linux-kernel - Re: [PATCH] tpm: Fix kexec crash due to access to ops NULL pointer (powerpc)

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Message-ID: <bad84f8a-9e07-0920-5c07-475e30e710e9@linux.ibm.com>
Date:   Mon, 20 Dec 2021 17:37:36 -0800
From:   Tyrel Datwyler <tyreld@...ux.ibm.com>
To:     Stefan Berger <stefanb@...ux.ibm.com>,
        Jason Gunthorpe <jgg@...pe.ca>
Cc:     jarkko@...nel.org, peterhuewe@....de,
        linux-integrity@...r.kernel.org, Korrapati.Likhitha@....com,
        pavrampu@...ibm.com, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, gcwilson@...ibm.com,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] tpm: Fix kexec crash due to access to ops NULL pointer
 (powerpc)

On 12/20/21 5:31 PM, Stefan Berger wrote:
> 
> On 12/20/21 20:13, Jason Gunthorpe wrote:
>> On Mon, Dec 20, 2021 at 08:05:58PM -0500, Stefan Berger wrote:
>>
>>> diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
>>> index ddaeceb7e109..4cb908349b31 100644
>>> +++ b/drivers/char/tpm/tpm-chip.c
>>> @@ -473,15 +473,8 @@ static void tpm_del_char_device(struct tpm_chip *chip)
>>>          mutex_unlock(&idr_lock);
>>>
>>>          /* Make the driver uncallable. */
>>> -       down_write(&chip->ops_sem);
>>> -       if (chip->flags & TPM_CHIP_FLAG_TPM2) {
>>> -               if (!tpm_chip_start(chip)) {
>>> -                       tpm2_shutdown(chip, TPM2_SU_CLEAR);
>>> -                       tpm_chip_stop(chip);
>>> -               }
>>> -       }
>>> -       chip->ops = NULL;
>>> -       up_write(&chip->ops_sem);
>>> +       if (chip->ops)
>> ops cannot be read without locking
> 
> This here could be an alternative:

I still think code de-duplication is a good thing. Maybe combine the two
approaches. Call tpm_class_shutdown from tpm_del_char_device and add a chip->ops
check in tpm_class_shutdown to ensure we only do the shutdown when chip->ops is
valid.

-Tyrel

> 
> diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
> index ddaeceb7e109..7772b475ebc0 100644
> --- a/drivers/char/tpm/tpm-chip.c
> +++ b/drivers/char/tpm/tpm-chip.c
> @@ -474,7 +474,7 @@ static void tpm_del_char_device(struct tpm_chip *chip)
> 
>         /* Make the driver uncallable. */
>         down_write(&chip->ops_sem);
> -       if (chip->flags & TPM_CHIP_FLAG_TPM2) {
> +       if (chip->ops && chip->flags & TPM_CHIP_FLAG_TPM2) {
>                 if (!tpm_chip_start(chip)) {
>                         tpm2_shutdown(chip, TPM2_SU_CLEAR);
>                         tpm_chip_stop(chip);
> 
>    Stefan
> 
> 
>>
>> Jason