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

[Juergen Gross <jgross@xxxxxxxx>]

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-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Marek Marczykowski-Górecki <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


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