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

["Daniel P. Smith" <dpsmith@apertussolutions.com>]

On 2/12/24 15:05, Jarkko Sakkinen wrote:
> 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?

ack

v/r,
dps