Re: [PATCH] gpio: cdev: fix a crash on line-request release

[Kent Gibson <warthog618@gmail.com>]

On Tue, May 23, 2023 at 05:51:01PM +0200, Bartosz Golaszewski wrote:
> When a GPIO device is forcefully unregistered, we are left with an
> inactive object. If user-space kept an open file descriptor to a line
> request associated with such a structure, upon closing it, we'll see the
> kernel crash due to freeing unexistent GPIO descriptors.
> 

nonexistent

But I'm not sure that works - gpiod_free() is null aware, so strictly
speaking "freeing nonexistent GPIO descriptors" isn't the problem.
You mean orphaned GPIO descriptors?

> Fix it by checking if chip is still alive before calling gpiod_free() in
> release callbacks for both v2 and v1 ABI.
> 
> Fixes: 3c0d9c635ae2 ("gpiolib: cdev: support GPIO_V2_GET_LINE_IOCTL and GPIO_V2_LINE_GET_VALUES_IOCTL")

The problem is also in v1, so do we want to consider backporting a fix
for that too?

> Reported-by: Kent Gibson <warthog618@gmail.com>
> Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
> ---
>  drivers/gpio/gpiolib-cdev.c | 22 ++++++++++++++++------
>  1 file changed, 16 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
> index 0a33971c964c..6830f668a1b0 100644
> --- a/drivers/gpio/gpiolib-cdev.c
> +++ b/drivers/gpio/gpiolib-cdev.c
> @@ -315,13 +315,19 @@ static long linehandle_ioctl_compat(struct file *file, unsigned int cmd,
>  
>  static void linehandle_free(struct linehandle_state *lh)
>  {
> +	struct gpio_device *gdev = lh->gdev;

It isn't clear to me what this is for.
The flow below now calls gpiod_free() less often, so not that.
It is there for the normal case??

>  	int i;
>  
> -	for (i = 0; i < lh->num_descs; i++)
> -		if (lh->descs[i])
> -			gpiod_free(lh->descs[i]);
> +	for (i = 0; i < lh->num_descs; i++) {
> +		if (lh->descs[i]) {
> +			down_write(&gdev->sem);
> +			if (gdev->chip)
> +				gpiod_free(lh->descs[i]);
> +			up_write(&gdev->sem);
> +		}
> +	}
>  	kfree(lh->label);
> -	gpio_device_put(lh->gdev);
> +	gpio_device_put(gdev);
>  	kfree(lh);
>  }
>  

lineevent_free() needs the fix too?

> @@ -1565,17 +1571,21 @@ static ssize_t linereq_read(struct file *file, char __user *buf,
>  
>  static void linereq_free(struct linereq *lr)
>  {
> +	struct gpio_device *gdev = lr->gdev;
>  	unsigned int i;
>  
>  	for (i = 0; i < lr->num_lines; i++) {
>  		if (lr->lines[i].desc) {
>  			edge_detector_stop(&lr->lines[i]);
> -			gpiod_free(lr->lines[i].desc);
> +			down_write(&gdev->sem);
> +			if (gdev->chip)
> +				gpiod_free(lr->lines[i].desc);
> +			up_write(&gdev->sem);
>  		}
>  	}
>  	kfifo_free(&lr->events);
>  	kfree(lr->label);
> -	gpio_device_put(lr->gdev);
> +	gpio_device_put(gdev);
>  	kfree(lr);
>  }
>  

TBH the fact you have to mess with sems here indicates to me the problem
lies in gpiolib itself.  As a gpiolib client, cdev should just be able to
release the desc back to gpiolib and have it cleanup the mess.

Not that I ever got my head around the whole gpiolib object lifecycle here
- for v2 I just followed what v1 did.

Also, gpiolib still reports an error when forceably removing chips that
have open requests:

    dev_crit(&gdev->dev,
			 "REMOVING GPIOCHIP WITH GPIOS STILL REQUESTED\n");

Any other gpiolib clients out there that this might impact?
Else why report that crit error if you expect it is dealt with?

So while this may fix the crash, I can't say I'm happy with it.

Cheers,
Kent.