no support at all for these keys,
> then you'll also get a zero key. That's why we have this check here.
Thanks for the clarification.
Cheers,
Ahmad
>
> Thanks,
> //richard
>
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