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

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Message-ID: <f1257a02-e383-484f-a0c4-90ac5b870dc1@gmx.de>
Date: Sat, 24 Feb 2024 03:06:44 +0100
From: Lino Sanfilippo <LinoSanfilippo@....de>
To: Alexander Steffen <Alexander.Steffen@...ineon.com>,
 Lino Sanfilippo <l.sanfilippo@...bus.com>,
 Jarkko Sakkinen <jarkko@...nel.org>,
 "Daniel P. Smith" <dpsmith@...rtussolutions.com>,
 Jason Gunthorpe <jgg@...pe.ca>, Sasha Levin <sashal@...nel.org>,
 linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: Ross Philipson <ross.philipson@...cle.com>,
 Kanth Ghatraju <kanth.ghatraju@...cle.com>, Peter Huewe <peterhuewe@....de>
Subject: Re: [PATCH 1/3] tpm: protect against locality counter underflow

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