Re: [PATCH 2/3] TPM: Close data_pending and data_buffer races

[Rajiv Andrade <srajiv@linux.vnet.ibm.com>]

On 06/12/11 16:29, Tim Gardner wrote:
> There is a race betwen tpm_read() and tpm_write where both chip->data_pending
> and chip->data_buffer can be changed by tpm_write() when tpm_read()
> clears chip->data_pending, but before tpm_read() grabs the mutex.
>
> Protect changes to chip->data_pending and chip->data_buffer by expanding
> the scope of chip->buffer_mutex.
>
> Reported-by: Seth Forshee<seth.forshee@canonical.com>
> Cc: Debora Velarde<debora@linux.vnet.ibm.com>
> Cc: Rajiv Andrade<srajiv@linux.vnet.ibm.com>
> Cc: Marcel Selhorst<m.selhorst@sirrix.com>
> Cc: tpmdd-devel@lists.sourceforge.net
> Cc: stable@vger.kernel.org
> Signed-off-by: Tim Gardner<tim.gardner@canonical.com>
> ---
>   drivers/char/tpm/tpm.c |   17 +++++++++--------
>   1 files changed, 9 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/char/tpm/tpm.c b/drivers/char/tpm/tpm.c
> index b366b34..70bf9e5 100644
> --- a/drivers/char/tpm/tpm.c
> +++ b/drivers/char/tpm/tpm.c
> @@ -1074,12 +1074,15 @@ ssize_t tpm_write(struct file *file, const char __user *buf,
>   	struct tpm_chip *chip = file->private_data;
>   	size_t in_size = size, out_size;
>
> +	mutex_lock(&chip->buffer_mutex);
> +
>   	/* cannot perform a write until the read has cleared
>   	   either via tpm_read or a user_read_timer timeout */
> -	while (atomic_read(&chip->data_pending) != 0)
> +	while (atomic_read(&chip->data_pending) != 0) {
> +		mutex_unlock(&chip->buffer_mutex);
>   		msleep(TPM_TIMEOUT);
> -
> -	mutex_lock(&chip->buffer_mutex);
> +		mutex_lock(&chip->buffer_mutex);
> +	}
>
>   	if (in_size>  TPM_BUFSIZE)
>   		in_size = TPM_BUFSIZE;
> @@ -1112,22 +1115,20 @@ ssize_t tpm_read(struct file *file, char __user *buf,
>
>   	del_singleshot_timer_sync(&chip->user_read_timer);
>   	flush_work_sync(&chip->work);
> -	ret_size = atomic_read(&chip->data_pending);
> -	atomic_set(&chip->data_pending, 0);
> +	mutex_lock(&chip->buffer_mutex);
> +	ret_size = atomic_xchg(&chip->data_pending, 0);
>   	if (ret_size>  0) {	/* relay data */
>   		ssize_t orig_ret_size = ret_size;
>   		if (size<  ret_size)
>   			ret_size = size;
>
> -		mutex_lock(&chip->buffer_mutex);
>   		rc = copy_to_user(buf, chip->data_buffer, ret_size);
>   		memset(chip->data_buffer, 0, orig_ret_size);
>   		if (rc)
>   			ret_size = -EFAULT;

What about just moving atomic_set(&chip->data_pending, 0); to here?
If I'm not missing anything, this would be cleaner.

Rajiv
> -
> -		mutex_unlock(&chip->buffer_mutex);
>   	}
>
> +	mutex_unlock(&chip->buffer_mutex);
>   	return ret_size;
>   }
>   EXPORT_SYMBOL_GPL(tpm_read);