linux-kernel - Re: [PATCH v9 13/19] tpm: Protect against locality counter underflow

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Message-ID: <1c3d7b31-89c0-400f-8f37-45089f7078e8@apertussolutions.com>
Date: Thu, 15 Aug 2024 14:52:33 -0400
From: "Daniel P. Smith" <dpsmith@...rtussolutions.com>
To: Jarkko Sakkinen <jarkko@...nel.org>,
 Ross Philipson <ross.philipson@...cle.com>, linux-kernel@...r.kernel.org,
 x86@...nel.org, linux-integrity@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-crypto@...r.kernel.org, kexec@...ts.infradead.org,
 linux-efi@...r.kernel.org, iommu@...ts.linux-foundation.org
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
 dave.hansen@...ux.intel.com, ardb@...nel.org, mjg59@...f.ucam.org,
 James.Bottomley@...senpartnership.com, peterhuewe@....de, jgg@...pe.ca,
 luto@...capital.net, nivedita@...m.mit.edu, herbert@...dor.apana.org.au,
 davem@...emloft.net, corbet@....net, ebiederm@...ssion.com,
 dwmw2@...radead.org, baolu.lu@...ux.intel.com, kanth.ghatraju@...cle.com,
 andrew.cooper3@...rix.com, trenchboot-devel@...glegroups.com
Subject: Re: [PATCH v9 13/19] tpm: Protect against locality counter underflow

On 6/4/24 16:12, Jarkko Sakkinen wrote:
> On Fri May 31, 2024 at 4:03 AM EEST, Ross Philipson wrote:
>> From: "Daniel P. Smith" <dpsmith@...rtussolutions.com>
>>
>> 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.
>>
>> Signed-off-by: Daniel P. Smith <dpsmith@...rtussolutions.com>
>> Signed-off-by: Ross Philipson <ross.philipson@...cle.com>
>> Reported-by: Kanth Ghatraju <kanth.ghatraju@...cle.com>
>> Fixes: 933bfc5ad213 ("tpm, tpm: Implement usage counter for locality")
> 
> Not sure if we have practical use for fixes tag here but open for
> argument ofc. I.e. I'm not sure what is the practical scenario to
> worry about if Trenchboot did not exist.

We can drop the fixes line.

>> ---
>>   drivers/char/tpm/tpm_tis_core.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/char/tpm/tpm_tis_core.c b/drivers/char/tpm/tpm_tis_core.c
>> index 176cd8dbf1db..7c1761bd6000 100644
>> --- a/drivers/char/tpm/tpm_tis_core.c
>> +++ b/drivers/char/tpm/tpm_tis_core.c
>> @@ -180,7 +180,8 @@ static int tpm_tis_relinquish_locality(struct tpm_chip *chip, int l)
>>   	struct tpm_tis_data *priv = dev_get_drvdata(&chip->dev);
>>   
>>   	mutex_lock(&priv->locality_count_mutex);
>> -	priv->locality_count--;
>> +	if (priv->locality_count > 0)
>> +		priv->locality_count--;
> 
> I'd signal the situation with pr_info() in else branch.

Ack.

>>   	if (priv->locality_count == 0)
>>   		__tpm_tis_relinquish_locality(priv, l);
>>   	mutex_unlock(&priv->locality_count_mutex);
> 
> BR, Jarkko