Re: TPM 2.0 device driver blocking open

[James Bottomley <jejb-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>]

On Fri, 2016-12-30 at 10:53 -0500, Ken Goldman wrote:
> It appears that an open() to the TPM doesn't block if another process
> has /dev/tpm0 open.  It returns -1, an error.
> 
> Questions:
> 
> Is this expected behavior?

It's enforced in drivers/char/tpm/tpm-dev.c by this check


	/* It's assured that the chip will be opened just once,
	 * by the check of is_open variable, which is protected
	 * by driver_lock. */
	if (test_and_set_bit(0, &chip->is_open)) {
		dev_dbg(&chip->dev, "Another process owns this TPM\n");
		return -EBUSY;
	}

so yes, it looks to be expected.

> Was this also true for 1.2?

In tpm 1.2 there was a single access broker daemon (tcsd) which opened
the device, so you could have multiple applications using the TPM but
only one device open.

> Is there any way to change it.  I didn't set O_NOBLOCK.  Is there 
> perhaps an ioctl()?
> Is this something that should be added?

I think for the 2.0 model of every application getting direct access,
we should make it so that every open gets a separate read/write stream
to the tpm which we send in via the locked version of tpm_transmit()
and just let the chip->tpm_mutex sort out the accesses.

I can code up a patch if no-one's already done it.

James


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