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

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Message-ID: <8692fcf6-2e67-45b4-b809-7723f30736a2@apertussolutions.com>
Date: Thu, 22 Feb 2024 20:56:33 -0500
From: "Daniel P. Smith" <dpsmith@...rtussolutions.com>
To: Lino Sanfilippo <l.sanfilippo@...bus.com>,
 Alexander Steffen <Alexander.Steffen@...ineon.com>,
 Jarkko Sakkinen <jarkko@...nel.org>, 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 15:54, 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.
> But this is rather code optimization and not really required to fix the reported bug.

Actually, doing so will break the TPM API. The function 
tpm_tis_request_locality() is registered as the locality handler, 
  int (*request_locality)(struct tpm_chip *chip, int loc), in the tis 
instance of struct tpm_class_ops{}. This is the API used by the Secure 
Launch series to open Locality2 for the measurements it must record.

v/r,
dps