linux-kernel - Re: [PATCH 2/4] KEYS: trusted: allow trust sources to use kernel RNG for key material

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Date:   Mon, 9 Aug 2021 12:56:47 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Ahmad Fatoum <a.fatoum@...gutronix.de>
Cc:     Sumit Garg <sumit.garg@...aro.org>,
        James Bottomley <jejb@...ux.ibm.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        David Howells <dhowells@...hat.com>,
        kernel <kernel@...gutronix.de>, James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Horia Geantă <horia.geanta@....com>,
        Aymen Sghaier <aymen.sghaier@....com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Udit Agarwal <udit.agarwal@....com>,
        Eric Biggers <ebiggers@...nel.org>,
        Jan Luebbe <j.luebbe@...gutronix.de>,
        David Gstir <david@...ma-star.at>,
        Richard Weinberger <richard@....at>,
        Franck LENORMAND <franck.lenormand@....com>,
        "open list:ASYMMETRIC KEYS" <keyrings@...r.kernel.org>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:SECURITY SUBSYSTEM" 
        <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH 2/4] KEYS: trusted: allow trust sources to use kernel RNG
 for key material

On Mon, Aug 09, 2021 at 09:52:20AM +0200, Ahmad Fatoum wrote:
> Hello Sumit,
> 
> On 22.07.21 08:31, Sumit Garg wrote:
> > On Wed, 21 Jul 2021 at 22:19, Ahmad Fatoum <a.fatoum@...gutronix.de> wrote:
> >>
> >> The two existing trusted key sources don't make use of the kernel RNG,
> >> but instead let the hardware that does the sealing/unsealing also
> >> generate the random key material. While a previous change offers users
> >> the choice to use the kernel RNG instead for both, new trust sources
> >> may want to unconditionally use the kernel RNG for generating key
> >> material, like it's done elsewhere in the kernel.
> >>
> >> This is especially prudent for hardware that has proven-in-production
> >> HWRNG drivers implemented, as otherwise code would have to be duplicated
> >> only to arrive at a possibly worse result.
> >>
> >> Make this possible by turning struct trusted_key_ops::get_random
> >> into an optional member. If a driver leaves it NULL, kernel RNG
> >> will be used instead.
> >>
> >> Signed-off-by: Ahmad Fatoum <a.fatoum@...gutronix.de>
> >> ---
> >> To: James Bottomley <jejb@...ux.ibm.com>
> >> To: Jarkko Sakkinen <jarkko@...nel.org>
> >> To: Mimi Zohar <zohar@...ux.ibm.com>
> >> To: David Howells <dhowells@...hat.com>
> >> Cc: James Morris <jmorris@...ei.org>
> >> Cc: "Serge E. Hallyn" <serge@...lyn.com>
> >> Cc: "Horia Geantă" <horia.geanta@....com>
> >> Cc: Aymen Sghaier <aymen.sghaier@....com>
> >> Cc: Herbert Xu <herbert@...dor.apana.org.au>
> >> Cc: "David S. Miller" <davem@...emloft.net>
> >> Cc: Udit Agarwal <udit.agarwal@....com>
> >> Cc: Eric Biggers <ebiggers@...nel.org>
> >> Cc: Jan Luebbe <j.luebbe@...gutronix.de>
> >> Cc: David Gstir <david@...ma-star.at>
> >> Cc: Richard Weinberger <richard@....at>
> >> Cc: Franck LENORMAND <franck.lenormand@....com>
> >> Cc: Sumit Garg <sumit.garg@...aro.org>
> >> Cc: keyrings@...r.kernel.org
> >> Cc: linux-crypto@...r.kernel.org
> >> Cc: linux-integrity@...r.kernel.org
> >> Cc: linux-kernel@...r.kernel.org
> >> Cc: linux-security-module@...r.kernel.org
> >> ---
> >>  include/keys/trusted-type.h               | 2 +-
> >>  security/keys/trusted-keys/trusted_core.c | 2 +-
> >>  2 files changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/include/keys/trusted-type.h b/include/keys/trusted-type.h
> >> index d89fa2579ac0..4eb64548a74f 100644
> >> --- a/include/keys/trusted-type.h
> >> +++ b/include/keys/trusted-type.h
> >> @@ -64,7 +64,7 @@ struct trusted_key_ops {
> >>         /* Unseal a key. */
> >>         int (*unseal)(struct trusted_key_payload *p, char *datablob);
> >>
> >> -       /* Get a randomized key. */
> >> +       /* Optional: Get a randomized key. */
> >>         int (*get_random)(unsigned char *key, size_t key_len);
> >>
> >>         /* Exit key interface. */
> >> diff --git a/security/keys/trusted-keys/trusted_core.c b/security/keys/trusted-keys/trusted_core.c
> >> index 569af9af8df0..d2b7626cde8b 100644
> >> --- a/security/keys/trusted-keys/trusted_core.c
> >> +++ b/security/keys/trusted-keys/trusted_core.c
> >> @@ -334,7 +334,7 @@ static int __init init_trusted(void)
> >>                         continue;
> >>
> >>                 get_random = trusted_key_sources[i].ops->get_random;
> >> -               if (trusted_kernel_rng)
> >> +               if (trusted_kernel_rng || !get_random)
> >>                         get_random = kernel_get_random;
> >>
> > 
> > For ease of understanding, I would prefer to write it as:
> > 
> >                   get_random = trusted_key_sources[i].ops->get_random ?:
> >                                          kernel_get_random;
> >                   if (trusted_kernel_rng)
> >                         get_random = kernel_get_random;
> > 
> > With that:
> > 
> > Acked-by: Sumit Garg <sumit.garg@...aro.org>
> 
> I don't think it improves readability to split up the conditional.
> At least I need to take a second pass over the code to understand
> the second conditional.

Ternary operators are pain to read, unless a super trivial case.

I'd stick to what you did.

/Jarkko