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

[Lino Sanfilippo <LinoSanfilippo@gmx.de>]

Hi,

On 20.02.24 19:42, Alexander Steffen wrote:
> On 02.02.2024 04:08, 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".
>
> check_locality() returns true, but __tpm_tis_request_locality() returns the requested locality. Currently, this is always 0, so the check for !ret will always correctly indicate success and increment the locality_count.

So how was the reported underflow triggered then? We only request locality 0 in TPM TIS core, no other locality.

>
> But since theoretically a locality != 0 could be requested, the correct fix would be to check for something like ret >= 0 or ret == l instead of !ret.

I thought that the underflow issue has actually been triggered and is not only a theoretical problem.
But now I wonder how this could ever happen.

BR,
Lino