linux-kernel - Re: [PATCH v2 1/6] hwrng: core: Freeze khwrng thread during suspend

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Date:   Wed, 17 Jul 2019 09:42:32 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Peter Huewe <peterhuewe@....de>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-integrity@...r.kernel.org,
        Andrey Pronin <apronin@...omium.org>,
        Duncan Laurie <dlaurie@...omium.org>,
        Guenter Roeck <groeck@...omium.org>,
        Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH v2 1/6] hwrng: core: Freeze khwrng thread during suspend

Quoting Jason Gunthorpe (2019-07-17 04:39:56)
> On Tue, Jul 16, 2019 at 03:45:13PM -0700, Stephen Boyd wrote:
> > The hwrng_fill() function can run while devices are suspending and
> > resuming. If the hwrng is behind a bus such as i2c or SPI and that bus
> > is suspended, the hwrng may hang the bus while attempting to add some
> > randomness. It's been observed on ChromeOS devices with suspend-to-idle
> > (s2idle) and an i2c based hwrng that this kthread may run and ask the
> > hwrng device for randomness before the i2c bus has been resumed.
> 
> You mean the TPM here right?

In my case yes, but in general it isn't the TPM.

> 
> Should we be more careful in the TPM code to check if the TPM is
> suspended before trying to use it, rather than muck up callers?
> 

Given that it's not just a TPM issue I don't see how checking in the TPM
is going to fix this problem. It's better to not try to get random bytes
from the hwrng when the device could be suspended.