Re: [PATCH 1/3] tpm: protect against locality counter underflow

[Lino Sanfilippo <l.sanfilippo@kunbus.com>]

On 01.02.24 23:21, Jarkko Sakkinen wrote:

> 
> On Wed Jan 31, 2024 at 7:08 PM EET, Daniel P. Smith wrote:
>> Commit 933bfc5ad213 introduced the use of a locality counter to control when a
>> locality request is allowed to be sent to the TPM. In the commit, the counter
>> is indiscriminately decremented. Thus creating a situation for an integer
>> underflow of the counter.
> 
> What is the sequence of events that leads to this triggering the
> underflow? This information should be represent in the commit message.
> 

AFAIU this is:

1. We start with a locality_counter of 0 and then we call tpm_tis_request_locality()
for the first time, but since a locality is (unexpectedly) already active check_locality() and consequently
__tpm_tis_request_locality() return "true". This prevents the locality_counter from being increased
to 1, see 

	ret = __tpm_tis_request_locality(chip, l);
	if (!ret) /* Counter not increased since ret == 1 */
		priv->locality_count++;

in tpm_tis_request_locality().

If now the locality is released the counter is decreased to below zero (resulting
in an underflow since "locality_counter" is an unsigned int.