[PATCH] xen/events: avoid NULL pointer dereference in evtchn_from_irq()

[Juergen Gross]

There have been reports of races in evtchn_from_irq() where the info
pointer has been NULL.

Avoid that case by testing info before dereferencing it.

In order to avoid accessing a just freed info structure do the kfree()
via kfree_rcu().

Cc: Marek Marczykowski-GÃrecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Marek Marczykowski-GÃrecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 drivers/xen/events/events_base.c     | 10 ++++++++--
 drivers/xen/events/events_internal.h |  3 +++
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/events/events_base.c b/drivers/xen/events/events_base.c
index 499eff7d3f65..838762fe3d6e 100644
--- a/drivers/xen/events/events_base.c
+++ b/drivers/xen/events/events_base.c
@@ -247,10 +247,16 @@ static void xen_irq_info_cleanup(struct irq_info *info)
  */
 unsigned int evtchn_from_irq(unsigned irq)
 {
+	struct irq_info *info;
+
 	if (WARN(irq >= nr_irqs, "Invalid irq %d!\n", irq))
 		return 0;
 
-	return info_for_irq(irq)->evtchn;
+	info = info_for_irq(irq);
+	if (info == NULL)
+		return 0;
+
+	return info->evtchn;
 }
 
 unsigned irq_from_evtchn(unsigned int evtchn)
@@ -436,7 +442,7 @@ static void xen_free_irq(unsigned irq)
 
 	WARN_ON(info->refcnt > 0);
 
-	kfree(info);
+	kfree_rcu(info, rcu);
 
 	/* Legacy IRQ descriptors are managed by the arch. */
 	if (irq < nr_legacy_irqs())
diff --git a/drivers/xen/events/events_internal.h b/drivers/xen/events/events_internal.h
index 82938cff6c7a..c421055843c8 100644
--- a/drivers/xen/events/events_internal.h
+++ b/drivers/xen/events/events_internal.h
@@ -7,6 +7,8 @@
 #ifndef __EVENTS_INTERNAL_H__
 #define __EVENTS_INTERNAL_H__
 
+#include <linux/rcupdate.h>
+
 /* Interrupt types. */
 enum xen_irq_type {
 	IRQT_UNBOUND = 0,
@@ -30,6 +32,7 @@ enum xen_irq_type {
  */
 struct irq_info {
 	struct list_head list;
+	struct rcu_head rcu;
 	int refcnt;
 	enum xen_irq_type type;	/* type */
 	unsigned irq;
-- 
2.16.4