Re: [PATCH] KEYS: asym_tpm: Switch to get_random_bytes()

[Jarkko Sakkinen]

On Fri, Oct 04, 2019 at 07:56:01PM +0000, Safford, David (GE Global Research, US) wrote:
> 
> > From: linux-integrity-owner@xxxxxxxxxxxxxxx <linux-integrity-
> > owner@xxxxxxxxxxxxxxx> 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