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

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Message-ID: <20191004201134.nuesk6hxtxajnxh2@cantor>
Date:   Fri, 4 Oct 2019 13:11:34 -0700
From:   Jerry Snitselaar <jsnitsel@...hat.com>
To:     James Bottomley <James.Bottomley@...senPartnership.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        David Safford <david.safford@...com>,
        linux-integrity@...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 19, Jerry Snitselaar wrote:
>On Fri Oct 04 19, James Bottomley wrote:
>>On Fri, 2019-10-04 at 11:33 -0700, Jerry Snitselaar wrote:
>>>On Fri Oct 04 19, James Bottomley wrote:
>>>> On Fri, 2019-10-04 at 21:22 +0300, Jarkko Sakkinen wrote:
>>>> > On Thu, Oct 03, 2019 at 04:59:37PM -0700, James Bottomley wrote:
>>>> > > I think the principle of using multiple RNG sources for strong
>>>> > > keys is a sound one, so could I propose a compromise:  We have
>>>> > > a tpm subsystem random number generator that, when asked for
>>>> > > <n> random bytes first extracts <n> bytes from the TPM RNG and
>>>> > > places it into the kernel entropy pool and then asks for <n>
>>>> > > random bytes from the kernel RNG? That way, it will always have
>>>> > > the entropy to satisfy the request and in the worst case, where
>>>> > > the kernel has picked up no other entropy sources at all it
>>>> > > will be equivalent to what we have now (single entropy source)
>>>> > > but usually it will be a much better mixed entropy source.
>>>> >
>>>> > I think we should rely the existing architecture where TPM is
>>>> > contributing to the entropy pool as hwrng.
>>>>
>>>> That doesn't seem to work: when I trace what happens I see us
>>>> inject 32 bytes of entropy at boot time, but never again.  I think
>>>> the problem is the kernel entropy pool is push not pull and we have
>>>> no triggering event in the TPM to get us to push.  I suppose we
>>>> could set a timer to do this or perhaps there is a pull hook and we
>>>> haven't wired it up correctly?
>>>>
>>>> James
>>>>
>>>
>>>Shouldn't hwrng_fillfn be pulling from it?
>>
>>It should, but the problem seems to be it only polls the "current" hw
>>rng ... it doesn't seem to have a concept that there may be more than
>>one.  What happens, according to a brief reading of the code, is when
>>multiple are registered, it determines what the "best" one is and then
>>only pulls from that.  What I think it should be doing is filling from
>>all of them using the entropy quality to adjust how many bits we get.
>>
>>James
>>
>
>Most of them don't even set quality, including the tpm, so they end up
>at the end of the list. For the ones that do I'm not sure how they determined
>the value. For example virtio-rng sets quality to 1000.

I should have added that I like that idea though.