linux-kernel - Re: [PATCH v10 2/7] KEYS: trusted: allow use of kernel RNG for key material

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Date:   Tue, 17 May 2022 18:25:08 +0200
From:   Ahmad Fatoum <a.fatoum@...gutronix.de>
To:     Mimi Zohar <zohar@...ux.ibm.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        David Howells <dhowells@...hat.com>
Cc:     kernel@...gutronix.de, Sumit Garg <sumit.garg@...aro.org>,
        Pankaj Gupta <pankaj.gupta@....com>,
        David Gstir <david@...ma-star.at>,
        Michael Walle <michael@...le.cc>,
        John Ernberg <john.ernberg@...ia.se>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Horia Geantă <horia.geanta@....com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Jan Luebbe <j.luebbe@...gutronix.de>,
        Eric Biggers <ebiggers@...nel.org>,
        Richard Weinberger <richard@....at>,
        Franck LENORMAND <franck.lenormand@....com>,
        Matthias Schiffer <matthias.schiffer@...tq-group.com>,
        keyrings@...r.kernel.org, linux-crypto@...r.kernel.org,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Theodore Ts'o <tytso@....edu>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: [PATCH v10 2/7] KEYS: trusted: allow use of kernel RNG for key
 material

Hello Mimi,

[Cc'ing RNG maintainers in case they want to chime in]

On 17.05.22 17:52, Mimi Zohar wrote:
> On Fri, 2022-05-13 at 16:57 +0200, Ahmad Fatoum wrote:
>>  static int __init init_trusted(void)
>>  {
>> +       int (*get_random)(unsigned char *key, size_t key_len);
>>         int i, ret = 0;
>>  
>>         for (i = 0; i < ARRAY_SIZE(trusted_key_sources); i++) {
>> @@ -322,6 +333,28 @@ static int __init init_trusted(void)
>>                             strlen(trusted_key_sources[i].name)))
>>                         continue;
>>  
>> +               /*
>> +                * We always support trusted.rng="kernel" and "default" as
>> +                * well as trusted.rng=$trusted.source if the trust source
>> +                * defines its own get_random callback.
>> +                */
>  
> While TEE trusted keys support was upstreamed, there was a lot of
> discussion about using kernel RNG.  One of the concerns was lack of or
> insuffiencent entropy during early boot on embedded devices.  This
> concern needs to be clearly documented in both Documentation/admin-
> guide/kernel-parameters.txt and Documentation/security/keys/trusted-
> encrypted.rst.

If a user decides to use kernel RNG for trusted keys, wait_for_random_bytes()
called first thing in the used get_random_bytes_wait() will (quoting
documentation) "wait for the input pool to be seeded and thus [is] guaranteed
to supply cryptographically secure random numbers."

Does this address your concerns about Kernel RNG use?

Thanks,
Ahmad

> 
> thanks,
> 
> Mimi
> 
>> +               get_random = trusted_key_sources[i].ops->get_random;
>> +               if (trusted_rng && strcmp(trusted_rng, "default")) {
>> +                       if (!strcmp(trusted_rng, "kernel")) {
>> +                               get_random = kernel_get_random;
>> +                       } else if (strcmp(trusted_rng, trusted_key_sources[i].name) ||
>> +                                  !get_random) {
>> +                               pr_warn("Unsupported RNG. Supported: kernel");
>> +                               if (get_random)
>> +                                       pr_cont(", %s", trusted_key_sources[i].name);
>> +                               pr_cont(", default\n");
>> +                               return -EINVAL;
>> +                       }
>> +               }
>> +
>> +               if (!get_random)
>> +                       get_random = kernel_get_random;
>> +
>>                 static_call_update(trusted_key_init,
>>                                    trusted_key_sources[i].ops->init);
> 
> 


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