Re: [PATCH v3 8/8] s390/vfio_ccw: implement a crw lock

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:

Pre-existing issues:
- [High] Missing cancellation of work items (`crw_work` and `io_work`) during device release leads to a Use-After-Free vulnerability.
- [High] Concurrent updates to eventfd contexts (`crw_trigger`, `io_trigger`) in `vfio_ccw_mdev_set_irqs()` lack synchronization, leading to a Use-After-Free of the `eventfd_ctx`.
--

--- Patch [8]: [PATCH v3 8/8] s390/vfio_ccw: implement a crw lock ---
commit 9d63fadad88983453bdf16e8b5f5b2ddd01df2c8
Author: Eric Farman <farman@linux.ibm.com>

s390/vfio_ccw: implement a crw lock

Unlike the channel_program struct, which covers synchronous I/O
submissions and asynchronous interrupts, the CRW region relies
exclusively on asynchronous events coming from hardware.

Implement a lock to manage the list of those payloads, to ensure
they are read cohesively.

Fixes: 3f02cb2fd9d2 ("vfio-ccw: Wire up the CRW irq and CRW region")
Cc: stable@vger.kernel.org
Cc: Farhan Ali <alifm@linux.ibm.com>
Signed-off-by: Eric Farman <farman@linux.ibm.com>

> diff --git a/drivers/s390/cio/vfio_ccw_drv.c b/drivers/s390/cio/vfio_ccw_drv.c
> index 1d8c2ed9da509..2c0a27851342c 100644
> --- a/drivers/s390/cio/vfio_ccw_drv.c
> +++ b/drivers/s390/cio/vfio_ccw_drv.c
> @@ -125,8 +125,10 @@ void vfio_ccw_crw_todo(struct work_struct *work)
>  
>  	private = container_of(work, struct vfio_ccw_private, crw_work);
>  
> +	spin_lock(&private->crw_lock);
>  	if (!list_empty(&private->crw) && private->crw_trigger)
>  		eventfd_signal(private->crw_trigger);
> +	spin_unlock(&private->crw_lock);

[Severity: High]
This isn't a bug introduced by this patch, but is it possible for this worker
thread to race with the VFIO_DEVICE_SET_IRQS ioctl handler?

Looking at vfio_ccw_mdev_set_irqs(), it updates the eventfd pointers and
releases old contexts via eventfd_ctx_put() without holding crw_lock.

Could a worker thread read a non-null crw_trigger here, but before
eventfd_signal() executes, the ioctl frees the eventfd context, resulting in a
use-after-free of the eventfd_ctx?

>  }
>  

[ ... ]

> diff --git a/drivers/s390/cio/vfio_ccw_ops.c b/drivers/s390/cio/vfio_ccw_ops.c
> index 04800cfa779b6..277138f14b251 100644
> --- a/drivers/s390/cio/vfio_ccw_ops.c
> +++ b/drivers/s390/cio/vfio_ccw_ops.c

[ ... ]

> @@ -138,10 +139,12 @@ static void vfio_ccw_mdev_release_dev(struct vfio_device *vdev)
>  		container_of(vdev, struct vfio_ccw_private, vdev);
>  	struct vfio_ccw_crw *crw, *temp;
>  
> +	spin_lock(&private->crw_lock);
>  	list_for_each_entry_safe(crw, temp, &private->crw, next) {
>  		list_del(&crw->next);
>  		kfree(crw);
>  	}
> +	spin_unlock(&private->crw_lock);

[Severity: High]
This is a pre-existing issue, but does this function need to call
cancel_work_sync() for private->crw_work and private->io_work before the
private structure is torn down and freed?

If a work item was asynchronously queued (e.g., via vfio_ccw_chp_event()),
could it execute after the device is released and dereference the freed
memory?

The flush_workqueue() in vfio_ccw_sch_quiesce() doesn't appear to be
guaranteed to run on all paths, and might not prevent new works from being
queued before the structure is freed.

>  
>  	kmem_cache_free(vfio_ccw_crw_region, private->crw_region);
>  	kmem_cache_free(vfio_ccw_schib_region, private->schib_region);

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260723174751.1180334-1-farman@linux.ibm.com?part=8