Re: [PATCH] usb: typec: Prevent setting invalid opmode value

[Sriharsha Allenki]

Hi Heikki,

Thanks a lot for the review.

On 10/26/2020 6:35 PM, Heikki Krogerus wrote:
> On Thu, Oct 22, 2020 at 03:12:14PM +0530, Sriharsha Allenki wrote:
>> Setting opmode to invalid values would lead to a
>> paging fault failure when there is an access to the
>> power_operation_mode.
>>
>> Prevent this by checking the validity of the value
>> that the opmode is being set.
>>
>> Cc: <stable@xxxxxxxxxxxxxxx>
>> Fixes: fab9288428ec ("usb: USB Type-C connector class")
>> Signed-off-by: Sriharsha Allenki <sallenki@xxxxxxxxxxxxxx>
>> ---
>>  drivers/usb/typec/class.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/usb/typec/class.c b/drivers/usb/typec/class.c
>> index 35eec70..63efe16 100644
>> --- a/drivers/usb/typec/class.c
>> +++ b/drivers/usb/typec/class.c
>> @@ -1427,7 +1427,8 @@ void typec_set_pwr_opmode(struct typec_port *port,
>>  {
>>  	struct device *partner_dev;
>>  
>> -	if (port->pwr_opmode == opmode)
>> +	if ((port->pwr_opmode == opmode) || (opmode < TYPEC_PWR_MODE_USB) ||
> You don't need to check if opmode < anything. opmode is enum which
> apparently means that GCC handles it as unsigned. Since
> TYPEC_PWR_MODE_USB is 0 it means opmode < TYPEC_PWR_MODE_USB is never
> true.
>
>> +						(opmode > TYPEC_PWR_MODE_PD))
>>  		return;
> You really need to print an error at the very least. Otherwise we will
> just silently hide possible driver bugs.
>
> To be honest, I'm not a big fan of this kind of checks. They have
> created more problems than they have fixed in more than one occasion
> to me. For example, there really is no guarantee that the maximum will
> always be TYPEC_PWR_MODE_PD, which means we probable should have
> something like TYPEC_PWR_MODE_MAX defined somewhere that you compare
> the opmode value to instead of TYPEC_PWR_MODE_PD to play it safe, but
> let's not bother with that for now (it will create other problems).
>
> Basically, with functions like this, especially since it doesn't
> return anything, the responsibility of checking the validity of the
> parameters that the caller supplies to it belongs to the caller IMO,
> not the function itself. I would be happy to explain that in the
> kernel doc style comment of the function.
>
> If you still feel that this change is really necessary, meaning you
> have some actual case where the caller can _not_ check the range
> before calling this function, then explain the case you have carefully
> in the commit message and add the check as a separate condition:
We had a bug that was setting this out of index opmode leading to the
mentioned paging fault, and as you have suggested we could have added
the range check there and prevented this. But there are many calls to
this function, and I thought it would be a good idea to abstract that
range check into this function to prevent adding multiple range checks
over the driver. And further motivation was also to prevent any
potential unknown bugs.

I will resend the patch with the suggested changes; separate condition and
anerror statement if the above justification is acceptable, else will
propose a patch to improve the documentation.

Thanks,
Sriharsha
>
> diff --git a/drivers/usb/typec/class.c b/drivers/usb/typec/class.c
> index 35eec707cb512..7de6913d90f9c 100644
> --- a/drivers/usb/typec/class.c
> +++ b/drivers/usb/typec/class.c
> @@ -1430,6 +1430,11 @@ void typec_set_pwr_opmode(struct typec_port *port,
>         if (port->pwr_opmode == opmode)
>                 return;
>  
> +       if (opmode > TYPEC_PWR_OPMODE_PD) {
> +               dev_err(&port->dev, "blah-blah-blah\n");
> +               return;
> +       }
> +
>         port->pwr_opmode = opmode;
>         sysfs_notify(&port->dev.kobj, NULL, "power_operation_mode");
>         kobject_uevent(&port->dev.kobj, KOBJ_CHANGE);
>
> Otherwise you can just propose a patch that improves the documentation
> of this function, explaining that it does not take any responsibility
> over the parameters passed to it for now.
>
>
> thanks,
>
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