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SubjectRe: [PATCH v3 1/2] tpm: fix reference counting for struct tpm_chip
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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

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