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Subject: Re: [PATCH v1 3/3] KEYS: trusted: Introduce support for NXP
 CAAM-based trusted keys
To:     Sumit Garg <sumit.garg@linaro.org>
Cc:     Mimi Zohar <zohar@linux.ibm.com>,
        =?UTF-8?Q?Horia_Geant=c4=83?= <horia.geanta@nxp.com>,
        Jonathan Corbet <corbet@lwn.net>,
        David Howells <dhowells@redhat.com>,
        Jarkko Sakkinen <jarkko@kernel.org>,
        James Bottomley <jejb@linux.ibm.com>,
        "kernel@pengutronix.de" <kernel@pengutronix.de>,
        James Morris <jmorris@namei.org>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Aymen Sghaier <aymen.sghaier@nxp.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "David S. Miller" <davem@davemloft.net>,
        Udit Agarwal <udit.agarwal@nxp.com>,
        Jan Luebbe <j.luebbe@pengutronix.de>,
        David Gstir <david@sigma-star.at>,
        Franck Lenormand <franck.lenormand@nxp.com>,
        "keyrings@vger.kernel.org" <keyrings@vger.kernel.org>,
        "linux-crypto@vger.kernel.org" <linux-crypto@vger.kernel.org>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        "linux-integrity@vger.kernel.org" <linux-integrity@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-security-module@vger.kernel.org" 
        <linux-security-module@vger.kernel.org>
References: <cover.56fff82362af6228372ea82e6bd7e586e23f0966.1615914058.git-series.a.fatoum@pengutronix.de>
 <319e558e1bd19b80ad6447c167a2c3942bdafea2.1615914058.git-series.a.fatoum@pengutronix.de>
 <01e6e13d-2968-0aa5-c4c8-7458b7bde462@nxp.com>
 <45a9e159-2dcb-85bf-02bd-2993d50b5748@pengutronix.de>
 <f9c0087d299be1b9b91b242f41ac6ef7b9ee3ef7.camel@linux.ibm.com>
 <63dd7d4b-4729-9e03-cd8f-956b94eab0d9@pengutronix.de>
 <CAFA6WYOw_mQwOUN=onhzb7zCTyYDBrcx0E7C3LRk6nPLAVCWEQ@mail.gmail.com>
From:   Ahmad Fatoum <a.fatoum@pengutronix.de>
Message-ID: <557b92d2-f3b8-d136-7431-419429f0e059@pengutronix.de>
Date:   Wed, 24 Mar 2021 15:07:30 +0100
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Hello Sumit,

On 24.03.21 11:47, Sumit Garg wrote:
> On Wed, 24 Mar 2021 at 14:56, Ahmad Fatoum <a.fatoum@pengutronix.de> wrote:
>>
>> Hello Mimi,
>>
>> On 23.03.21 19:07, Mimi Zohar wrote:
>>> On Tue, 2021-03-23 at 17:35 +0100, Ahmad Fatoum wrote:
>>>> On 21.03.21 21:48, Horia Geantă wrote:
>>>>> caam has random number generation capabilities, so it's worth using that
>>>>> by implementing .get_random.
>>>>
>>>> If the CAAM HWRNG is already seeding the kernel RNG, why not use the kernel's?
>>>>
>>>> Makes for less code duplication IMO.
>>>
>>> Using kernel RNG, in general, for trusted keys has been discussed
>>> before.   Please refer to Dave Safford's detailed explanation for not
>>> using it [1].
>>
>> The argument seems to boil down to:
>>
>>  - TPM RNG are known to be of good quality
>>  - Trusted keys always used it so far
>>
>> Both are fine by me for TPMs, but the CAAM backend is new code and neither point
>> really applies.
>>
>> get_random_bytes_wait is already used for generating key material elsewhere.
>> Why shouldn't new trusted key backends be able to do the same thing?
>>
> 
> Please refer to documented trusted keys behaviour here [1]. New
> trusted key backends should align to this behaviour and in your case
> CAAM offers HWRNG so we should be better using that.

Why is it better?

Can you explain what benefit a CAAM user would have if the trusted key
randomness comes directly out of the CAAM instead of indirectly from
the kernel entropy pool that is seeded by it?

> Also, do update documentation corresponding to CAAM as a trusted keys backend.

Yes. The documentation should be updated for CAAM and it should describe
how the key material is derived. Will do so for v2.

Cheers,
Ahmad

> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/jarkko/linux-tpmdd.git/tree/Documentation/security/keys/trusted-encrypted.rst#n87
> 
> -Sumit
> 
>> Cheers,
>> Ahmad
>>
>>>
>>> thanks,
>>>
>>> Mimi
>>>
>>> [1]
>>> https://lore.kernel.org/linux-integrity/BCA04D5D9A3B764C9B7405BBA4D4A3C035F2A38B@ALPMBAPA12.e2k.ad.ge.com/
>>>
>>>
>>>
>>
>> --
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> 

-- 
Pengutronix e.K.                           |                             |
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