Re: [PATCH v2] iio: trigger: fix use-after-free in current_trigger_store() on concurrent writers

[Andy Shevchenko]

On Mon, Nov 02, 2026 at 01:45:32PM +1000, Yu Zhang wrote:
> current_trigger_store() holds iio_dev_opaque->mlock only around the
> INDIO_BUFFER_TRIGGERED / trig_readonly checks and then drops it. The rest
> runs unlocked: it reads the current trigger into oldtrig, acquires the new
> one by name, stores it into indio_dev->trig, and drops the old trigger's
> reference with iio_trigger_put().
> 
> sysfs does not serialize writes to the same attribute issued through
> different open file descriptors (each open has its own kernfs_open_file),
> so two threads can run current_trigger_store() on the same
> .../trigger/current_trigger concurrently. Both read the same oldtrig, both
> overwrite indio_dev->trig, and both call iio_trigger_put(oldtrig). The
> extra put drops the trigger's refcount to zero while it is still in use,
> frees it, and a concurrent writer then dereferences the freed trigger.
> 
> With two sets of writers hammering current_trigger between two software
> triggers on an iio_dummy device, KASAN reports:
> 
>   refcount_t: addition on 0; use-after-free.
>   WARNING: CPU: 3 PID: 3467 at refcount_warn_saturate+0xbb/0xe0
>    kobject_get+0xf6/0x100
>    current_trigger_store+0x15c/0x4b0
>   ...
>   BUG: KASAN: slab-use-after-free in current_trigger_store+0x168/0x4b0
>   Freed by task 3468:
>    kobject_put+0x125/0x220
>    current_trigger_store+0x2cc/0x4b0
>   Allocated by task 3454:
>    __iio_trigger_alloc+0xc7/0x100
>   ...
>   Oops: general protection fault
>    __module_get+0x27/0xd0
>    current_trigger_store+0x192/0x4b0
> 
> Hold mlock across the whole operation so that reading oldtrig, swapping
> indio_dev->trig and dropping the reference are atomic with respect to
> other writers (and to buffer enable, which the -EBUSY check guards).
> Acquiring the new trigger with iio_trigger_acquire_by_name() under mlock
> is safe: it only takes iio_trigger_list_lock, and the buffer path already
> calls iio_trigger_{attach,detach}_poll_func() with mlock held.
> 
> The trigger swap itself keeps its goto based cleanup of the newly
> acquired trigger, so move it into a helper and take mlock with guard()
> in current_trigger_store() around the mode checks and the helper call,
> rather than mixing guard() and goto in one function.

...

> +static ssize_t current_trigger_store(struct device *dev,
> +				     struct device_attribute *attr,
> +				     const char *buf, size_t len)
> +{
> +	struct iio_dev *indio_dev = dev_to_iio_dev(dev);
> +	struct iio_dev_opaque *iio_dev_opaque = to_iio_dev_opaque(indio_dev);
> +
> +	guard(mutex)(&iio_dev_opaque->mlock);
> +
> +	if (iio_dev_opaque->currentmode == INDIO_BUFFER_TRIGGERED)
> +		return -EBUSY;

+ Blank line.

> +	if (iio_dev_opaque->trig_readonly)
> +		return -EPERM;
> +
> +	return iio_trigger_set_current(indio_dev, buf, len);
> +}

> +

Drop this blank line.

>  static DEVICE_ATTR_RW(current_trigger);

-- 
With Best Regards,
Andy Shevchenko