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

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Date:   Mon, 7 Oct 2019 03:05:20 +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 07:56:01PM +0000, Safford, David (GE Global Research, US) wrote:
> 
> > From: linux-integrity-owner@...r.kernel.org <linux-integrity-
> > owner@...r.kernel.org> On Behalf Of Jarkko Sakkinen
> > Sent: Friday, October 4, 2019 2:27 PM
> > Subject: EXT: Re: [PATCH] KEYS: asym_tpm: Switch to get_random_bytes()
> > 
> > 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.
> 
> And if the entropy pool has a bug or is misconfigured, you lose everything.
> That does not sound like defense in depth to me. In the real world
> I am not aware of a single instance of RNG vulnerability on a TPM.
> I am directly aware of several published vulnerabilities in embedded systems 
> due to a badly ported version of the kernel random pool. In addition, 
> the random generator in a TPM is hardware isolated, and less likely to be
> vulnerable to side channel or memory manipulation errors. The TPM
> RNG is typically FIPS certified.  The use of the TPM RNG was a deliberate
> design choice in trusted keys.

Hmm... so is RDRAND opcode FIPS certified.

Kernel has the random number generator for two reasons:

1. To protect against bugs in hwrng's.
2. To protect against deliberate backdoors in hwrng's.

How TPM RNG is guaranteed to protect against both 1 and 2?

If I would agree what you say, that'd be argument against using kernel
random number generator *anywhere* in the kernel. Even with the entropy
issues it is least worst thing to use for key generations for better
or worse.

/Jarkko