Re: [PATCH 2/2] tpm: Fix crash on tmprm release

[Jarkko Sakkinen <jarkko@kernel.org>]

On Tue, Jun 15, 2021 at 11:14:09AM +0200, Vincent Whitchurch wrote:
> If the tpm_tis module is removed (or a system shutdown is triggered)
> while the tpmrm device is use, the kernel crashes due to chip->ops being
> NULL:
> 
>  # exec 3<>/dev/tpmrm0
>  # rmmod tpm_tis
>  # exit
>  ==================================================================
>  BUG: KASAN: null-ptr-deref in tpm_chip_start+0x2d/0x120 [tpm]
>  Read of size 8 at addr 0000000000000060 by task sh/994
> 
>  Call Trace:
>   kasan_report.cold.13+0x10f/0x111
>   tpm_chip_start+0x2d/0x120 [tpm]
>   tpm2_del_space+0x2c/0xa0 [tpm]
>   tpmrm_release+0x3f/0x50 [tpm]
>   __fput+0x110/0x3c0
>   task_work_run+0x94/0xd0
>   do_exit+0x683/0x13e0
>   do_group_exit+0x8b/0x140
>   do_syscall_64+0x3c/0x80
>  ==================================================================
> 
> Fix this by making tpm2_del_space() use tpm_try_get_ops().  The latter
> already includes the calls to tpm_chip_start() and tpm_chip_stop().

This lacks explanation why migrating to tpm_try_get_ops() fixes the
issue. Saying that doing something fixes something is not good enough
explanation. So, can you extend this paragraph just a bit in the next
version?

> 
> Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
> ---
>  drivers/char/tpm/tpm2-space.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm2-space.c b/drivers/char/tpm/tpm2-space.c
> index 784b8b3cb903..e1111261021f 100644
> --- a/drivers/char/tpm/tpm2-space.c
> +++ b/drivers/char/tpm/tpm2-space.c
> @@ -58,12 +58,10 @@ int tpm2_init_space(struct tpm_space *space, unsigned int buf_size)
>  
>  void tpm2_del_space(struct tpm_chip *chip, struct tpm_space *space)
>  {
> -	mutex_lock(&chip->tpm_mutex);
> -	if (!tpm_chip_start(chip)) {
> +	if (!tpm_try_get_ops(chip)) {
>  		tpm2_flush_sessions(chip, space);
> -		tpm_chip_stop(chip);
> +		tpm_put_ops(chip);
>  	}
> -	mutex_unlock(&chip->tpm_mutex);
>  	kfree(space->context_buf);
>  	kfree(space->session_buf);
>  }
> -- 
> 2.28.0
> 
> 

/Jarkko