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
