[PATCH net] 9p/xen : Fix use after free bug in xen_9pfs_front_remove due to race condition

[Zheng Wang]

In xen_9pfs_front_probe, it calls xen_9pfs_front_alloc_dataring
to init priv->rings and bound &ring->work with p9_xen_response.

When it calls xen_9pfs_front_event_handler to handle IRQ requests,
it will finally call schedule_work to start the work.

When we call xen_9pfs_front_remove to remove the driver, there
may be a sequence as follows:

Fix it by finishing the work before cleanup in xen_9pfs_front_free.

Note that, this bug is found by static analysis, which might be
false positive.

CPU0                  CPU1

                     |p9_xen_response
xen_9pfs_front_remove|
  xen_9pfs_front_free|
kfree(priv)          |
//free priv          |
                     |p9_tag_lookup
                     |//use priv->client

Fixes: 71ebd71921e4 ("xen/9pfs: connect to the backend")
Signed-off-by: Zheng Wang <zyytlz.wz@xxxxxxx>
---
 net/9p/trans_xen.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/9p/trans_xen.c b/net/9p/trans_xen.c
index c64050e839ac..60adb3aadd63 100644
--- a/net/9p/trans_xen.c
+++ b/net/9p/trans_xen.c
@@ -274,16 +274,21 @@ static const struct xenbus_device_id xen_9pfs_front_ids[] = {
 static void xen_9pfs_front_free(struct xen_9pfs_front_priv *priv)
 {
 	int i, j;
+	struct xen_9pfs_dataring *ring = NULL;
 
 	write_lock(&xen_9pfs_lock);
 	list_del(&priv->list);
 	write_unlock(&xen_9pfs_lock);
 
 	for (i = 0; i < priv->num_rings; i++) {
+		ring = priv->rings[i];
 		if (!priv->rings[i].intf)
 			break;
 		if (priv->rings[i].irq > 0)
 			unbind_from_irqhandler(priv->rings[i].irq, priv->dev);
+
+		cancel_work_sync(&ring->work);
+
 		if (priv->rings[i].data.in) {
 			for (j = 0;
 			     j < (1 << priv->rings[i].intf->ring_order);
-- 
2.25.1