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

From: Stephen Boyd
Date: Wed Jul 17 2019 - 12:42:36 EST

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.