[PATCH v1] vfio/ccw: Flush notoper_work before returning from reset

[Yuho Choi <oss.patchbox@gmail.com>]

vfio_ccw_dma_unmap() is the DMA invalidation callback, which must unpin
the pages covering the range before it returns.  It calls
vfio_ccw_mdev_reset(), which raises a CLOSE event.

On the normal path fsm_close() calls cp_free() itself, so the pages are
unpinned synchronously.  But if the subchannel cannot be disabled or
quiesced, fsm_close() falls through to a NOT_OPER event, and fsm_notoper()
only queues notoper_work, which is where cp_free() then runs.  Nothing
waits for that work, so the callback returns with the pages still pinned.

Because notoper_work runs asynchronously, vfio_dma_do_unmap() can exhaust
its 10 retries before cp_free() completes, hitting BUG_ON(++retries > 10)
and taking the host down.  A further CLOSE cannot help either, as both
CLOSE and OPEN are fsm_nop in the NOT_OPER state.

Flush notoper_work after the CLOSE event, as vfio_ccw_mdev_close_device()
already does for the same reason.  The flush returns immediately when the
work was never queued, so the normal path is unaffected.

Fixes: ce4b4657ff18 ("vfio: Replace the DMA unmapping notifier with a callback")
Signed-off-by: Yuho Choi <oss.patchbox@gmail.com>
---
Found by code review.  Cross-compiled for s390 with gcc 14.3.0, W=1 clean,
but not tested on s390 hardware and I have no reproducer.

VFIO_DEVICE_RESET also reaches vfio_ccw_mdev_reset() and benefits from
this flush, ensuring channel program memory is cleanly released before
the reset ioctl returns.

On locking: the flush is not called with io_mutex held.  cp_iova_pinned()
takes and drops it before vfio_ccw_mdev_reset() runs, vfio_ccw_fsm_event()
takes no lock, and the VFIO_DEVICE_RESET path holds nothing.  This matches
vfio_ccw_mdev_close_device(), which already flushes the same work.

 drivers/s390/cio/vfio_ccw_ops.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/s390/cio/vfio_ccw_ops.c b/drivers/s390/cio/vfio_ccw_ops.c
index 5ce91285c7d5..8e0a2edbd34a 100644
--- a/drivers/s390/cio/vfio_ccw_ops.c
+++ b/drivers/s390/cio/vfio_ccw_ops.c
@@ -25,6 +25,14 @@ static int vfio_ccw_mdev_reset(struct vfio_ccw_private *private)
 	 * and re-opening the mdev, return an error.
 	 */
 	vfio_ccw_fsm_event(private, VFIO_CCW_EVENT_CLOSE);
+
+	/*
+	 * A failed CLOSE leaves the FSM Not Operational and defers cp_free()
+	 * to notoper_work. Wait for it, so the channel program pages are
+	 * unpinned before this returns.
+	 */
+	flush_work(&private->notoper_work);
+
 	vfio_ccw_fsm_event(private, VFIO_CCW_EVENT_OPEN);
 	if (private->state == VFIO_CCW_STATE_NOT_OPER)
 		return -EINVAL;

base-commit: f0100363d8c374bd8e9ea7c9ba02744f0b802ca4
-- 
2.43.0