linux-kernel - Re: [PATCH v3 1/2] tpm: fix reference counting for struct tpm

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Date:   Fri, 5 Feb 2021 15:55:09 +0100
From:   Lino Sanfilippo <l.sanfilippo@...bus.com>
To:     Jason Gunthorpe <jgg@...pe.ca>,
        Lino Sanfilippo <LinoSanfilippo@....de>
Cc:     peterhuewe@....de, jarkko@...nel.org, stefanb@...ux.vnet.ibm.com,
        James.Bottomley@...senpartnership.com, stable@...r.kernel.org,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] tpm: fix reference counting for struct tpm_chip

Hi,

On 05.02.21 14:05, Jason Gunthorpe wrote:

>>
>> Commit fdc915f7f719 ("tpm: expose spaces via a device link /dev/tpmrm<n>")
>> already introduced function tpm_devs_release() to release the extra
>> reference but did not implement the required put on chip->devs that results
>> in the call of this function.
> 
> Seems wonky, the devs is just supposed to be a side thing, nothing
> should be using it as a primary reference count for a tpm.
> 
> The bug here is only that tpm_common_open() did not get a kref on the
> chip before putting it in priv and linking it to the fd. See the
> comment before tpm_try_get_ops() indicating the caller must already
> have taken care to ensure the chip is valid.
> 
> This should be all you need to fix the oops:
> 
> diff --git a/drivers/char/tpm/tpm-dev-common.c b/drivers/char/tpm/tpm-dev-common.c
> index 1784530b8387bb..1b738dca7fffb5 100644
> --- a/drivers/char/tpm/tpm-dev-common.c
> +++ b/drivers/char/tpm/tpm-dev-common.c
> @@ -105,6 +105,7 @@ static void tpm_timeout_work(struct work_struct *work)
>  void tpm_common_open(struct file *file, struct tpm_chip *chip,
>                      struct file_priv *priv, struct tpm_space *space)
>  {
> +       get_device(&priv->chip.dev);
>         priv->chip = chip;
>         priv->space = space;
>         priv->response_read = true;

This is racy, isnt it? The time between we open the file and we want to grab the
reference in common_open() the chip can already be unregistered and freed.

As a matter of fact this solution was the first thing that came into my mind, too,
until I noticed the possible race condition. I can only guess that this was what
James had in mind when he chose to take the extra reference to chip->dev in
tpm_chip_alloc() instead of common_open(). 


>> diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
>> index ddaeceb..3ace199 100644
>> +++ b/drivers/char/tpm/tpm-chip.c
>> @@ -360,8 +360,7 @@ struct tpm_chip *tpm_chip_alloc(struct device *pdev,
>>  	 * while cdevs is in use.  The corresponding put
>>  	 * is in the tpm_devs_release (TPM2 only)
>>  	 */
>> -	if (chip->flags & TPM_CHIP_FLAG_TPM2)
>> -		get_device(&chip->dev);
>> +	get_device(&chip->dev);
>>  
>>  	if (chip->dev_num == 0)
>>  		chip->dev.devt = MKDEV(MISC_MAJOR, TPM_MINOR);
>> @@ -422,8 +421,21 @@ struct tpm_chip *tpmm_chip_alloc(struct device *pdev,
>>  	rc = devm_add_action_or_reset(pdev,
>>  				      (void (*)(void *)) put_device,
>>  				      &chip->dev);
>> -	if (rc)
>> +	if (rc) {
>> +		put_device(&chip->devs);
>>  		return ERR_PTR(rc);
> 
> This isn't right read what 'or_reset' does
> 
 
In case of failure installing the action handler devm_add_action_or_reset() puts
chip->dev for us. But we also have put chip->devs since we have retrieved a 
reference to both chip->dev and chip->devs. Or do I miss something here?

> Jason
> 

Regards,
Lino