linux-kernel - Re: [PATCH] KEYS: asym_tpm: Switch to get_random

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Date:   Fri, 4 Oct 2019 21:27:11 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     "Safford, David (GE Global Research, US)" <david.safford@...com>
Cc:     Mimi Zohar <zohar@...ux.ibm.com>,
        "Wiseman, Monty (GE Global Research, US)" <monty.wiseman@...com>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        "open list:ASYMMETRIC KEYS" <keyrings@...r.kernel.org>,
        "open list:CRYPTO API" <linux-crypto@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KEYS: asym_tpm: Switch to get_random_bytes()

On Fri, Oct 04, 2019 at 01:26:58PM +0000, Safford, David (GE Global Research, US) wrote:
> As the original author of trusted keys, let me make a few comments.
> First, trusted keys were specifically implemented and *documented* to
> use the TPM to both generate and seal keys. Its kernel documentation
> specifically states this as a promise to user space. If you want to have 
> a different key system that uses the random pool to generate the keys,
> fine, but don't change trusted keys, as that changes the existing promise
> to user space. 

TPM generating keys (i.e. the random number) would make sense if the key
would never leave from TPM (that kind of trusted keys would not be a
bad idea at all).

> There are many good reasons for wanting the keys to be based on the
> TPM generator.  As the source for the kernel random number generator
> itself says, some systems lack good randomness at startup, and systems
> should preserve and reload the pool across shutdown and startup.
> There are use cases for trusted keys which need to generate keys 
> before such scripts have run. Also, in some use cases, we need to show
> that trusted keys are FIPS compliant, which is possible with TPM
> generated keys.

If you are able to call tpm_get_random(), the driver has already
registered TPN as hwrng. With this solution you fail to follow the
principle of defense in depth. If the TPM random number generator
is compromissed (has a bug) using the entropy pool will decrease
the collateral damage.

> Second, the TPM is hardly a "proprietary random number generator".
> It is an open standard with multiple implementations, many of which are
> FIPS certified.
> 
> Third, as Mimi states, using a TPM is not a "regression". It would be a
> regression to change trusted keys _not_ to use the TPM, because that
> is what trusted keys are documented to provide to user space.

For asym-tpm.c it is without a question a regression because of the
evolution that has happened after trusted keys. For trusted keys
using kernel rng would be improvement.

/Jarkko