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

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Message-ID: <88b75c9c-98ab-4474-8112-6a27d11a2fdf@apertussolutions.com>
Date: Thu, 22 Feb 2024 20:58:40 -0500
From: "Daniel P. Smith" <dpsmith@...rtussolutions.com>
To: Jarkko Sakkinen <jarkko@...nel.org>,
 Lino Sanfilippo <l.sanfilippo@...bus.com>,
 Alexander Steffen <Alexander.Steffen@...ineon.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

On 2/20/24 17:23, Jarkko Sakkinen wrote:
> On Tue Feb 20, 2024 at 8:54 PM UTC, Lino Sanfilippo wrote:
>> Hi,
>>
>> On 20.02.24 19:42, Alexander Steffen wrote:
>>> ATTENTION: This e-mail is from an external sender. Please check attachments and links before opening e.g. with mouseover.
>>>
>>>
>>> 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.
>>>
>>
>> Will the TPM TIS CORE ever (have to) request another locality than 0? Maybe the best would
>> be to hardcode TPM_ACCESS(0) and get rid of all the locality parameters that are
>> passed from one function to another.
> 
> Usually, or at least use cases I'm aware of, localities are per
> component. E.g. Intel TXT has one and Linux has another.
> 
> There's been some proposals in the past here for hypervisor specific
> locality here at LKML they didn't lead to anything.
> 
> If you are suggesting of removing "int l" parameter altogether, I
> do support that idea.
> 
>> But this is rather code optimization and not really required to fix
>> the reported bug.
> 
> Just adding here that I wish we also had a log transcript of bug, which
> is right now missing. The explanation believable enough to move forward
> but I still wish to see a log transcript.

That will be forth coming.

v/r,
dps