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

[Jarkko Sakkinen]

On Tue Feb 20, 2024 at 8:42 PM EET, 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.
>
> 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. Then the counter will also be incremented correctly for localities 
> != 0, and no underflow will happen later on. Therefore, explicitly 
> checking for an underflow is unnecessary and hides the real problem.

Good point.

I think that the check should contain info-level klog message of the
event together with the check against the underflow. I think this is
very useful info for live systems.

BR, Jarkko