2019-12-13 13:21:49

by Paul Durrant

Subject: [PATCH net v2] xen-netback: avoid race that can lead to NULL pointer dereference

In function xenvif_disconnect_queue(), the value of queue->rx_irq is
zeroed *before* queue->task is stopped. Unfortunately that task may call
notify_remote_via_irq(queue->rx_irq) and calling that function with a
zero value results in a NULL pointer dereference in evtchn_from_irq().

This patch simply re-orders things, stopping all tasks before zero-ing the
irq values, thereby avoiding the possibility of the race.

Fixes: 2ac061ce97f4 ("xen/netback: cleanup init and deinit code")
Signed-off-by: Paul Durrant <[email protected]>
---
Cc: Juergen Gross <[email protected]>
Cc: Jakub Kicinski <[email protected]>
Cc: Wei Liu <[email protected]>
Cc: "David S. Miller" <[email protected]>

v2:
- Add 'Fixes' tag and re-work commit comment
---
drivers/net/xen-netback/interface.c | 24 ++++++++++++------------
1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/drivers/net/xen-netback/interface.c b/drivers/net/xen-netback/interface.c
index 68dd7bb07ca6..f15ba3de6195 100644
--- a/drivers/net/xen-netback/interface.c
+++ b/drivers/net/xen-netback/interface.c
@@ -628,18 +628,6 @@ int xenvif_connect_ctrl(struct xenvif *vif, grant_ref_t ring_ref,

static void xenvif_disconnect_queue(struct xenvif_queue *queue)
{
- if (queue->tx_irq) {
- unbind_from_irqhandler(queue->tx_irq, queue);
- if (queue->tx_irq == queue->rx_irq)
- queue->rx_irq = 0;
- queue->tx_irq = 0;
- }
-
- if (queue->rx_irq) {
- unbind_from_irqhandler(queue->rx_irq, queue);
- queue->rx_irq = 0;
- }
-
if (queue->task) {
kthread_stop(queue->task);
queue->task = NULL;
@@ -655,6 +643,18 @@ static void xenvif_disconnect_queue(struct xenvif_queue *queue)
queue->napi.poll = NULL;
}

+ if (queue->tx_irq) {
+ unbind_from_irqhandler(queue->tx_irq, queue);
+ if (queue->tx_irq == queue->rx_irq)
+ queue->rx_irq = 0;
+ queue->tx_irq = 0;
+ }
+
+ if (queue->rx_irq) {
+ unbind_from_irqhandler(queue->rx_irq, queue);
+ queue->rx_irq = 0;
+ }
+
xenvif_unmap_frontend_data_rings(queue);
}

--
2.20.1

2019-12-15 19:43:13

by Jakub Kicinski

Subject: Re: [PATCH net v2] xen-netback: avoid race that can lead to NULL pointer dereference

On Fri, 13 Dec 2019 13:20:40 +0000, Paul Durrant wrote:
> In function xenvif_disconnect_queue(), the value of queue->rx_irq is
> zeroed *before* queue->task is stopped. Unfortunately that task may call
> notify_remote_via_irq(queue->rx_irq) and calling that function with a
> zero value results in a NULL pointer dereference in evtchn_from_irq().
>
> This patch simply re-orders things, stopping all tasks before zero-ing the
> irq values, thereby avoiding the possibility of the race.
>
> Fixes: 2ac061ce97f4 ("xen/netback: cleanup init and deinit code")
> Signed-off-by: Paul Durrant <[email protected]>

> v2:
> - Add 'Fixes' tag and re-work commit comment

I've added Wei's Ack from v1, if the code doesn't change substantially
please keep people's Acks.

Applied, thanks.