Potential bug in gpiolib-cdev.c in v1 notification about line info changes

[Gabriel Knezek <gabeknez@xxxxxxxxxxxxx>]

Hello GPIO maintainers,

While upgrading our system from the 5.4 to 5.10 kernel release, we noticed this potential defect in the gpiolib-cdev.c file: https://github.com/torvalds/linux/blob/master/drivers/gpio/gpiolib-cdev.c#L2255

In the lineinfo_watch_read routine,

} else {
                struct gpioline_info_changed event_v1;
                gpio_v2_line_info_changed_to_v1(&event, &event_v1);
                if (copy_to_user(buf + bytes_read, &event_v1,
                                                event_size))
                                return -EFAULT;
}

if userspace requests a GPIO v1 line info changed event, the kernel populates and returns the event_v1 structure. That structure (https://github.com/torvalds/linux/blob/5bfc75d92efd494db37f5c4c173d3639d4772966/include/uapi/linux/gpio.h#L367) contains 5 words of padding at the end of the structure that do not appear to be initialized in the gpio_v2_line_info_change_to_v1 routine (nor its subordinate routines):

struct gpioline_info_changed {
                struct gpioline_info info;
                __u64 timestamp;
                __u32 event_type;
                __u32 padding[5]; /* for future use */
};

It appears that this could be a potential kernel information leak to userspace, and could be fixed by zeroing out the padding field before copying the structure to userspace.

We wanted to get your thoughts on if you feel this is actually a bug, or if we overlooked something.
We're proposing to fix this issue by memsetting the entire structure to zero before calling the conversion routine; if you agree that that's a valid approach, I'm happy to submit an official patch.

Thanks!
-Gabe Knezek
gabeknez@xxxxxxxxxxxxx
Azure Sphere Team, Microsoft Corporation