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

[Jarkko Sakkinen]

On Fri Feb 2, 2024 at 5:08 AM EET, Lino Sanfilippo wrote:
>
>
> 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. 

Thanks, Daniel, can you transcript this to the commit message?

BR, Jarkko